Monday, January 23, 2012

Cone 6 glazes, bowls, tiles, and tomato sex!!

I have been doing a lot of glaze testing over the last few weeks.  Whilst this has been somewhat exhausting, I have gained a lot of useful information about some glazes that are good for around cone 6 to cone 7, let's say in the region of 1205 - 1230 Centigrade 2201- 2246 Fahrenheit, all depending on how quickly your kiln climbs up to temperature.  

To suit my clay I am firing until cone 6 is completely down and cone 7 is starting to bend.  There is sufficient heat in the kiln to bring cone 7 down as the kiln is cooling. 

Glaze tests and a commissioned set of bowls that I completed recently.
The most useful base glaze that I have found for cone 6 is a very easy to remember one. Danny Moorwood, a potter friend of mine recommended it, and I also found it on the (glossy base glaze) site; as follows:
20 Wollastonite
20 Fritt 3134 (F 4108)
20 Potash Feldspar
20 Silica
20 China Clay

Danny uses ball clay rather than china clay.  I have been mostly using a 50/50 mix of ball clay and china clay.  The substitution of ball clay for china clay does slightly lower the maturing temperature of the glaze.  I use fritt 4108, as 3134 is not available here.  I think they are fairly similar though, essentially they are a soft borax fritt.

This clear glossy glaze fits the clay I use very well, with no sign of crazing.  I did also try two other variations on the glaze that mention on their site, one was a low expansion version (to further assist in glaze fit) and the other was a satin version. To be honest I found neither as good as the simple one that I have recorded above.  Due to the high amount of clay in this glaze, it is possible to use it as a raw glaze on dry pots, and I have been doing this with some success.

Wollastonite is an interesting material, it is calcium silicate, and as such provides both calcium and silica to a glaze.  I suppose that it is really a natural fritt.  Wollastonite is used as a flux and has the advantage over calcium carbonate (whiting) in that it does not have to get rid of lots of carbon dioxide in the course of the firing, which can be a potential source of pin-holing in glazes.

These all look very similar, but have significant changes to the base glaze.
As an experiment, I did a version of the glaze with fritt 4110 (an alkaline fritt), soda feldspar and increasing amounts of petalite, my aim was a more alkaline base that would give me blue-greens with copper.  It didn't..... In fact my variations made almost no discernible difference (as shown in the photo above), but a complete substitution of petalite for feldspar made for a crazed glaze that was not quite mature.

One simple test that was fun to do (and far more rewarding), was the one in the photo above.  On the right is the base glaze with 0.4 percent of chromium oxide.  In the center is the base with 0.4 chromium oxide, and 0.5 cobalt carbonate.  On the left is the base with 0.4 chromium oxide, 0.5 cobalt carbonate, and 4 percent tin oxide.  The tin oxide is trying its best to make red from the chromium oxide, but the cobalt blue is making the red turn violet.

I also did a variation with increasing additions of dolomite.  The dolomite was to encourage some little crystals to grow in the glaze to make it more interesting to look at (a bit like looking into a rock pool at the beach), you might be able to see this process in the main sequence of tiles in the photo above.

The first complete tile on the left of the photo has a green translucent glaze that you can see right through even though it is fairly dark.  The glaze has 3 percent copper carbonate and two percent iron in it, but no dolomite. The tile to the right of it has 5 percent dolomite added to the glaze, which lightens it a little, and the one to the right of that has 5 percent more dolomite, notice how the glaze gets darker again, and there is a dusting of tiny pale crystals over it. By the time we get to the tile at the right end the glaze is looking a sugary and paler blue-green colour. This tile has15 percent dolomite added to the glaze.  I used the 10 percent dolomite version of this glaze to put on some bowls that I was commissioned to make someone recently.

It is always good to try the effect of putting one glaze over another, and it is especially good to try this when you are doing test tiles in a controlled and orderly sort of way.  When I am glazing pots I often put one glaze over another, and frequently cannot remember afterward what glazes I combined together in this way!  I always mean to write it down, but forget every time!  The combination in the photo above is chrome-tin red over "floating blue".  On its own the floating blue came out rather a muddy grey-brown for me (the sort of colour that a child's paint box goes if all the colours are muddled up together), but this was transformed into a lovely blue where the red glaze flowed over the top!

under-glaze.  The iron glaze tends to bubble up through the top glaze, and both go for a slide and create the lacy pattern that you see.

chrome-tin red on a 9 inch bowl (230mm).

One of the most exciting cone 6 glazes that I tested, and have started to use, is the chrome-tin red that I found on June Perry's web site,  She has posted a wonderful collection of glazes there for cone 6 and for cone 10, and it is a great resource for any potter who wants to test glazes.  I have used the chrome-tin glaze in the bowl in the photo above.  It is a bad photo unfortunately, but you can see from it that the glaze develops an opalescent purple where it is thick, rather like the "bloom" of a ripe red plum.

The glaze is as follows:
21 Gerstley Borate
16 Nepheline syenite
11 China Clay
20 Whiting
32 Silica
5 Tin oxide
0.15 Chromium oxide

In my first test of this glaze I put in too much chromium oxide.... not sure how much, but I know it was wrong because my second batch is a much paler green when I apply it to the pot.  When it is fired, the first version of this glaze comes out a darker purple-red; the second "correct" batch is a much brighter red.  Nice purples can be made by adding very small quantities of cobalt to the glaze, or, if the chromium oxide is left out, good blues can be made with additions of cobalt.

I glazed up some goblets and used the chrome-tin red on a number of them. One other splendid characteristic of this red is that it does "break" nicely over detail, so the throwing rings of the goblets show up well. The blue version of this glaze also shows detail well too.

I have been experimenting with some 4 inch tiles.  I have spared little thought for the tourist trade over the years, most of the time I have been fully absorbed in my task of learning to make pots, kilns, and other important things, but it is also a good idea to try to stay afloat financially, so......

I thought that sheep, cows, and kiwis would be a good start.  Here I am just incising a design into a leather hard tile with a couple of wooden tools and a needle.  Over the top I put a clear glaze with a bit of copper and iron in it.

I do like old Persian pottery, and I copied part of a Persian design when I decorated the tile below.  I have done hardly any under-glaze type decoration before, so it seemed a good way to practice.

Laura has been helping in the studio lately, and she made tiles too, and most of the test tiles.  She decorated the tile below whilst I was doing my "Persian" one.

It is supposed to be summer here, but has been quite cold and un-summery the last two days.  There was even talk of snow on some of the high hills....  In spite of all that, we have a very large crop of plums ripening on the trees in our garden, and these show in the photo of the lily that Laura took recently (below).

My tomatoes are heading towards being an expensive failure.  They did grow, but few tomatoes have shown any sign of setting.  Now that the weather is getting chilly, we will be rather lucky to have anything from them.  Anyway, in desperation I did some research into the personal habits of tomatoes, and found out that they are supposedly self fertile, but the flowers do need to be shaken around a bit, and the best time is the middle of the day!  So...., I have been out amongst the tomatoes around noon each day, giving them as much encouragement as I can!  I call it tomato sex, and I really hope that they enjoy it enough to make some baby tomatoes!

One useful discovery that I have made is that it is possible to use a scanner to record the glaze tests (one advantage of doing tests on flat tiles).  I also was able to record the recipes by flipping the test tiles over and scanning the reverse side.  The only thing to be really careful of is of scratching the glass on the scanner.
I write all the recipes on the backs of the tiles before they are fired with a paintbrush dipped in a watered down mixture of iron oxide and manganese.

Anyway, hope some of that is helpful.  I have been working rather long days lately, basically from dawn to when I go to bed, so I have not been able to find much time to spend on the Internet.  I am afraid that I am neglecting you all!
Kind Thoughts, P.


Cat's Ceramics said...

Absolutely fascinating! Thank you so much for sharing your tests! So helpful and very interesting. I love the Chrome-tin Red- very beautiful.
Happy tomato sex!!

Anonymous said...

to protect the glass of the scanner get an overhead projecter transparency... it's clear + thick enough plastic that it shouldn't effect the scan quality I think...
the bowls look awesome. :-)
Gotta run, have an exam later :S

(ps. merry christmas... it's a bit late but yeah i've been a slacker *i mean a forgetful studying person*)

Peter said...

