High Fire Glazes Q and A

High Fire Glazes Q and A

I am happy to see that the High Fire Glaze page has been of use to some people, and that there have been a number of questions and comments. Now that the comment tally has risen to above 30 the comment section is getting a bit hard to navigate so I thought I would select some of the questions and answers and arrange them here under headings so that they are easier for everyone to read. 

Understanding Glaze Recipes 

I want to try out one of your glazes and need some help with my measurements to weigh these correctly. 
February 9, 2014 at 1:14 AM 
susie wong said... 
Hi I'm sue, 
I'm keenly interested in mixing my own glazes. I bought some ingredients and appropriate scales. But now I'm stuck. I want to try out one of your glazes and need some help with my measurements to weigh these correctly. It reads De Boos 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 ingredients except colorants which equal 95 and divided 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 desperately need your advise. 
Thanks kindly 

De Boos 144 + 3 rutile and 2 copper carbonate on stoneware body, electric fired to cone 10

February 9, 2014 at 11:52 AM 
Peter said... 
Hi Sue, 
Glaze recipes can be confusing, and you ask a very 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. 

Firing Schedules

I was wondering if you have a firing schedule with a slow cool or a hold at 900?
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. 

March 2, 2014 at 1:14 AM 
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. 


We fire everything at a gas fired cone 10 reduction. How will these glazes work in that situation? 
January 10, 2015 at 1:29 AM
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. 

January 10, 2015 at 10:38 AM
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. 

De Boos 144 + 3 rutile and 2 copper carbonate on stoneware body. Wood fired in reduction to Cone 11
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 

Glaze Defects - "Pitting"

What causes pitting? Does this happen in the cooling stage? I am getting too much pitting right now. 
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 

November 15, 2013 at 8:00 AM 
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 porous 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. 

Buttermilk.... blisters and crystals!

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), …..Crystals I know nothing about, not even micro ones. Do you know? 
November 17, 2014 at 11:40 PM 
Maggie said... 
Hi, 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! 
Regards, Maggie 

November 18, 2014 at 2:32 PM 
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, which could also give magnesium silicate crystals. 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!) Peter 

Buttermilk... Success!

December 19, 2014 at 3:12 PM 
Maggie Jean Gray said... 
Peter, 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!
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!!! Cheers, Maggie 

December 19, 2014 at 9:16 PM
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, Peter 
Maggie later emailed me some photos of her lovely work with this glaze. It interested me that she used the glaze over an underglaze design. Also the "buttermilk" Maggie uses is without the zircopax. It is nice to see variations on this useful glaze.

Wine cup by Maggie Jean Gray of Canada.
Maggie wrote...
Hiya Peter,
I took a couple of snaps of a wine cup that I hauled out of the kiln this morning, to show off the buttermilk. I hope the texture and depth shows in them (over a lighter coloured buff stoneware). The glaze typically glosses up quite a bit over the stain-painted/underglazed areas (I suspect they're fluxing it, just a little). Also attached a darker stoneware knitting bowl, earlier firing.

Incidentally, someone at the Guild transcribed the glaze incorrectly at one point, and the zircopax was left out - so mine doesn't have it.

Stoneware knitting bowl by Maggie Jean Gray with buttermilk glaze.

Body Clay 

Have you tried any of these glazes on a porcelain body? 
July 27, 2014 at 11:26 AM 
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!

July 27, 2014 at 4:04 PM 
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. 

Black Tenmoko on porcelain with rutile swirl decoration. Electric fired to cone 10.
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. 

Chrome red on porcelain. Cone 10 electric fired.
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 2 percent cobalt carbonate on porcelain. Cone 10 electric fired.
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 

PTM and clay body

PTM over stoneware, electric fired to 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 clay body is the PTM being used on? 
November 10, 2013 at 6:19 AM 
lwcoffey said... 
What clay body is the PTM being used on in the photos at the top of the page??? I really would like to try it and hope for a similar finish... thank you.. wonderful work you do.... :) 

November 10, 2013 at 3:39 PM 
Peter said... 
Hi Iwcoffey, Good to hear from you. The clay for that pot was almost certainly Southstone made by Southern Clays Dunedin NZ. The glaze also works well on Southern Clays Stoneware. I am not sure if you will have access to the same clay as me, as I don't know if you are writing from NZ or a different country, but Southstone is a fairly sandy, open,stoneware clay that, I suspect may have a fairly high amount of iron and other "goodies" in it. The Southern Clays Stoneware is less sandy, and may also have a little less iron in it (I am guessing here), but is not a "white" stoneware clay. 
I suspect that you should have good results over many stoneware bodies with this glaze, and you will need to fire to about cone 9. For best results it would be worth experimenting with a half hour soak at about 950 degrees C (1742F) as the kiln is cooling. Keep the glaze fairly thin in bottoms of bowls, as this glaze does move a bit and will tend to collect in the lowest part of a bowl. If too thick, this can become a bit ugly. 

Do let us know how you get on with the glaze. 
Best Wishes, P. 

A pleasant surprise.. PTM and Nuka..

May 6, 2014 at 3:48 PM 
Larry Coffey said... 
I had a very pleasant surprise when I glazed a mug using your PTM and added a triple dip to the rim of white Nuka, a glaze I saw on D.Michael Coffee's web site... the results were a drip pattern that I could never have done by hand or deliberately... I don't know how to post a picture here so I will attempt to email it to you and hope that you find it worthy of sharing on your page... many thanks.... Larry Coffey 

May 6, 2014 at 9:56 PM
Peter said...  
Hi Larry, Thank you so much for your comment, I am delighted that PTM has worked well for you, the combination with Nuka sounds a great idea. I would be really pleased to see a photo of the mug, so do email one to me.

Larry kindly sent me 2 photos of a mug he made with the Nuka glaze over the PTM glaze, and I am delighted to include one here.
Mug by Larry Coffey with Nuka glaze over PTM

Other Glaze Questions 

Raw Glaze

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. 
September 2, 2014 at 6:29 PM 
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. 

September 3, 2014 at 1:38 PM 
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. http://stevenhillpottery.com/StevenHillPottery/Articles_files/An%20Approach%20to%20Single%20Firing.pdf 
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 

Cone 6 request...

Do you have a cone 6 recipe for your striking blue glaze?
February 18, 2013 at 5:35 AM
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

Blue Chun glaze. Stoneware electric fired to cone 10.

February 18, 2013 at 6:06 PM 
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. http://opopots.blogspot.co.nz/2012/01/cone-6-glazes-bowls-tiles-and-tomato.html

No comments: