High Fire Glazes

These are glazes that I find useful for my work, and I will keep adding to the list as time permits, and as I discover more.  Some glazes are recipes from books or from on line, and I have tried to credit the source where I can. Many of the glazes are later modified by me as I adapt them to my use and to the materials that I have available in my part of the world.

All glazes should be tested before you use them on anything that matters. Your kiln, your clay, your glaze materials, your scales, your water, the thickness you apply your glazes, and your firing schedule are different from mine, and all these things make a difference to the end result!

I have attempted to list the glazes according to colour. This, of course, is unsatisfactory as it is possible to use the glazes to produce other colours as well, but it is a start!

  • Underglazes
  • Tenmoko Glazes
  • White Glazes
  • Red Glazes
  • Green Glazes
  • Crawled Glazes


I often put one glaze over another, all sorts of interesting inter-mixtures as a result of this, and because there are so many variables in terms of glaze thickness and what glaze goes over what, it is difficult to ever get two pots quite the same as each other with this method.

Two underglazes that I use frequently are both by the New Zealand potter, Len Castle, and are to be found near the back of a glorious book that was published in 2002 by Sang Architects and Company Limited to celebrate Len's work called, "Len Castle Potter".

These glazes can be fired in oxidation or reduction.

Len Castle underglaze 1 Cone 9 -11
Potash feldpar  24
Kaolin  6
Ball clay  3
Calcite  6
Silica  12
Red iron oxide 6

This is a very dark brown and fairly static satin glaze at cone 10.

Len Castle underglaze 2  Cone 9 -11
Mixed or potash feldspar  50
Kaolin  6.5
Calcite  15
Silica  19
Red iron oxide  9.5
Cobalt carbonate  0.5

This is more fluid than underglaze 1 and is nearly black

Tenmoku Glazes

These are dark brown to black glazes that usually "break" to a straw colour, mid brown, or iron red where they are thin.  I have two that I use quite a bit in the electric kiln, and I "inherited" them from my teacher, Peter Watson, when I started potting.  I use these glazes on their own or under other glazes.

I have fired these in oxidation and in reduction.  If anything my results were darker and nicer in my electric kiln. In my wood fired kiln the glazes were not as dark, and were a little "muddy", and I do wonder if the wood ash in the atmosphere tended to bleach out the iron somewhat??

Black Tenmoku Cone 10 - 11

Black Tenmoko on stoneware body. Fired to cone 10 in the electric kiln.

Talc 15,
Wollastonite 15,
China Clay 10,  (I used Ball Clay)
Silica 15,
Potash Feldspar 55, (I use Nephaline Syenite if I want the glaze to mature at around cone 9 - 10, ball clay in place of china clay also helps with this)
and red iron oxide 8.

Red Tenmoku Cone 9-10

This was fired to cone 9, and works OK with a bit of a red  showing around the rim.

Potash Feldspar  50
China Clay  10
Whiting  5
Dolomite  15
Silica  30
Red Iron Oxide  8

White Glazes

This glaze worked well in my wood fired kiln, and was good in my electric kiln. In the wood fired kiln, the clay body had more influence on the appearance of the glaze.

A28 Orton cone 10
Potash feldspar  33
Silica  27
washed wood ash  40
standard borax fritt  5

This was my own attempt at a "nuka" style glaze.

A28 woodfired to cone 10 in reduction.
The real thing is mostly made from rice straw ash, but East Otago New Zealand is not a great rice growing area, so an alternative needed to be found.  This works well in oxidation and reduction, has an interesting flecked appearance due to the wood ash, and is also good over underglazes such as the Len Castle underglazes that I have shown above.

A28 over Len Castle underglaze 2 fired to cone 10 in oxidation.

Magnesium Matt Glazes

I have only fired this one in my electric kiln, but suspect it would work well in reduction.

Buttermilk Orton cone 10
Potash Feldspar  29.3 (the recipe says Custer Feldspar but we don't have that in our part of the world).
Silica  24.1
Whiting 9
Kaolin 6.8
Dolomite 6.8
Talc  13.5
Gerstley Borate 10.5 (I still have some of this, but you could use Gillespie Borate or another substitute)
Zircopax  8

The glaze base is a white glaze that is in John Britt's Cone 10 Glaze book (The Complete Guide to High-Fire Glazes).

Buttermilk cone 10 in oxidation.  The colour stripes are manganese, cobalt, and red iron oxide.

The glaze has an attractive texture something like an egg shell, but whiter and a touch more shiny.  The glaze is nice on its own, but I had a hunch that it would make a nice cobalt violet, because the glaze contains quite a lot of magnesium*, and magnesium and cobalt tend to produce purple, or violet.  I really like this variation.

For violet with a dark "break" over rims or throwing rings, add 1 percent cobalt carbonate.


An excellent book called "The New Potter's Companion", by Tony Birks gives a list of several useful stoneware glazes. One dolomite matt glaze has the mysterious name, "BNO".

I have fired BNO in my electric kiln and in my wood fired kiln, both with good results.

BNO, oxidised 1250 Centigrade (2282 F). Source, Tony Birks, "New Potter's Companion".

Cornish Stone 50
China Clay 25
Dolomite 20
Silica 10
Whiting 5

This is a nice glaze that I have found useful in the past, both in oxidation in my electric kiln and in wood firings. The glaze had a smooth eggshell feel to it. Then something happened to my Cornish Stone..., not only did the price in my part of the world become prohibitive, but the material itself changed and became more refractory and not so interesting in glazes. I looked around for a substitute. I tried directly substituting Potash Feldspar, or Soda Feldpar for the Cornish Stone, and it did not work so well as the original. With the help of  Insite glaze software from digitalfire.com, I became more adventurous. Attempts to do a perfect substitution where the theoretical formula was the same, resulted in a glaze that melted... just.., but looked "thin" and too full of clay. Following my hunch and doubling the amount of Soda Feldspar in my substitute, gave me a nice glaze that may not be the BNO that I enjoyed a few years ago, but it is a glaze that looks like it will be useful.

My BNO, cone 9-10

Soda Feldspar 23.8
Potash Feldspar 9.5
China Clay 26
Dolomite 15.8
Silica 19.4
Whiting 5.5

My BNO over Peter's Red.

And..., what does BNO mean???  I have no idea!

Red Glazes

I have only fired PTM and Bailey's Red in my electric kiln thus far, but the Janet DeBoos Iron Red has worked well both in electric and wood firings, with wood firing being the best of all.

PTM Probably best fired to Orton cone 9.
Potash Feldspar  41
Ball Clay  13
Silica  13
Talc  10
Bone Ash  13
Red Iron Oxide  10

+ Dolomite  11
+ Lithium Carbonate  2

What I like about PTM, is it is a little "wild".  It moves and changes colour as it goes from thick to thin.  Throwing rings show up well, and close to, or under magnification, the colour is very complex, showing flashes of plum, tomato, blue, orange, and brown.  At time of writing I have only ever fired this in an electric kiln. If fired too high, or put on too thickly, the glaze can pool in an unattractive brownish puddle in the bottom of a bowl or cup.  

Close up of PTM with a little brushed on swirl of dolomite and water that provides extra fluxing

Note that with iron reds it is well worth experimenting with slow cooling.  A hold of about 45 minutes at  950 degrees Centigrade (1742 F) allows time for the iron to re-oxidise and iron crystals to grow.  A better, stronger red can be gained this way.

More about the origins of PTM can be found here on my blog post for July 24 2011.

Bailey's Red Orton cone 10.
Custer Feldpar 47  (I used potash feldspar) 
Silica  13
Kaolin  13  (I used ball clay to help lower the maturing temperature to cone 9)
Talc  10.5
Bone Ash  14
Lithium Carbonate  2.5

Red Iron Oxide  8

Bailey's Red Cone 10 oxidation on Southstone body.

Bailey's red inside the bowl shown above.

This is easier to use than PTM above, and produces fine, reliable iron reds that definitely benefit from a 45 minute soak at 950 Centigrade (1742 F) when cooling.  

As noted in the recipe, I substitute ball clay for the Kaolin, then I can fire this successfully at cone 9, which is kinder on the kiln and suites one of my clay bodies better than cone 10.

Janet DeBoos Iron Red Orton cone 10
Soda feldspar  50
Silica  20
Bone Ash  10.5
Whiting  6.5
Ball Clay  7 
Talc  6
Red iron oxide 11

This gives an orange red in the electric kiln and it likes heat.  In the wood fired kiln I have fired this to about cone 12 with reduction, and it is redder and more interesting than in the electric kiln.  Higher than cone 10, expect some movement.  I added extra ball clay to this by mistake in one glaze batch (probably ball clay went from 7 to 10 or 11 parts), and the glaze improved.

You will find the original glaze in Glazes For Australian Potters by Janet DeBoos.

My Wen Red Orton cone 10, oxidation.
This beautiful saturated iron red is based on a recipe that may go back to the Chinese Song Dynasty. I found it in a 1997 Ceramics Monthly article about a wonderful potter from Taiwan called Wang Chun Wen.

I modified the recipe, the original called for 10% red iron oxide and 4% Edgar Plastic Kaolin. I added 9 % yellow ocher to supply some of the iron, and all of the clay to the recipe. I also adjusted the silica and feldspar amounts to take account of the change of clay..

Cone 10 oxidation

47  Potash feldspar
10  Bone Ash            
8  Talc                    
17  Silica                    
9  Yellow Ocher      
8  Red Iron Oxide  

The original Wen/Conrad translation was as follows
48 Feldspar
10 Bone Ash
8 Talc
20 Silica
4 Edgar Plastic Kaolin
10 Iron Oxide

Chrome Red

I have developed good chrome red for cone 9-10. I tested this on a vase for the first time in October 2013, and this comes as a result of a series of tests on tiles over several firings that I did earlier in the year.

This has only been tested in my electric kiln thus far.

Peter's Red Cone 9-10

Silica 35
China Clay 10
Gerstley Borate 9
Nepheline syenite 18.5
Whiting 20
Dolomite 10
Bentonite 2

+ Tin oxide 5
   Chromium oxide 0.5

Peter's Red, fired to cone 9. 

This glaze is nice on its own, but also may prove useful under other glazes.

Peter's Red under My BNO.

Peter's red was appied as an inch band around the top of this vase before a crystalline glaze was applied. 

Chrome red under a crystalline glaze makes the crystalline glaze go brown where it is thin, and gives blue and purple colours where it is thicker. A small dab of the red glaze will travel a long way, from rim to foot of a vase, so very little is needed!

Green Glazes

This is for oxidation firings.

Green/blue Orton cone 10
Potash feldspar  8
Soda feldspar  8
Silica  28.25
Dolomite  18
Whiting  17
China Clay  16
Bone Ash  4
Tin Oxide  2
Copper Carbonate 1

This green/blue glaze is one that I developed recently.  It is a substitute for one that is in a Thames and Hudson glaze book (The Glaze Book A Visual Catalogue of Decorative Ceramic Glazes by Stephen Murfitt).

Here is the glaze from the Thames and Hudson glaze book that I based my own glaze on.  This glaze is listed as being for 1290 degrees C or 2354 degrees F with an hour soak.
Cornish Stone  28
Silica  20
Dolomite  18
Whiting  16
China Clay 12
Bone Ash  4
Tin Oxide 2
Copper Carbonate 1

I love the look of the glaze in the book, it is a green that shades through to clear on rims and edges.  The glaze crazes, but it does so attractively.  There is a watery clarity to the glaze that reminds me of a cold copper stained pool high in the mountains.  The original glaze uses Cornish stone, and this is a very expensive glaze ingredient in this part of the world, so, much as I like it, I cannot afford to use it in anything other than small amounts.  I set about making an alternative.  Unfortunately you cannot simply substitute potash feldspar weight for weight with Cornish stone, because both are quite different materials.  It is necessary to attack on several fronts simultaneously!   I fired up the Insight glaze software that I purchased recently, and fiddled around until I had something that was chemically very similar to the original glaze, but used soda and potash feldspar in place of Cornish stone, and adjusted the other ingredients in the glaze, both the china clay and the silica had to be increased appreciably.

When I tested the glaze on a tile I found that, to my surprise, my substitute was much better than the original, giving superior coverage when applied with a brush (the extra china clay in my substitute would have helped with this).  The colour was almost identical, as was the crazing.  This is a glaze with character, and I suspect that it will do well as a copper red if fired in reduction, but I have not had the opportunity to test that yet.  At cone 10 the glaze will move, and it pools attractively in the bottom of bowls, giving a bluer colour where thick, green where medium and almost clear where thin.  It is not really suitable for the sort of potter where everything has to be the same, but it is a lovely addition to my glaze repertoire.

An Update!
Late December 2012 I made an improvement to my own glaze.  My original as shown above does have a bit of a tendency to bubble, especially on rims.  I suspect that Whiting might be the culprit.  Whiting is calcium carbonate, and this releases a lot of carbon dioxide when firing and this gas has to bubble out of the glaze.  I took out the Whiting and introduced Wollastonite.  Wollastonite is calcium silicate, a naturally occurring mineral that is one of nature's "fritts".  This form of calcium has already got rid of the carbon dioxide, and is a bubble-free way of providing calcium in a glaze.  Wollastonite introduces silica along with the calcium, so other sources of silica in the glaze need adjustment.  My improved version of my green/blue glaze is as follows:

Wollastonite  20
Silica  18
Dolomite  18
China Clay  16
Soda Feldspar  8
Potash Feldspar  8
Bone Ash  4
Tin Oxide  2
Copper Carbonate  1

I was very pleased with how this performed.  The bubbling appears to have gone, and the craze pattern of the glaze is a little more subtle.  I noticed that the glaze has a greater tendency to give a response to any iron in the clay body, so there were some nice iron spots and a tendency to produce a warm rust colour where the glaze was very thin.  Although this recipe has the same theoretical chemistry as my previous one when put through Insight glaze software, the colour is somewhat greener than my previous glaze.

My green/blue glaze  Cone 10 same firing: Left, Wollastonite version, Right, Whiting version.

A Useful Base Glaze (especially good for copper green and rutile blues)

This is good in both electric and wood firings. In wood firings the range of colours is very lovely indeed, rutile giving blue, and copper giving a hint of purple.

Janet DeBoos glaze 144 Cone 9 - 10

Potash Feldspar 40
Silica 25
Whiting 15
Ball Clay 10
Zinc Oxide 5

I did not much like this glaze when I first made it. The glaze was supposed to be white, and for me it never was...., but I found later that it really makes a good base that will cope with a huge range of firing temperatures, and it becomes much nicer if red iron oxide, copper carbonate, or rutile is added, either individually, or together.  Some examples are given below.

De Boos 144 with 3% rutile, 3% copper carbonate, and 1% cobalt carbonate.

De Boos 144 with 3% rutile and 2% copper carbonate.
 This gives green in oxidation as in the photo above, and a range of colours in reduction through rutile blue to purple.  I have fired this to cone 12 in the wood fired kiln.

De Boos 144 with 3% rutile and 2% copper carbonate over Len Castle underglaze 2.
 Using this glaze over a dark iron-rich underglaze can give attractive blues.

De Boos 144 with 15% dolomite and 6% red iron oxide.
In this variation pollen-like yellow crystals form in the glaze.  Expect this one to run!  Where the glaze pools, expect blue chun effects.

Crawled Glazes

I have only tried these so far in my electric kiln.

Crawling of glazes is something to be avoided most of the time!  But it can be used as a special effect, particularly for sculpture.  I did some simple experiments with equal quantities of Nepheline Syenite and Magnesium Carbonate made into a glaze and applied over various under glazes.

Magnesium carbonate shrinks madly as it is being fired.  If you make a simple glaze with equal parts of Magnesium carbonate and Nepheline Syenite and put it over another glaze, you can get effects like the ones above.  I fired all the tests to Orton Cone 9
Top left 50/50 Magnesium Carbonate and Nepheline Syenite.   Top Right the 50/50 mix over PTM.
Bottom left the 50/50 mix over a tenmoku glaze.  Bottom right the 50/50 mix thickly over PTM.
All fired to Orton cone 9.

You are very welcome to ask questions and add to the comments on this page. From time to time I will select some of the questions and answers from here and add them to the High Fire Q and A Page so that it becomes a resource for people to use.


John said...

Very nice idea Peter,
I like the illustrative pics.

Diane said...

Thank you for sharing these, Peter. I will give these a try and let you know how I go.

Peter said...

Hello John (dad) and Diane, good to hear from you both. Do let me know how the glazes work for you Diane, it is always helpful to see how they go on other clay and fired in a different kiln. Good luck! P

mintyfairy said...

Hi thanks so much for the in depth descriptions and recipes

Peter said...

Hi Mintyfairy,
A Pleasure. I'll be adding more glazes soon. P.

chris houghton said...

Hi Peter my wife and I have a pottery studio in Rockport Texas USA WE like your glazes ....do you have a cone 6 recipe for your striking blue glaze CHRIS HOUGHTON

Peter said...

Hi Chris,
Thanks for writing in. Sadly I probably won't be able to be of much help with that as most of the work I do is with cone 10 or earthenware, so I have not had the need to reformulate my cone 10 recipes for cone 6.

I did a commission that needed cone 6 a year or so ago and did quite a bit of cone 6 glaze testing for that back then. Here is a link to the post I wrote about the testing.. and I included recipe or two in the post, however I suspect that you will know them already.


June Perry has very generously published a lot of recipes for cone 6 and cone 10 glazes on her web site, and it is well worth a visit.


Nina Roberts said...

Hello Peter, I just wanted to thank you for sharing your glazes. I'm pretty new to ceramics and I'm been looking for high fire glaze recipes that give a good result in an electric kiln - I was so happy to find your page. I mentioned it in my latest post on my own blog (which is very fledgling, just like my work - www.riverwifeclay.blogspot.com in case you want to see what I wrote about your glazes). Thanks again. Nina

Peter said...

Hello Nina,
Welcome to my blog (and thank you for the kind mention on yours), it is really nice that you are finding the high fired glaze recipes of some help to you. I had not really thought about it, but there does seem to be a bit of an information "gap" out there for high fired glazes in the cone 8-10 area that work well in electric kilns, most electric kiln users seem to be doing midfire glazes at cone 6 these days. I really must add some more glazes to this (I started with good intentions!!). Anyway, do feel free to ask questions if you need more information. Best Wishes, P

Mark said...

Thanks for the blog... I am finding it helpful. My wife is an accomplished potter and I am the neophyte kiln-tech. We live off-the-grid in BC and as such we are using a propane fired kiln.
i have done several firings and am definitely slowly gaining more control over the process. Glaze firings have been so far to cone 5. One thing that I am dealing with is that when the middle of 3 shelves hits cone 5 the top shelf is just reaching cone 4, so I am trying to sort the shelves accordingly. What causes pitting? Does this happen in the cooling stage? I am getting too much pitting right now.
still finding that certain glazes handle the propane better than others. All help will be massively appreciated. Thanks, mark

Peter said...

Hi Mark,
Good to hear from you, and I am glad that you are finding the blog helpful. Here are a few random thoughts that come to mind when I think about pitting, pinholing, and uneven kiln temperatures.

I don't think that pitting would be caused by the cooling of the kiln, but firing too fast can sometimes cause pitting and other glaze faults.

With a fault like pitting there are two things to consider, the clay that the pot is made of, and the glaze that goes over it. Pitting may be caused by gasses being released by something in the clay body and the glaze not healing over. There could also be a glaze ingredient that is the source of the problem. In either case, firing slowly for the last couple of hundred degrees, or "soaking" the kiln for half an hour or so at the peak temperature may solve the problem by allowing sufficient time for the gas to pass out of the pot and the glaze to heal over. You may also find that slowing the kiln down could help the temperatures even out between the middle and top shelves.

Pinholing can sometimes be caused by applying a glaze to a pot that has not been fired high enough in the bisque firing. In this case, the pot is so porus that numerous bubbles form in the glaze as it is applied to the pot, and these can sometimes lead to pinholing in the glaze.

At certain temperatures Whiting (calcium carbonate), can cause problems in glazes, as carbon dioxide is released and can cause pinholing. Substituting Wollastonite for the calcium carbonate is one solution. Wollastonite is calcium silicate, and would supply silica to the glaze as well as calcium, so the glaze recipe would have to be adjusted to compensate for this.

Another "problem" glaze material is colemanite. This can spit glaze in little drops on the kiln shelf and can sometimes cause adhesion problems, especially if fired too quickly.

A few glaze materials do not like reduction atmospheres, lead bisilicate glazes can blister, or glazes high in zinc can form a surface like the moon!

To sum up...
My hunch is that you are probably firing too fast, and should slow things down as the kiln approaches peak temperature.

Do let me know if any of this helps and how you get on.

Good luck, Peter.

Amber said...

Beautiful pictures and great post! Thanks!

Peter said...

Thanks for that Amber, glad you enjoyed it. P

Anonymous said...

Thanks for sharing your experience
The crackled glazes are really nice

Peter said...

Thank you "Anonymous",
It is good of you to write in. P.

susie wong said...

Hi I'm sue, I'm keenly interested in mixing my own glazes. I bought some ingredient and appropriate scales. But now I'm stuck. I want to try out one of your glazes and need some help with me measurements to weigh these correctly. It reads deeboos glaze 144
Potash feldspar. 40
Silica 25
Whiting. 15
Ball clay 10
Zinc oxide. 5

Do I weigh up 40gms of the feldspar etc. or do I add all the ingredient except colorants which equal 95 and dived each ingredient by that number. Eg 95 divide by 40 = 2.735 of the feldspar????
And then add 3% rutile Does this mean 3 grams of rutile
I deparely Ned your advise. Thanks kindly sue

Peter said...

Hi Sue,

Glaze recipes can be confusing, and you ask a very a good question. It is fairly common practice for glaze recipes to show the base recipe first. The base recipe is really the glaze at its simplest with no colouring added to it.

Oxides added to the base for colouring are often shown as an extra amount, and this is often expressed as an percentage.

In an ideal world, potters would make sure that the base glaze ingredients added up to 100, but often glazes get modified and become "untidy", sometimes adding up to slightly less than 100, or sometimes more!

In the example of glaze 144, which you rightly have added up to 95, the total is so close to 100 that there is really no need to recalculate the rutile, or other colouring ingredients that you might want to add to the base glaze. The slight oversupply of the amount of rutile will make almost no difference.

So, if something says 3%, use 3 grams if you have weighed out the other glaze ingredients in grams.

Some recipes also add CMC or bentonite after the base recipe is shown. They might be written as "+ 2% bentonite", or "plus 2 percent CMC". Again, no difficult maths needs to be done, if you have weighed out the other glaze ingredients in grams, then use grams for these amounts also, if you have used some other measure, such as ounces then you would use these.

Hope this helps! Sometimes I find it easier to think of glaze making as cooking, rather than science! Do let me know how you get on. P.

susie wong said...

Hi Pete, thanks for your reply. I mixed up my glazes. Some turned out ok. I've just made up a few cone 10 glazes. I was wondering if you have a firing schedule with a slow cool or a hold at 900. Thanks for you help.

Peter said...

Hi Susie,
Good to hear back from you. Glad that you are trying some glazes and some have turned out OK.

When I am firing regular cone 10 glazes in my electric kiln I usually switch off and let the kiln cool naturally after peak temperature is reached, however if I am firing an iron red glaze, then I do find that it can be beneficial to do a 45 minute hold at about 900 degrees, as this really can improve the red.

My electric kiln is lined with insulating fire brick and cools slower than a kiln that has been constructed entirely of fibre. Some people with fibre kilns find it beneficial to "fire down" for the first 200 degrees, keeping the kiln on a low power setting to slow the cooling, especially when firing matt glazes.

The general rule is to cool quickly for the first 200 degrees if you want shiny glazes, and consider slowing cooling if matt glazes are too glossy.

Some potters always do a hold at peak temperature, to even out temperatures in the kiln and to let glazes settle and get rid of any gas bubbles. My kiln is usually quite slow near the peak temperature, and it may take more than an hour and a half with a full load to climb from 1200 degrees to cone 10. If I am having any problems with bubbles in glazes I slow things down from cone 8 until cone 10 is down.

Best Wishes, P.

studio wendy said...

Peter, I am so pleased to have found your blog! I have been working with Cone 6 (Orton) electric firings but have been curious about cone 9/10. The simplicity of your glazes with the ease of your explanation is terrific. One quick question before I start my tests, have you tried any of these glazes on a porcelain body? Thanks a bunch!

Peter said...

Hi Wendy,

Good to hear from you, I am glad that you are enjoying the blog. I am using porcelain for a lot of my work these days, and have used several of the high fire glazes that I have featured here.

I use the Black Tenmoko frequently as a liner glaze for my crystalline glazed work, and also as a glaze on its own. It really needs cone 10, and probably benefits from a short soak at that temperature, and can develop a lovely deep black if given enough time.

My chrome red works very well on porcelain.

Buttermilk with 1 or 2 percent cobalt carbonate looks better to me on porcelain than on stoneware, and will give blue to purple colours that vary depending on glaze thickness.

Buttermilk with copper carbonate on porcelain seems less successful than the same glaze used on stoneware, it just makes rather a bright and uninteresting green.

I prefer the iron red glazes on stoneware as I think that the colour of the stoneware body is more sympathetic than the pure white of porcelain.

I also prefer the look of the green/blue glaze on stoneware, as I think that the stoneware body makes the areas where the glaze is thin more interesting.

It is worthwhile trying any of these glazes, and good to experiment with layering one over another. Often the most exciting things happen where glazes overlap.

Hope this encourages you to experiment. Do keep in touch and let me know how you get on! P

Melissa Mead said...

Hi Peter - I really appreciate your blog and your work! I'm new to pottery but have learned to throw well enough to provide myself with functional ware. Glazing and particularly raw glazing fascinates me. I'd like to develop a high fire cone 8-9 raw glaze for once firing. I am wondering the most effective way to start this process and if you could suggest any books or articles. Google searches are useless. I'm most keen for the raw glazes simply out of practicality - the loading and unloading of a kiln not at my studio is an undertaking and a half not to mention the lag time between firings.

I look forward to hearing from you.

Peter said...

Hi Melissa,
Thank you for writing in, it is good to hear from you. It is a real achievement to be able to throw well enough to make your own domestic ware. There is something really nice about being able to use your own mugs and bowls in the home!

It is a bit late in the evening here as I write so this is just to let you know that I have read your comment and will try to put a more helpful reply together for you tomorrow. Raw glazing is an interesting topic, and can be well worth the effort to master. I have experimented a bit with it myself, and I have come across a couple of good books on the subject, and will track down the titles for you.
I will be in touch soon,
Best Wishes, P

Peter said...

Hi Melissa,
Here are a couple of book titles for you. I have personally used both books at various times, and they should give you some useful ideas about how potters approach raw glazing, and some handy tips. I see that both are available on Amazon.com, but your library may have them.

The first is a fairly old book, and suited to a "pioneer" approach with useful ideas about kilns and firing as well as glazing.

A Potter's Guide to Raw Glazing and Oil Firing Hardcover – January 1, 1980
by Dennis Parks

The second is a newer book that has very good information and you will probably find this the most useful of the two books.

Single Firing: The Pros and Cons (Ceramics Handbooks) Paperback – 28 Jun 1996
by Fran Tristram

I did a short web search and found a an article by Stephen Hill that appeared in Ceramics Monthly in 1986. This may also be of help to you too.

I think that it is true to say that most potters who raw glaze either glaze at the leather hard stage of drying or when work is bone dry. It used to be thought that raw glazes had to have 20 - 30 percent of clay content, but this is definitely not the case, especially if glazing bone dry pots. Often ordinary glaze recipes can be adapted to raw glazing, merely by substituting China Clay for Ball Clay, or adding 3 or 4 percent bentonite. The glaze has to match the shrinkage of the clay body as it dries. If you glaze at leather hard, the glaze will need a higher clay content than if you glaze when the work is bone dry.

Some clay bodies are easier to raw glaze than others, and you need to be prepared to experiment. A very "open" sandy clay may take up too much moisture from the glaze and collapse.

Raw glaze can be applied by spraying, brushing, dipping or pouring. All methods are possible, but some potters find spraying the easiest way to raw glaze. The potter Lucie Rie (1902 - 1995) brushed all her raw glazed pots. You might like to do an image search for her work. Lucie Rie began raw glazing for practical reasons fairly early in her life of potting. I seem to remember that she had to take her unfired work on a bus ride from one part of town to another, and it was easiest to do that only once!

It is best to begin with simple objects until experience is gained. Success is more likely if objects are evenly thrown, and are a medium weight, with not too much variation in thickness. Overly thin objects could be very difficult to glaze. A good, generous rim round a bowl will help.

Have you got a favourite cone 8-9 recipe that you use already, or are you needing to start from scratch with that?

Do keep in touch and let me know how you are progressing with this.

Best Wishes, P

Maggie said...


I'm Maggie - I'm a potter from Ontario, Canada. I've been using Buttermilk pretty exclusively in my electric kiln for the last year to a hard cone 9/soft 10 (any higher and it blisters), and have enjoyed some of the lovely effects I've had with it, and all it's subtle variations. I got the recipe from my local potters guild, and they use it exclusively in the gas kiln. Anyway, as it's 4 am, I woke up thinking about how to encourage the crystals that I sometimes get, and thought to google the glaze name. Lucky that your blog came up.

Anyhow, on to my questions - I've always assumed that the microcrystalline surface I sometimes manage to produce with it come from the borate. I'm hoping that you might know how to encourage that to occur more regularly - I tend to get some difference in the appearance of the glaze from top to bottom of kiln (more crystalline on the bottom, more regular/matte on the top), but haven't managed to pinpoint the conditions that are allowing for them. I'm wondering if they occur more often when I'm a few minutes late to shut up the kiln after it's fired off (computerized). Help. Crystals I know nothing about, not even micro ones. Do you know?

Anyhow. Also, lovely to see that you've coloured the glaze with cobalt. I'd been idly musing lately that I'd like to try colouring it with cobalt to see if I could produce any pinky violets, or copper to produce soft minty greens, and it seems that you're way ahead of me. Pretty!



Peter said...

Hi Maggie,
4am and thinking of glazes, you sound like a potter after my own heart! It is funny how, in the "wee small hours of the night", the mind starts to try to piece together the "whys" and the "hows" of glazing. Thank you for writing in. I am interested to hear of your experiences with Buttermilk, I have never encountered the blistering problem with this glaze, and have taken it to cone 10 really flat on a number of occasions. The glaze does get a little glossy where thin at that temperature and will move a little too. You may well have tried this solution, but a possible remedy for you if you really need to fire to a full cone 10, would be to soak the kiln from cone 9 until 10 comes down. That may give a little more time at top temperature and more chance for bubbles and blisters to heal.

Regarding the microcrystalline surface, slow cooling will help a lot to encourage crystal formation. I don't use a controller on my kiln, so I'm not sure what is possible for you, but can you "fire down" with your kiln, so that you have some gentle heat from the elements slowing the cooling? My intuition tells me that you would get the most benefit from slow cooling between 1150 and 800 C (2100 - 1500 F), but someone may know more precisely. The more crystalline results that you are getting in the lower part of the kiln could well be caused by the kiln furniture of the kiln storing heat and slowing the cooling there. You will also find the glaze looking more opaque if it is not reaching the same temperature as the top of the kiln (my electric kiln fires almost a cone lower on the bottom shelf).

I think that your crystals are most likely calcium silicate which will be due to the abundance of calcium in this glaze. The calcium is provided by the Calcium carbonate, Gerstley Borate, and Dolomite in this glaze.

Another important ingredient in the glaze is magnesium, and this is provided here by Dolomite and Talc. Magnesium will be responsible for much of the milky opacity and silky feel of the glaze.

There is a wonderful technical explanation of how high temperature Matte glazes work on http://digitalfire.com/4sight/glossary/glossary_matte_glaze.html

I had better get back to making bowls in my studio, but I have enjoyed reading and responding to your question. Do write in and let me know any further progress or discoveries with this.

Best Wishes (hope you get some sleep!)

Maggie Jean Gray said...


I've had lots of sleep. Well, mostly. November and December are busy months, as you know! I don't wake up in the middle of the night thinking about glazes now, I wake up trying to calculate how many items of stock I can make and sell between now and the holiday :p

With a little experimentation, I've managed to consistently produce microcrystalline surfaces with Buttermilk. You were absolutely right in your surmise to fire to cone 9 with a long soak to bring that last 10 cone down. I'm now soaking from cone 9 for 30-45 minutes, and it's working just like a charm. The glaze is producing lots of little crystals, and breaking to a thin clear gloss on the edge. If I could figure out how to attach photos, I would. I'm actually really enjoying this glaze on what would be a fairly toasty stoneware, if fired in a gas kiln, that is. The contrast of the white glaze crystals with the soft grey clay beneath gives the surface much more depth. Combined with underglaze decoration, it has a soft, lacy, feminine feel, and works well for what I'm doing right now. You're absolutely right about it getting a little glossy at higher temps (and moving much more), and as I'm half a cone out from bottom to top (now getting a solid ten on the bottom, and a soft ten on top), the pots on the bottom of the kiln aren't always my favourites. Regardless, I'm so pleased, and thank you so much for your thoughts!

Anyhow, Happy Christmas to you and yours!!!


Peter said...

Hi Maggie,
Thank you so much for your comment,it was very nice of you to write back, and I am really pleased to hear that the glaze is now working for you. It is interesting what can be done with minor changes in the way the kiln is fired. Regarding stock... the cobalt variation on buttermilk is a good seller for me, and always brightens up my display of pots! The white version can be beautiful though, and I like the lacy things that can happen where it overlaps other glazes.

Do send me a photo at opogallery AT gmail DOT com (with the necessary change to the email address!), it would be lovely to see what you are doing with the glaze.

Happy Christmas to you,


Mike said...

Peter, Great stuff! I am a novice potter and do all my work at a public studio. I am attempting to influence them to increase their glaze offerings. We fire everything at a gas fired cone 10 reduction. How will these glazes work in that situation? Mike K.

Peter said...

Hi Mike,
Good to hear from you, thank you for writing in. Well done for wanting to make things more interesting with the glazes at the public studio. It is always a challenge working in with shared kilns and so on!

I am confident that glazes such as the Len Castle underglazes, the black tenmoko, the wood ash glazes and magnesium matts all work well in reduction, as I have used them in my own wood fired kiln and in a friend's gas kiln.

The Janet de Boos iron red goes well with reduction, but I haven't as yet tried my PTM iron red or my chrome red glazes in reduction.

The green glazes have only been fired in oxidation thus far, but I would be interested to see if they would give copper red in reduction, and hope to try that myself one day.

The "Useful base glaze" de Boos 144 with 3 percent rutile and 2 percent copper carbonate was a really nice glaze in reduction, and I have used that one from cone 9 to cone 12. It is worth also trying that one over an underglaze such as the Len Castle number one.., you might get an optical blue that way.

Do keep in touch and let me know how you get on.

Kind Regards, P

Anonymous said...

I have a question. I am new at this and was wondering if I can underglaze high fire porcelain without glazing.
Like underglaze bisque to cone 06, then fire again to cone 9. I have some stuff i'd like to glaze in the same firing but some stuff I don't want to glaze just underglaze.

Peter said...

Hi "Anonymous",
Good to hear from you. If you fire porcelain to the temperature it is designed to mature at, then it should be vitrified (glassy) and should not leak or absorb moisture. Really the glaze for porcelain is to give it a particular colour or feel, rather than to make it waterproof. Thus, it is not strictly necessary to glaze porcelain, although most people do. It should be perfectly OK to just underglaze bisqued porcelain, then fire it to maturity, if that is the "look" that you want it to have. If the work is intended for domestic use, it still may be better practice to put a clear glaze over the underglaze, to make quite sure that it will not leach metal oxides into food or drink.

susie wong said...

Hi pete, I have used this clear glaze recepe and fired but it has turned out a milk foggy. potash feldspar 230 gms; whiting 110 gms; talc 150gms; Silica 130gms; bone ash 40gms ( I had no Kaolin so I used 150gms of Ball clay ; and 130 gm ball clay. I thick the viscosity of my glaze needed more water ...but My problems is I want to REFIRE these pieces to get a better result. The schedule I used was for a cone 9 stoneware. Celcius 200/100 ; 600/200 ; 1280/150 ; 1280/5. Can you suggest a refiring schedule that will help my glaze go clear which it was meant to be. Will a fast fire help also?? I don't know what to do. Help please

Peter said...

Hi Susie,
Good to hear from you. Agggggh, the frustrations of glazing pots! I think that all potters have had a similar experience to you with a "clear" glaze turning out cloudy!

Dealing with viscosity first...
These days I always weigh the water that I put in a glaze... I didn't for the first few years that I was making pots, but find it helpful now.

As a very rough starting point, I usually find that for every kilogram of dry glaze materials I use 1000 mls of water (which also weighs a kilo!).

If a glaze has more than 10 percent clay content in it, particularly if that is ball clay.., then I would almost certainly have to add more water... For a glaze such as the one you were using that has a lot of ball clay, I might be looking at 1.2 ltrs of water, possibly more, to 1 kg of dry glaze materials.

If a glaze recipe has almost no clay component in it, then sometimes I might need to have only 900 mls of water. In the case of crystalline glazes, where I need very thick glaze application, the water content might be as low as 800 mls of water to 1 kg of dry glaze materials.

A thin (watery) glaze usually goes on thinly.... A thick glaze usually goes on thickly.

A clear glaze will usally fire clear if it is put on thinly, and if it is fired to the correct maturing temperature.

It isn't always possible to correct a "milky" clear glaze if it is on really thick, however... a second glaze firing can sometimes help a lot.

If the glaze looks underfired.., for example it is chalky or matte where it should be shiny, then you may have to go to cone 10 to correct it.

If the glaze looks properly fired at cone 9... the right amount of gloss, then you could go to cone 9 again, but this time hold it for longer at the peak temperature. Your current schedule shows quite a rapid rise to 1280 and only a 5 minute hold at 1280. It may well be that the glaze has not had long enough at peak temperature to mature fully. Try extending the hold at 1280 to half an hour. Fast firing is the exact opposite of what you have to do to clear the glaze! The glaze needs time! Also, do put a cone 9 and a cone 10 in the kiln to verify the temperature that you have got to.

Hope all that helps, do let me know how you get on, or if you want further help with this.

Best Wishes, P

susie wong said...

Oh gosh, thank you for taking the time to explain that to me. Your feedback is extremely helpful. Thanks again for the adviice I will try again with the firing tips you suggest. I will let you know how it goes. Thanks again.

Theresa Yondo said...

Hello Peter, my name is Theresa Yondo and I wanted to thank you for posting your crawl glaze recipe. I work cone 10 electric and have been looking for a crawl glaze for a bit and this recipe worked beautifully. Again, thanks a million for your generous posting of this recipe. Theresa

Peter said...

Thank you Theresa for getting in touch, it is really encouraging to hear that the crawl glaze has worked for you. Good too to read of someone else working at cone 10 electric! (Cone 6 seems so popular these days!). P

Laurel said...

Hi Peter,

Thank you for posting such detailed glaze information - your blog is a treasure and I am enjoying it immensely. You are a gentleman and a scholar, as my father would say! :)

I am amazed to see your Chrome Red glaze, truly amazed. Is it really possible that I can achieve such a beautiful colour in oxidation in my electric kiln? Even a colour half and nice will be amazing, I can hardly wait to try it.

How important is the Gerstley in the Chrome Red? I live in Coastal, Central Queensland, and see Australian catalogues are already showing that it is a limited supply.

I am using a porcelaneous white stoneware, Walker PB103, but my glazing colleague has some Southern Ice on hand. Is porcelain critical to your Chrome Red?

Your Buttermilk glaze looks lovely, and I can't wait to try it either. You have made me a very happy and inspired potter! :D

Cheers and kudos from another Laura. :)

Peter said...

Hi Laurel,
Thank you so much for your kind and enthusiastic comment! It is always really nice for me to know that the glazes I have put up here may be of help to someone. The Chrome red has been one of my most useful glazes really as it gives me an interesting red that fills an important "gap" in the spectrum of colours that are obtainable in oxidation firings. I use this glaze in my electric kiln, and it really can look almost identical to a reduction fired copper red (and frequently confuses people!!).

I have used the glaze on porcelain, on white and buff stoneware. Whilst it is probably best over a pale clay, it can still be interesting over darker clays and is worth trying over, or under, other stoneware glazes too!

If Gerstley borate is hard to get, you could try Gillespie borate, I think it likely to work without having to do major adjustments to the glaze. It is possible that a calcium borate frit may work, but you would have to do some testing. My reading around the subject suggests to me that fine tuning the levels of borate in the glaze should affect how purple the red is, more borate = more purple. The calcium part is essential for the development of the red.

Regarding the calcium in the glaze...
When I was developing this glaze I did several tests with Wollastonite to see if I could use it to supply calcium and silica, and even though the glaze chemistry worked out the same, I could not get more than the palest pinks, it seemed essential that the calcium be supplied as whiting. The Gerstley would also be supplying calcium and I am not sure if using a calcium borate frit instead would also make the red harder to achieve??

Do keep in touch and let me know if you try the chrome red. I find it useful to apply it fairly thickly, and the colour can go from the palest hint of green where extremely thin, through red, to a slightly purple red where thick, sometimes with a hint that "bloom" that you get on ripe plums or grapes!

You can also play with slightly reducing the chromium content from 0.5 to 0.3 and seeing what that does. It is amazing just what colour such a small amount will achieve.

Best Wishes,

Stephen Cheek said...

Looking at your glazes - really lovely. As a beginner I am a bit confused by the frequent inclusion of Silica in your recipes, but I can't find this raw material listed in my suppliers' supply list. Is it the same as Silica Sand? I suspect not.

Peter said...

Hi Stephen,
Thank you for writing in. Your question regarding silica is a very good one as such things are confusing. You may find that your supplier lists quartz or flint, either of those would be just what you need for supplying the silica for the glaze recipe. (I use quartz). You would want 200 to 300 mesh quartz or flint for a glaze. Silica sand is usually too coarse for glaze.

It is always a bit difficult to think of an appropriate term for some materials, as different suppliers sometimes use different names for them; for instance, Whiting, Calcite are both names used on supply lists and recipes, but they are really the same thing, both supplying calcium carbonate.

Hope this helps! P :-)

Stephen Cheek said...

Thanks so much for your explanation, I was quite embarrassed at having to ask! I notice that your recipe raw materials for the tin-chrome red do not add up to 100. Is this because they are just actual weights rather than percentages? If so, are the figures for the tin & chrome oxide also absolute weights rather than % ?

Peter said...

Hi Stephen,
No need for embarrassment about asking questions, it is really good for me to be able to find out when things on this site are confusing for people, and this discussion may help others too!

There is quite a good convention regarding glaze recipes that the base recipe should be calculated to add up to 100 and the additional oxides that are added to modify the base for colour, or for other reasons are given as a percentage. All this is good in theory, and does help when comparing one recipe with another! The problem is that recipes often end up evolving over time, and frequently fail to be re calculated for the base to add up to 100 again.

I have noticed that many potter's recipes that have their origins before computers and glaze software, often start out adding up to all sorts of strange amounts. It is actually quite interesting (and fun) to look at old recipes, as you can sometimes trace their evolution from a simple beginning, to something that has been heavily modified.

You might find something like this,
Feldspar 80
Whiting 20
China Clay 10
Silica 15
You can speculate that a potter started with Feldspar 80, and Whiting 20, and probably made quite a nice stoneware glaze with it, but the glaze was a little tricky to work with. It would tend to form a hard mass in the bottom of the glaze bucket, and be easy to dust off the pot before it was fired. So the potter added 10 of china clay, which helped solve the problems of working with the glaze. At some future time, he tried to cure the crazing in the glaze and make it fit the clay body he was using, so added 15 silica!

Anyway, in answer to your question....
In the case of my own recipes (and probably the majority of other ones that you will find in books and on line) it is safe to assume that the numbers all work when taken as absolute weights.
Sorry about the confusion!
Best Wishes, P.

Stephen Cheek said...

That's pretty well what I expected, but thanks once again for being so willing to share your knowledge. I find that potters generally are all so nice to each other, and don't get too protective about their successes! Away, I have just mixed up a small batch of the chrome-tin red and will be having a glazing session tomorrow. I'll let you know how things go! Stephen :)

Peter said...

Hi Stephen,
Other potters have certainly been a great encouragement to me over the years, and I always think of them with thankfulness when I can pass knowledge on. Good luck with the chrome-tin test, I find this glaze a really useful addition to the glazes that work well for me in the electric kiln. You probably do this already, but it is worth trying a test with the glaze on thick and the glaze on thin, as glaze thickness makes quite a difference to this glaze. Do let me know how you get on.

Stephen Cheek said...

Hi Peter
Just thought that I would let you know how I got on with the red chrome-tin glaze. I fired three pieces in my electric kiln (bottom, middle & top shelves). Now, my kiln is a bit temperamental, and the temperature varies quite a lot. The top shelf, which fired to cones 8/9 yielded a plain muddy chrome green, the middle shelf (fired to cone 9) had touches of red on the breaks, and the bottom (which strangely is the hottest, fired to cone 9/10) had 50% red coverage. So, it seems that for me it definitely needs to fire to cone 10, which I shall try again soon. What fun! Have been having a change today and playing with the Raku kiln!
Best wishes, Stephen.

Peter said...

Hi Stephen,
Really great to hear from you about the chrome-tin tests. Thank you too for the link to the photo. I have downloaded a copy of the photo, and wondered if you would mind if I put it up on the blog here sometime? I think that "temperamental" kilns that have a wide temperature variation can be quite helpful when testing, in that you can fire once, and tests glazes at 3 temperatures!

My own experience with this glaze is that the colour becomes more opaque and paint-like at lower temperatures, but it stays red... however, I haven't done much in the cone 8-9 range, so it is new territory for me as well and it is very interesting to hear your results. Certainly temperature does affect things, and this glaze is probably best around cone 9.5 - 10.5, but I am suspecting that the green-red thing that you have going on might have an additional cause and my hunch is that it may be to do with glaze preparation and sieving!

I always sieve my glazes and make sure that they pass easily through an 80 mesh sieve 2 or 3 times prior to applying to the pot. I suspect that it is very important for chrome-tin reds to be sieved quite finely in order for them to work as they should. This would be particularly important if the firing temperature was a little on the low side as larger particles take heat and time to go into solution in the glaze. What mesh do you sieve to, and do you put the glaze though the sieve more than once?

The other thing that can dramatically affect colour is the quantity of chromium oxide that is used. If you are "heavy handed" in the amount of chromium oxide it is possible that you could get "muddy" colours or green instead of red. I think you really do have to have between 0.3 and 0.5 chromium in the glaze, any more is likely to be counter productive.

Hope you have a good day with the Raku kiln, they can be great fun!
Best Wishes, Peter

Stephen Cheek said...

Hi Peter
Please use the picture in any way you wish! I think that I'm probably a bit lazy in the glaze preparation - just passed once through an 80 sieve. Also, as I mixed up only a small amount of glaze it was quite difficult to accurately weigh the low percentage of chrome oxide. All things to pay a bit more attention to methinks! However, I shall persevere, as any red in an electric kiln is worth the effort. The raku session was most rewarding, and I had some fabulous metallic reductions. The only problem is that I smell like a smoked kipper!

Peter said...

Hi Stephen,
Glad that the raku sesson went well,the "smoked kipper" aroma could give inspiration for a future reducing agent for raku.... kipper reduction!! It would make a change from sawdust or newspaper!!
The weighing problem with chromium oxide is a challenge when doing small amounts. I only recently bought some digital scales, which have helped enormously. Prior to that I was weighing 5 grams, then dividing a line of chromium oxide powder with a knife... it worked, but did seem like something out of a B grade crime movie!
I'm off to make mugs and cups, although your mention of raku does make me tempted to go and have a fiddle with the wood fired kiln! P

Stephen Cheek said...

Hi Peter. I have just had a test firing (at Cone 9) using your PTM Red Stoneware glaze slightly modified. I didn't have any Ball Clay, so substituted China Clay, and used Flint for the Silica (I am not sure whether this is best, or possibly Quartz?). Anyway, the results were really pleasing - a bit 'wild' as you suggest, but quite beautiful with so many colours when examined closely. Next time I'm going to also try the Bailey's Red, which looks very similar except for the lack of Dolomite in the recipe. Thanks for publishing these. Stephen

Peter said...

Hi Stephen, I was delighted to read of your successful test of PTM, thank you for writing in about it. The china clay substitution may even help with this glaze as it will probably "stiffen" it just a little and might make it behave in a slightly more civilized fashion!! Glad you also found the beautiful colours when examined closely. I think that the dolomite may well help with those, it really is a most useful addition to glazes giving calcium and magnesium, and almost certainly some other helpful "impurities" that will add interest. If you substitute calcium carbonate and magnesium carbonate for dolomite, you probably will get a different result, even if theoretically the same chemically.

The flint for silica is definitely OK!!

I have found Bailey's Red a very useful glaze, but it seems to lack some of the complexity of PTM when looked at closely (not so many of the "beautiful colours"!).

Best Wishes, P

Claudia Mardel said...

Thank you, Peter, for you sharing all these amazing glazes.
I am pretty new to pottery and have fired at the Community Studio where glazes were provided and we fired in a gas kiln. I just got myself an electric kiln - Oh my, what did I get myself into :)?
Just a big thank you to you. I will be trying out some of your glazes after going on a shopping spree for ingredients this coming week...
All the best,

Peter said...

Hi Claudia, thank you so much for getting in touch to say "thank you"! It is really nice to hear from you. Congrats on getting an electric kiln, you will find that you can make huge amounts of progress by having to do the firings yourself. You are most welcome to write in and ask questions if you are stuck with anything.

I still fire to cone 10 in my electric kiln, but many potters and community studios are reluctant to do so because it does wear out the heating elements faster, and can be hard on kiln shelves. A potter friend of mine simply lowered his favorite cone 10 glazes a couple of cones by adding about 5 - 10 percent standard borax frit to his cone 10 recipes (I keep meaning to try this myself... but...). Anyway, you might like to do some experiments yourself if there are cone 10 recipes that you want to try at about cone 8. You can also often substitute ball clay for china clay in recipes to slightly lower maturing temperatures without changing glaze chemistry that much.

Do use Orton Cones as much as you can to check your peak temperature even if you have a nice shiny new controller for your kiln! Cones are a wonderful way of knowing how heat and time have affected the work that is in the kiln!

Anyway, very best wishes to you,
Have fun!


Thank you so much for taking time to write this, it is extremely helpful, I love glazing this is one of my favourite parts of making but it could be extremely confusing and tiring. Thank you Peter!

Peter said...

Hi Kyriaki,
Thank you for your "Thank you"! It is good to hear from you and to know that what I have written here might be of help to people. I must update this page with some more information and glaze recipes!
Best Wishes,


Claudia Mardel said...

Thank you Peter, I am amazed how you keep up with your blog. In the meantime, well, I have had the first few pieces break while bisquing - probably too impatient and the bigger pieces not dry yet.... Now I really love your site and I keep remembering someone was trying out one of your Blue's on Cone 6 and even wrote in what he/she changed and I cannot find it. I got all the ingredients for Cone 10 from the store and have now decided to instead fire Cone 6... I will let you know if I can actually find the comment on the recipe on your site :) and in case I do - what the results are. I used some purchased glazes but I am already over brushing it on from the little jars.... and while they are drying I am thinking maybe I can mix up some glazes to try it....
Anyways, Thank you so much for your offer, I I do get stuck I will be sure to ask. Really, Thank you.
All the best,

Peter said...

Hi Claudia,
Good to hear from you, thank you for keeping in touch. Regarding work breaking whilst bisquing..

If the work has broken because it has been too damp, then it usually breaks with some force, pieces tend to move apart or be blown into small fragments. When bisque firing I usually take my kiln to 100 Celsius (212F) and keep it there for about 4 hours before continuing the firing. I check to see if there is steam coming through the top vent in the kiln by holding a mirror over it and seeing if it mists up. I wait until the mirror doesn't mist. I probably am overcautious with this, but I rarely have anything break.

Other cracking can be caused by firing too fast, or by cooling too quickly. I have known of people having problems with cracking because they fire with the spy holes that are on the side of most electric kilns left open throughout the firing, this can cause work to crack due to cold air hitting the work in the kiln. I leave the bungs out whilst steaming out the moisture from the kiln, then cover all side spy holes for the firing itself. I often leave the one in the lid open.

In the comments to http://opopots.blogspot.co.nz/2009/03/chun-glaze.html Mike wrote about how he adjusted a cone 10 blue chun glaze and its underglaze so he could use them at cone 6, he used lithium carbonate and some extra feldspar to lower the maturing temperature of the glaze.

Best Wishes,


carmel said...

hey Peter, I'm using a white glaze in a reduction cone 10 gas firing. i really like the colour on a standard stoneware however it gets pretty bad crazing and comes off slightly to matte for my liking. iv been told by the teachers to try and alter the glaze recipe but i don't know where to start :)

potash feldspar %38
china clay %22
dolomite %14
whiting %16
silica %10
zirconium silicate %10

(iv never made a glaze before)

can i replace the potash with soda feldspar or reduce it down and replace with something else? I'm trying to figure this all out with just the internet proving to be a challenge, any advice would be much appreciated!


Peter said...

Hi Carmel,
Good to hear from you.
You don't need to replace the potash feldspar, that isn't the problem in this case....

The usual "quick fix" to cure a bad crazing problem is to add extra silica to a glaze. I wanted to check if this was likely to help with your particular glaze so I had a happy hour or so looking at your glaze with the help of Insight Glaze Software.

What the software tells me is that your glaze has quite a high thermal expansion, which is why it will be crazing badly, the glaze is also very high in Alumina, which is one reason it will come out matte looking. The most interesting thing is that the glaze is very low in silica, and this does confirm for me that the most effective way to lessen the crazing in this glaze and to make it less matte is to increase the silica. You might also have to reduce the amount of alumina in the glaze slightly if the glaze is still too matte, you would do this by reducing the amount of china clay in the glaze.

If you can, make some little test tiles in the clay that you use and do some tests of the glaze with additions of 10 parts extra silica, 20 parts extra silica, and 30 parts extra silica. Observe what happens as the silica increases. I suspect that you will need at least 20 parts extra silica.

Once you get the glaze to work as you want to, you can re-total it so that -leaving out the zirconium silicate - all the other ingredients add up to 100.

Let me know how you get on! Good Luck!

carmel said...

oh my thank you peter i think I'm about to cry, that is the most helpful and kind information i have ever received from asking a question of a potter :) i would love to visit one day next time I'm in the south island, what a cool dude will let ya know how i get on!

Peter said...

Hi Carmel,
Happy to help! :-)
Do keep in touch, and do visit if ever in the South Island. If you are firing at Cone 10, you might find John Britt's Cone 10 glaze book helpful Here's a link to it on his site


There are some nice glazes in the book that would be good to try and quite a useful introduction to mixing glazes and how they work.

carmel said...

sweet :) will check that out, I'm new to pottery but very keen :)

about to make my own porcelain paper clay tomorrow! (if my 2 year old son allows) let me know if you and your wife are ever in the waikato! we have spare accommodation nice wine and food if your into that stuff, can talk glazes all night. thanks again Peter your the best!

Peter said...

Thanks Carmel, for kind offer of hospitality! Wine, food, and glaze talk sound a winning combination! Have fun with the paper clay! Best, P

Melissa Mead said...

Hi Peter
I'm trying to get more "professional' with my pottery. I practiced throwing about 50 mugs and even pulled handles (more fun than expected). I've got some questions regarding glazing if you would permit and a few other questions - I've asked you somethings before -I'd like to send you some pics but I'm not able to work out how. I'm going to put them up on my blog http://melissamead.blogspot.co.za so you can take a look. My immediate questions are…. I was trying to once fire at cone 6 but it seems my clays mature at cone 8 and higher - therefore I did my last once firing at cone8. My glazes were painted on (I did spray last time - most rims came out smooth) - I thought the painting on would give me a thicker glaze coat giving better coverage. Most of my rims are rough and sharp (I did finish them with chamois after throwing). Then I seem to have a lot of crawling - I used an established glaze but I had an accident with my mixer and it threw some black gook into my tests (I think it was oil). I used them anyway because I was curious if it would cause crawling or color changes. Do you agree that would cause crawling? Thank you so much for sharing your recipes and your amazing help to me and others as I learn such so much. I am in NZ later this month - is there anywhere in Auckland I can see your pottery in person?? We won't manage the South Island this trip.

Peter said...

Hi Melissa,
Lovely to hear from you again.

Well done with throwing 50 mugs and pulling handles. Pulling handles certainly gets easier with practice!

I put a little comment on your blog with my email, so you can always email me the photos of your work if you would like to.

Regarding glaze application...
I do most of my glazing by dipping work in the glaze or by pouring the glaze over the work, but most of my work has been bisque fired. Pouring or dipping are certainly much easier methods than brushing, but I am assuming that you are brushing your glazes because you are raw glazing your work and are trying to avoid your mugs splitting?

It could be that rough rims may have something to to with brushing glaze onto raw work, the brush behaving a bit like a sponge and disturbing or removing the fine clay particles as you apply the glaze?? I think that your comment that when you spray glaze that most of the rims are smooth confirms that the brushing is likely to be part of the problem here.

Raw glaze can sometimes soak into the clay body excessively over the course of the firing and this may also cause some roughness around edges and rims. I would imagine that a glaze that is high flux could do this (I am thinking sodium or wood ash here).

Glaze formulation....
Another interesting idea would be that it could have something to do with the glaze formulation. If you could write in with the glaze recipe you are using, this would be very helpful here.

A glaze that crawls badly may be one that shrinks a lot during the drying and/or firing and is too "stiff" to heal over again when it is at peak temperature. There are several glaze ingredients that can prompt a glaze to have this tendency, ones that come immediately to my mind are, magnesium carbonate, zinc oxide, tin oxide, ball clay, and other clays... A "problem" glaze can sometimes be cured by calcining some of the "difficult" ingredients. Calcining glaze materials usually involves heating the powder to a good red heat in the kiln. My crystalline glazes usually have about 25 percent zinc oxide in them, and it is almost essential to calcine this before the glaze can be used (I find 800 - 900 degrees Celsius adequate for zinc oxide or for clay ingredients. Calcining gets rid of the molecularly bound water content of the material, and effectively pre-shrinks it.

Glazes with a High Clay content....
Many glazes that are suitable for raw glazing have a high clay content to help them match the shrinkage of the unfired object that they are applied to. Some high clay glazes should be applied to leather hard clay, others are designed to be applied to dry clay.. and it is important for the shrinkage of the glaze to match that of the object, or the glaze may pull away from the object as it dries, or craze like a dry river bed and possibly not heal again over the course of the firing. You would probably notice this before the work went in the kiln.

Low expansion glazes... Shivering...
Glaze ingredients such as Magnesium carbonate, or lithium carbonate, can make for a low expansion glaze. This can be a good thing, in that it can help prevent crazing due to the glaze being in a state of compression, but excessive compression can cause peeling or shivering of the glaze. This is most likely to show first around rims, handles or other sharp edges. Where the glaze breaks away, due to excess compression, it will leave a really sharp edge like broken glass.

You ask about probable oil contamination of the glaze causing crawling.. If this was going to cause a problem I think it would have shown up as the glaze was applied or as it dried. I don't think the oil in itself would have caused the crawling that you describe.

Not sure if any of the above will have diagnosed the problem you describe, but I hope that something might be of help!

Regarding Auckland...
Sadly I don't have work in any galleries there, I am mostly to be found South of Christchurch in the South Island.

Hope you have a lovely time in NZ.

Claudia Mardel said...

Thank you Peter for your kind advice on bisque firing and responding to all of us newbies and puzzled potters!!! I really really appreciate your effort and time you are putting into this. I shall try the mirror method next time! and preheat like you said - I had it at 180 degree F which I guess is not hot enough....

And thank you for posting the Cone 6 adjustment link to your blue. I will try this next time...
I went ahead and mixed the Opal Blue from John Britts Cone 6 Book and it turned out VERY matte... Of course I went back to the book and right next to it is a recipe for SHINY OPAL - I guess I missed that little detail. So maybe I will add Feldspar to that matte Opal Blue since I made quite a bit of it or just start over. Ooops.....

Peter said...

Thanks Claudia,
Hope the Opal Blue can be persuaded to lose some of the matte, you might indeed find more feldspar helps... certainly worth a try!

Stephen Cheek said...

Hi Peter - greetings all he way from merry England! I have been having so much fun using your Bailey's Red, PTM and Chrome Red recipes over the last few months, that I am now trawling through all your other goodies. I think that I have a knowledge gap in my understanding of the basic differences between Ball Clay, China Clay & Kaolin. I had always thought that Kaolin & China Clay were the same thing just with different names, and that China Clay itself was a specific 'variety' of Ball Clay. Reading your recipes you use them all individually, so obviously there must be significant differences. Can you enlighten me?
Regards, Stephen.

Peter said...

Hi Stephen,
Oh to be in England... and all that! I guess you should be having the first Spring flowers bravely popping their heads up and trying to see if the world out there is full of snow, hail, rain, or sweet sunshine!!
Glad you are having fun with the recipes, it is lovely for me to think of someone making use of them, and I have more to put up there soon.

China Clay and Kaolin are the same thing. Many recipes say Kaolin (which may mean "high hill" in Chinese??), but I tend to write China Clay and abbreviate this to CC on scraps of paper when writing recipes, because I don't want to confuse it with the element K (potassium). Others, more clever than myself, use "china clay" because they don't want "kaolin" confused with kaolinite which is an "ideal" clay material.

Ball Clay and China Clay both supply alumina and silica and are often given the same chemical formula as Kaolinite for glaze calculations. However.... both materials have very different physical properties and ball clay usually contains more impurities than China Clay.

If you looked under a microscope you would see that ball clay has a very fine structure, and china clay had larger, much coarser structure. Damp ball clay is very plastic and tends to shrink a lot when drying and lose moisture slowly, damp china clay is "short" and probably nearly impossible to work with on the potters wheel. China clay shrinks less than ball clay when drying.

Due to impurities and its small particle size, ball clay tends to be slightly less refractory than china clay as a glaze material, and substituting ball clay for china clay in a typical cone 10 glaze recipe may mean the glaze will be able to mature half a cone lower or so, or make the glaze more glossy if fired at the same temperature.

Ball clay may also assist in helping glazes keep in suspension in the glaze bucket. Ball clay often gives a harder surface to an unfired glaze than china clay, which can be an advantage. Ball clay is very useful in glazes intended for raw glazing, as its shrinkage is a closer match to that of an unfired clay body.

The trade off is that the higher shrinkage of ball clay when drying can cause cracking problems (particularly if used thickly over a bisque fired pot) that might not heal over in the firing and trigger crawling of the glaze.

My own "rule of thumb" is that I like to use ball clay in a glaze that has a low clay content, say less than 7 percent. On bisqued pots I would be cautious using it above about 10 percent, especially if the glaze is to be applied thickly, and may calcine some of the clay content in order not to have excessive drying shrinkage. Over bisque, in glazes with a higher clay content, 10 percent and greater, china clay makes things a bit easier.

Calcine...??? Simple, just put a heap of ball clay (or china clay) in a wide bisque fired bowl and include it in your next bisque firing. The ball clay will lose its water content, including that which is chemically bound, and be "pre shrunk"! Be careful when handling this material as is ultra fine and dusty after calcining.

Just my thoughts, probably slightly unscientific but practical!

Best Regards, Peter

Unknown said...

Hi Peter,

I'm very excited to try out your chrome red. I'm new to glaze making, have a couple of questions:
for the silica, you mentioned in a previous post that you recommend 200 - 300 mesh, my supplier can offer: 200, 325. I fire to 1260ºC, what effect would the different mesh size make?

And the same question would apply to the whiting where they offer 15 µm and 10 µm. What effect would the different particle size make?

I'm based in South Africa, do you think the source of the raw materials would make a difference to the final result? Do you know of anything in particular to look out for when sourcing raw materials?

Thanks for all the great insights!



Peter said...

Hi Ephraim,
It is good to hear from you, thank you for writing in and welcome to the blog!
The finer mesh sizes help silica and whiting go into solution in the glaze more easily. In theory a fine mesh size should mean that the glaze will need less heat and time to reach maturity. If you made two test glazes, one with 200 mesh and one with 300 mesh, and fired them together in the same kiln, you might find that the 300 mesh glaze was more shiny or fluid than the 200 mesh, however, they might both look the same!

The chrome red recipe is really an Orton cone 10 recipe, but will fire lower than that. I think that the finer mesh and particle size could be an advantage to you as you are firing to 1260 C which is probably nearer to cone 9 in most kilns than cone 10.

The source of raw materials can make a difference, and this is one of the delights and frustrations of making pottery! The glaze materials, the clay that the pot has been made from, how thickly the glaze has been applied, and how long and hot the firing has been, all have an affect on the finished pot.

It does help if you know where the raw materials have come from, and to be sure that they really are what they are supposed to be! Occasionally raw materials can have the wrong label, or none at all, and as you get more experienced at making glazes you will find that you can sometimes identify materials by their feel or weight! Talc feels different to silica. Magnesium carbonate is fantastically light in weight, zirconium is heavy. Do try to notice what a material looks and feels like when you use it in a glaze, it is a good thing to learn.

A good glaze recipe will make success more likely, but every potter needs to test a new glaze to see how it behaves with the materials and kiln that he or she has. Test on small things first, in different parts of the kiln. My electric kiln fires a whole cone lower on the bottom shelf than in the middle of the kiln. This is useful when testing glazes as I can test at a range of temperatures in one firing!

I hope you have good success with the chrome red glaze, do let me know how you get on.

Best Wishes,

w Hutchinson said...

Hello, Peter! Firstly, just wanted to thank you again for your Red Tenmoku recipe which works beautifully on English Porcelain (Tucker's Pottery Supply, Richmond Hill, Ontario Canada!), at cone 9 in oxidation, using EPK for the china clay. This past weekend, I tried a test of the Black Tenmoku with the recommended adjustment for cone 9, but I think I need to adjust it a little more to get the glossy melt. Then again, as I said, the Red works so well, I'm quite happy to stay with it!

Meanwhile, I also tried a test of the Janet De Boos 144 with 3% rutile, 3% copper carb, and 1% red iron oxide, again on the porcelain at cone 9 in oxidation. Another success! I see great potential with this formula with variations both over and under the Hensley Clear (Pietro Maddalena, Certaldo, Italy) that I use for the interior of cups and such.

Thanks again for all your generous efforts!

Wendy H.

Peter said...

Hi Wendy,
Very nice to hear from you and that the glazes are working well for you. The red tenmoko is definitely a better one for cone 9 than the black one, which does like a bit more heat. The black TM also really likes a soak at the top temperature to develop the black. You could try slowing things down from cone 8 onwards and seeing if that helps. Failing that, the glaze might respond to 5 percent standard borax frit, but I haven't tried that one!

The De Boos 144 is a very useful place to start for a nice little range of glazes. I have had interesting results with additions of dolomite to that glaze base. Rutile blues are also possible in reduction atmospheres.

Thank you so much for writing in,

Eyca said...

Hello Peter.
My name is Eyca.
Thank you for sharing your glazes and knowledge. I used your Peter's Red Cone 9-10 glaze and it turned out pink. I fire to Cone 10 (2345 F) on my electric kiln. Do you have any suggestions to get red? Thank you again.

Peter said...

Hello Eyca,
Thank you for writing in and for trying the "Peter's Red" recipe. By "pink" I am thinking that you mean that it is pink and rather pale?

There are several things that might affect the colour and success of this glaze, I'll list some of them. I am sure you will have done most of these things correctly, but I'll put them here because they might help somebody.

1) Make sure that the ingredients are right and are weighed out correctly. The amount of chromium oxide in this glaze is so small that it can be tricky to weigh out (I have been known to make mistakes with this!), digital scales can help!

2)Fire to Orton cone 10 and no more. Turn the kiln off as soon as the cone is down. I once shared a firing with a friend who said she was firing to cone 10. What she didn't tell me was that she fires to cone 10 then holds the kiln at that temperature for 45 minutes or so.... Needless to say, some of my glazes were over fired as I think that she would have had cone 11 down and cone 12 bending if she had been using them!

3)Glaze thickness (and this is the most likely to be the culprit).

It would be helpful to try a test with the glaze where you dip half of the test piece once with the glaze, and the other half you dip a second time (so that the glaze is twice as thick). Glaze thickness does make a difference with this glaze, and it is possible that you are using it too thin. The glaze test would help confirm this... hopefully, your test will come out pink where thin and red where thick.

4)Try a different clay body.
I have used this glaze successfully over several clay bodies, however, one that I used did affect this glaze badly. It is possible that there is something about the clay you are using that is affecting the colour.

5)Eeeek, it could be something else that I haven't thought of! :-)

Do let me know how you get on,
Good Luck,

Stephen Cheek said...

Hi Peter. Your Bailey's Red is proving a very useful glaze, and it works well on the clay bodies that I'm using. My only observation is that I seem to get a lot more black in it than your examples! If you look at the third picture on my own website (a jug) you'll see what I mean (http://www.stephenthepotter.artsociety.net/my_gallery.html). Any ideas about this? Also, whilst talking about this glaze, do you consider that the Lithium content would render it not food-safe? There seems to be a lot of confusing comment about this!
Do you have a good recipe for a clear glaze for cone 9?
Cheers, Stephen

Peter said...

Hi Stephen,
Thank you for keeping in touch. It was really nice to be able to have a look at your work, you are doing some great things with high fired glazes!

Regarding Bailey's Red, I think that your kiln is most likely cooling a bit faster than my one. Saturated iron reds do actually reduce when at high temperatures, even in an electric kiln, then re-oxidize as they cool. These glazes can benefit a lot from doing a 40 to 60 minute hold as the kiln is cooling, when the temperature is about 980 degrees C (1800 F). This can help develop more red through re-oxidation, and also grow iron crystals to a greater size. It is definitely worth experimenting with this when you next fire the kiln.

Unfortunately I have no way of testing lithium release in glazes, but I think Bailey's red has every chance of being perfectly safe. The glaze has a sensible level of silica and alumina, it is high fired to maturity and does not contain lead or copper. I would be more cautious about a "soft" earthenware temperature glaze that contained lithium, lead and copper. Copper encourages lead release, and I would think lithium would like to keep the lead company in those circumstances. In a good high temperature glaze, where the levels of silica, alumina, and flux are in sensible proportions, very little of anything is likely to release.

I'll get back to you about a clear glaze for cone 9.

Best Wishes, P

Geff Gomez said...