Hello Cat,
The Chrome-tin red is a real treasure, I'm looking forward to trying it over coloured slips and doing some other minor tweaks with it. I'll play sweet music to the tomatoes, I think that might help them!

Hello Eleanor,
Happy Christmas to you to, and New Year... Don't worry about slackness, it is lovely to hear from you whatever the time of the year, and... I've been utterly hopeless this Christmas with not cards going out to people. Just soooo busy all at the wrong time. Feel bad about it, but... Great idea regarding the overhead projector transparency, thanks for that, I'll definitely use a transparency next time I try scanning the tiles. With ideas as good as that you'll pass your exams with no problems! Good luck with it all.
Love from Peter & Laura xx

Laura said...

Very, very interesting. Congratulations!

Linda Starr said...

Beautiful results and love Laura's and your tiles, that chrome tin red is great, June has a great web site.

Michèle Hastings said...

you definitely got my attention with the "tomato sex" title! I hope you get some babies soon.

you have been busy... i leave the glaze testing to Jeff, but I really should start doing more on my own. When we moved to NC we discovered that between the two of us we probably have a life time supply of wollastonite. None of our ^10 glazes use it! Our plan is to mix up some ^6 glazes and some electric firing. Thank you for sharing your tests.
Love the goblets!

Anonymous said...

pity that question didn't come up in my exam... Hopefully i said enough to pass, it's hard to tell here... will see in Feb if I did ok. At least I have an excuse for failing (if I do). I mean who does tertiary exams in a foreign language less than 3 years after starting to learn that language?!? :P ah I'm a little crazy. Just 1 more to go! I will then have to catch up on my emails.


Angie said...

As you know the technical side is alittle beyond me but your results are again stunning ...those bowls I hope, are appreciated by the purchaser... they are gorgeous. The tiles are amazing the fun factor of the cleverly sculpted animal ones ...and the touch of art nouveaux with a hint of the continental, ones are brilliant too. How lovely that you shared this task.

As for Tomatoe sex lol ....I start with the highest flowers and brush them with my finger tips or palm and then work down, ending at the top again ...this way pollen is distributed all over. I do this every few days when the flowers are blooming and have a 75% setting ...except for the odd plant that seems infertile!!! and nothing sets.xx

DirtKicker Pottery said...

Fantastic glaze testing and detailed info. Now I'm really inspired to start some glaze testing. Thanks for an awesome post.

Interesting fact about growing tomatos ;)

Melissa Rohrer said...

Appreciate the information. I would like to experiment with those purples, although that might interfere with my resolution to whittle down my glaze choices.

cookingwithgas said...

all this and time to fondle tomatoes! You are a busy man.
I am doing some lower fire glazing/firing but it seems to me I need a good cone 5 recipe any suggestions on lower the temps to 2185?
I would love a good base and go from there.
One day soon I hope to explore this temp range.
Shake it up! I had always heard that tom's like at least 50 degree's at night to set fruit... maybe a bedding down along with the shaking?

smartcat said...

Hi Peter....we used a varient of this glaze in my old pottery co-op. We named it after the member who brought it to us.....she may have been responsible for the variation......this was over thirty years ago! We fired it from ^6-^8....looked good at all those temps.

Jane's Base:
Frit 3134.........1000
Ball Clay..........1000

As I recall it's a most forgiving glaze which is excellent for beginners. Those who worked at ^6++ developed lots of variations. Superpax and/or tin and rutile with oxides made many interesting variations. (EX: Warm Green....500 pax; 150 dark milled rutile; 150 copper carbonate. The rutile gives it a nicely mottled appearance)

You have a good set of tests...lots to think about.

(my word verification is comment!)

Linda B said...

Lovely glaze tests. The reason you don't get turquoise blue from copper is that the alumina is high -you would have reduce it to 5 or 10% to get turquoise.

Armelle said...

Beautiful glazes Peter, they are really nice, I love them all and specially on the gobelets, your red chrome.
Must go now, I will send you "my chün recipe" by mail soon, as I go away tomorrow.

Best wishes


Hi Peter! I use a similar Cone 6 base glaze. I found that adjusting the Wollastonite but keeping the firing temperature consistent gave me a more matte glaze, which was fun to experiment with.

Your garden looks lovely! I am sitting watching it snow and feeling quite jealous...

Peter said...

Firstly, a big thank you to you all for writing in, and welcome to those of you writing a comment on this blog for the first time.
It was really good to have helpful suggestions regarding pottery, and tomato sex!!

Eleanor's idea of using overhead projector transparency for protecting the glass on a scanner when scanning test tiles is one that I will adopt straight away.

Thank you Linda B for the information about alumina needing to be low in glazes to achieve turquoise from copper. That does explain a lot, as there is considerable alumina in this glaze because it contains so much china clay. If I had really thought things through I should have remembered that the lovely runny Persian copper blue glazes that I admire so much on Iranian 12th - 13th Century pots were low alumina glazes. It will be interesting as a little exercise to do some "unscientific" line tests and see if I can reduce the clay content of this glaze to the point where I achieve a copper turquoise, whilst still maintaining a useful, practical glaze.

Thank you Smartcat for sharing a variation of this glaze base. It great that it worked right up to cone 8, which certainly makes it a most useful, and forgiving glaze for people with kilns that may have cool spots. I loved your word verification word... some of them are really collectible!!

Meredith (cooking with gas) and Angie both gave good nuggets of information regarding the romantic side of tomatoes. Meredith, I did wonder if the nights are getting a little cool here (which is frustrating as we are supposed to be enjoying summer!!), but I am not sure of the temperature that we are actually dropping to, it just feels like frost is almost a possibility some mornings! I think I'll have to supply a nice blanket for the plants to snuggle up in.

Angie, at 75% success rate with your tomatoes, I think you must have the magic touch!!

Armelle, your chün recipe would be much appreciated, thank you.

Best Wishes to you all, I must dash now and throw an enormous pot on the wheel! P.

Peter said...

Hi Julia (Henhouse Pottery,
Good to hear from you (you must have been writing as I was putting together my comment), adjusting the Wollastonite does sound well worth a try. It is exciting being able to extend what a simple base glaze will do. I also suspect that doing some slow cooling may be worthwhile to explore, as the Wollastonite should give some calcium crystals. I had some nice things happen to glazes that are on the bottom shelf of my kiln, where it tends to cool more slowly than elsewhere. I'll put some more flower photos up soon to encourage those of you that are "blessed" with snow! P.

Mieke van Sambeeck said...

what a load of information thank you so much, and now finding the time to try some out, I usualy try new glaze samples in every firing!
So these wil keep me busy for a while.

Peter said...

Hi Mieke,
Lovely to hear from you. Time..., a most valuable commodity, certainly a good idea to put new glaze samples in every firing (probably much saner than doing a huge lot all at once!!).
I'll be most interested to see any variations on the glazes that you try.

This morning, in a Ceramics Monthly (of 1981), I found an article about the variation of the glaze that Smartcat mentioned in her comment. It was most interesting to see the many combinations of oxides that had been tried with the glaze. It is certainly a well proven glaze base and very useful for mid fired temperatures. A small addition to the amount of frit could drop the maturing temperature to cone 5.

angela walford said...

ab fab peter loving the iron bubbling thru the white glaze is that like a tenmoku with a chun over??? prob not at cone 6 though.... love the hares fur look!!'
and great timing with the arrival of my wee electric kiln i was about to embark on some c6 tests....really intrigued with the overlapping of glaze combos v nice work peter!!!!

gz said...

I'd look at Mike Bailey's book on cone 6 glazes

Peter said...

Gidday Ang,
You're right with the iron bubbling through the white glaze. It is cone 6, and I'll send you the glaze combination for you to try when I next fire up my computer and have the glaze tests handy. The overlapping glazes are really very interesting, and I'm sure that there's a huge lot of possible things to try with that for cone 6.

Hi Gwynneth,
I'm promising myself a copy of Mr Bailey's book if I am on my best behavior this month! It looks a really useful book and I have been reading reviews of it for the last few weeks. Good of you to recommend it.

angela walford said...

hey thanks peter that would be great....the testing begins :))

Tiles and Pots said...