Hi Peter,
Geff from Perth here. I can only re-iterate your generosity with sharing glaze recipes and also following up with the many followers you have!
I tried the 1) Buttermilk; 2) Buttermilk with Cobalt Carb; 3) Bailey's Red and the 4) Deep Blue Speckle with Rutile and Copper Carb.
I was very happy with the results of 1), 2) & 4). Unfortunately the Baileys Red turned out a mottled dirty brown (3 out of 4 ain't bad!)
I fired to 1280c, electric and the clay body has a "reasonable" amount of iron ("LGH"). Do you have any advice please? Thanks in advance.

Peter said...

Hi Geff, all the way from Perth!!
Nice to hear from you, thank you for following the blog and for getting in touch. Glad that 3 out of 4 have worked, and interesting about the "mottled dirty brown" from Baileys Red.

Some ideas of things to test are as follows. Always a good idea to do tests where one thing at a time is changed if possible! Fear not, you don't have to do all the tests I have suggested all at once, but try some over the next few firings that you do.

1)Ingredients must be all present in the right quantity and correct!
Once sure of that then the two ingredients that I think are most likely to have an effect are the feldspar and the Red Iron Oxide. Your feldspar may be different to the one I use, it may help to try a Soda Feldspar rather than a Potassium Feldspar for better colour. I know that some Iron Oxides can be more powerful than others in a glaze, it may be worth testing with 10, 12.5, and 15 percent Red iron oxide.

2)Glaze application. It is worth doing a test piece with the test single dipped and double dipped in the glaze so as to see if the dirty brown is the same where glaze is thick or thin.

3)Clay body. It could be that there is something about the clay that is not suiting this glaze. Do try a test on something else if you can.

4)Firing. The cooling rate of the kiln could be part of the problem. Have a read of the reply that I wrote to Stephen (Just above your question), where I wrote about trying a hold at 980 degrees C as the kiln is cooling.

It would be interesting for you to try the cone 9 PTM glaze that I have given. A bit tricky in terms of glaze thickness this one as it can run and pool (so don't glaze right to the foot of something as the glaze may stick things to the kiln shelf!) but it would be useful to see if this will give you a red.

PTM Probably best fired to Orton cone 9.
Potash Feldspar 41
Ball Clay 13
Silica 13
Talc 10
Bone Ash 13
Red Iron Oxide 10

+ Dolomite 11
+ Lithium Carbonate 2

Best Wishes, P

Anna said...

Just posted a link to my Fb page.. hope that was OK.. you might get a few more lookers

Peter said...

Hi Anna,
Much appreciated, thank you!

Geff Gomez said...

Thanks for that comprehensive answer Peter. I will try your suggestions out and let you know - it will be a while though 😁.
Cheers, Geff

Peter said...

Hi Geff,
I put another iron red recipe up on this page, "My Wen Red", you might like to experiment with that one too. Good luck with testing (and try to enjoy it!!). P

Stephen Cheek said...

Hi Peter

Just had my first results using your Janet de Boos base with added copper/cobalt carnbonates. A lovely gloss finish with a good colour variation. Comes out best on my good quality (i.e. excpensive!) white stoneware clay, but has lots of 'pinpricks' on clay with high iron content. Really pleased with it - take a look at www.stephencheek.com/pics/SauceJug1.jpg & www.stephencheek.com/pics/SauceJug2.jpg

Stephen Cheek said...

Your "My Wen Red" looks amazing. Do you think that it could be satisfactorily modified to work at cone 9? Also, is Yellow Ocher the same thing as Yellow Iron Oxide?

Peter said...

Hi Stephen,
I have just been having a look at your photos of the sauce jug with the Janet de Boos base and it looks a really lovely result. Most impressive! Thank you for sharing the photos, it is very interesting to see how glazes work for someone else. The "pinpricks" on clay of high iron content could be from a number of causes, but I am guessing that there is something gassing out from the clay body. Some high iron clays benefit from a slow and careful bisque firing, with lots of time taken between say 800 and 1000 degrees Celsius to allow impurities to burn out. Sometimes firing to a higher than usual bisque temperature is also helpful. I have had this trouble at time with other clays and glazes, and also try to cure it by going slower whilst nearing the peak temperature of the glaze firing, or by giving a "soak" at top temperature. You have to be careful with this not to over fire the piece, so using cones to judge heat work is essential.

Regarding the "My Wen Red" glaze. You might find that it does work at cone 9 in its current form, in that it does slightly move at cone 10. Other simple modifications could be substituting all or part of the potash feldspar for nepheline syenite, or soda feldspar.

The good thing about Yellow Ochre is that it is a clay as well as being a useful supplier of iron oxide and other impurities. I think yellow iron oxide may be a synthetic version, and may miss out on the interesting impurities that the natural Ochre may contain.

Have fun with My Wen Red, it is a starting point for experimentation!

Stephen Cheek said...

Hi Peter. I have had a try with your Wen Red glaze, but using a modified version of the original Wen/Conrad recipe.
24 Potash Feldspar
24 Soda Feldspar
10 Bone Ash
08 Talc
20 Silica (Flint)
04 Ball Clay
10 Red Iron Oxide
Fired to cone 9 I am quite pleased with first results, although if anything I would say that it is slightly 'pinkier' than the Bailey's Red, which I am also using. Here's a link to a picture of the Wen's - www.stephencheek.com/pics/WensRed.jpg
Next one for me to experiment with is the Janet DeBoos Iron Red (cone 10), but do you think I should make any adjustments for cone 9, which is my maximum?

Peter said...

Hi Stephen,
Good to see your modified Wen/Conrad glaze, it is very pleasing to see that it is behaving itself very well at cone 9 with your modification. It would be interesting to see if you can do some further variations on this. You could try replacing the ball clay with some earthenware clay, the sort that would fire a good brick red at about cone 04. You could try adding more red iron oxide, or another source of iron such as crocus martis (Anhydrous Ferrous Sulphate FeSo4).

Regarding the DeBoos Iron Red, I have fired this at cone 9 in the electric kiln and it has worked. The electric kiln seems to give a more orange red than when fired in the wood fired kiln with some reduction. In general the glaze seems to improve at higher temperatures, but you should get something from it at cone 9. If the glaze really wasn't looking very nice at cone 9, my instinct would suggest adding 1 or 2 percent lithium carbonate to this one to lower the maturing temperature, lithium may also give a "redder" red.

Stephen Cheek said...

Hi Peter. As ever, many thanks for your constructive and helpful reply to my last post! I'll certainly try some further variations of the Wen's Red when next having a mixing session, and also have a stab at the JDB Iron Red as well. I'll keep you updated on results! Off to apply some handles to a batch of mugs now.
All the best, Stephen

ruby said...

Hi Peter,

repeating what many have already stated in the comments, thank you SO much for sharing your breadth of knowledge with the world. As a beginner potter who is starting to look into creating my own glazes this has been incredibly helpful. However, before I go out and purchase a bunch of these materials I wanted to ask you some questions..

I was looking at your Magnesium matt/Nuka style glaze and am wondering if they are food safe to be used on functional ware? Is a glaze only not food safe if it uses toxic chemicals? Your BNO glaze is really beautiful, i googled Dolomite glazes and Digitalfire is going on about how Dolomite creates creamy glazes that often aren't food grade...but can be sometimes?? Assume then it's not dolomite that is toxic, but that it's sensitive in glazes and prone to things like crazing?

Also, is Kaolin and china clay the same thing?? Can't find a solid answer on google and none of the pottery supply shops in my area have Kaolin, only china! I heard somewhere that it can be used as a substitute?

Thanks again

Peter said...

Hi Ruby,
Welcome to the blog, thank you for getting in touch. Dealing with your last question first, Kaolin and China clay are the same thing. (In fact I once had a bag of China clay from China, and it had Kao Lin written on it!) China clay can vary somewhat, depending where it comes from. Occasionally a particular brand of China clay may be specified in a glaze recipe, for example, "Grolleg" or "EPK", but, most glaze recipes don't require you to be that precise. China clay provides alumina and silica to a glaze, and having some clay in a glaze makes it easier to apply to a pot.

Regarding Dolomite glazes. Dolomite is a type of limestone, and there is nothing much that is toxic about it. Digitalfire is a wonderful site, and there is great information there for the studio potter and also for those who work on an industrial scale. It is important to remember that some glazes may well be wonderful for a studio potter, but not at all suitable for a large commercial manufacturer of dinnerware. A wood fired stoneware hand made mug may look even nicer for having some crazing, crawling and other glaze issues, but such things would not be appropriate on mass produced mugs that you buy in a department store. Dolomite glazes can work well on domestic ware and they have a lovely satin eggshell quality to them which is pleasing to touch. Being high fired cone 10 glazes these are fairly simple and robust, and do cope with the rough and tumble of life quite well. The glazes can craze, especially if applied thickly, but provided the pot is fired to maturity and the clay well vitrified, this need not be a health hazard. It is more a question of aesthetics.

Regarding the Nuka glaze. I used this on various things, including a teapot, some years ago, and they have remained craze free.

Do feel free ask more questions if you need more help.

Best Wishes,


Miyuki said...

I am new to mixing glazes and had a couple of questions about the raw materials. When recipes call for Red Iron Oxide(RIO), I have a choice of two types. RIO 521 and RIO 4284. Not sure what the differences are between the two. Also when a recipe calls for silica, it comes in two different mesh sizes, is it better to get the finer one?


Peter said...

Hi Shirley,
Good to hear from you, thank you for your questions. RIO 521 is natural red iron oxide and RIO 4284 is synthetic red iron oxide. The synthetic is more pure than the natural and would have a stronger colouring effect when added to a glaze recipe. I would suggest using the RIO 4284 unless you had a particular reason for preferring the natural.

Red iron oxide is a comparatively inexpensive oxide, and you might like to purchase a small quantity of them both, and make some tests so you can compare them.

Silica. Somewhere between 200 and 400 mesh will work in glazes. I prefer 300 mesh. A fine mesh silica should dissolve faster, more completely, and at lower temperature than a course mesh silica.

Substituting a 400 mesh silica for a 300 mesh silica may mean that you could slightly lower the maturing temperature of the glaze.

400 mesh is not usually necessary, although some potters who specialise in crystalline glazes think it helps with those glazes.

Be careful when using silica as the dust is hazardous, especially the really fine dust that you can't easily see! It is a good idea to wear a face mask when handling glaze materials.