Hello Peter,
I stumbled upon your blog while doing a search on the web. What a great source of information you have here.
I love the green glaze test tiles you got.
You mentioned you used frit 4110 instead of 3134. I usually use 3134 like in the recipe. Do you think the glaze will look similar with 3134 and what if I use a different feldspar. I guess I have to do some testing. (not my favorite part of the process.)
Another question I have: I am doing a large tile project. Usually I make the tiles out of a cone 6 porcelain clay but the customer wants them in a red clay. I have never used red clay for tiles and so I don't know what glazes to use and whether I should make them out of terracotta or a cone 6 clay. They have very specific colors they are looking for: teal, olive green, mustard yellow, Indian red, aubergine. Since the tiles are in relief the glazes need to show the design so an opaque glaze would not work well here.
Do you have any suggestions for glazes that work well on red clay that show off the designs well either low fire or cone 6 would be OK with me.
Thanks so much,

Peter said...

Hi Chaya,
Good to hear from you, and welcome to my site. I did do some testing of the glaze recipe that has 4110
where I changed the feldspar and found little difference between Potash Feldspar or Soda Feldspar in the limited testing that I did. With copper carbonate for the green, I really struggled to detect a difference between the two feldspars.

As far as changing the frit, I haven't tried as yet. I know that 4110 is an alkaline frit and has a lot in common with 4124. I seem to remember that 3134 is a borax frit and has a slightly higher maturing temperature than 4110. You may find that makes a slight difference in the amount of gloss that the glaze has at cone 6, or it might make no difference at all! You might also see slight differences in colour response when using oxides and stains, however I suspect that this glaze base is reasonably "forgiving" of substitutions like the one you are proposing. The glaze will also tolerate quite a temperature range, so far I have taken it from about cone 5 where it is a bit "underdone" to just over cone 7, where it is perfectly happy. I note that Smartcat (who commented on this post) had an interesting variation on the glaze, and wollastonite has been replaced by Dolomite, and 4110 has made way for 3134, so do give it a try.

Regarding the red clay commission, I am full of sympathy, I always seem to be asked to do things that I don't currently do...., this is always very educational, but it does mean lots of extra testing. Moving to a red earthenware clay will very likely mean that you will have to glaze fire to somewhere between cone 04 and cone 02, this would mean a lot of testing of glazes to get one that fits your clay at that temperature. Your clay supplier may know of a commercially made clear glaze that is designed to fit your clay, and this could save you some time. You could adapt this by adding zircon for opacity, and oxides or stains for colour.

If you are currently working at cone 6, and already have some glazes that are "good friends" at that temperature, I would stay with that if you can and and try to find a clay that will give a good red at that temperature.

I've just had a quick search of cone 6 red clays and see that Laguna clays do a WC-420 Redstone clay that may be the sort of thing that you are after.

A glaze base such as the cone 6 one that I have been playing with in this post, is certainly a good starting point for a glaze that will do the things that you want. I noticed that most oxides that I tried show detail very well, and some give an attractive "break" over detail. Some stains are less helpful, as they can make things rather opaque.

Not sure if any of the above is very helpful.. I don't feel that I have been able to offer an easy "quick fix", and what ever you do will almost certainly require lots of testing! Good Luck, and do report back with your progress. P

Tiles and Pots said...

Hello Peter,
Thanks for all the info. I will definitely try this glaze recipe with the copper carb. the green I am currently using is one I mix up myself but it sometimes crazes and gets pinholes. I'll post the recipe when I get a chance.

As far as the red clay, I was also leaning towards sticking with a cone 6 clay. And yesterday I got in touch with another tile artist that I found online and she recommended the sculptural series by Georgies. They have some really beautiful glazes. I hope they look good on Bas relief. She said they do.
The opening of the show is tomorrow evening and My husband and I are going with some neighbors. I'm looking forward to seeing all the great cups and mugs.
Thank you once again.
Happy potting :)

Peter said...

Hello Chaya,
Thank you so much for getting back in touch. It will be very interesting to see what you do with the tiles. Good luck with the exhibition, it will be fun to see everything on display, and always nice to see lots of cups and mugs on show.
Best Wishes, P.


Hi Peter I'm novice in glaze making. On one of your green sample,(the first one on the first row on the left side), you write PU..2.. can you tell me what PU means?

Thank you

Peter said...

Hi Boutriack,
Good to hear from you. My writing is terrible and even I have difficulty understanding it sometimes...., so what you thought was a P was really an R! RU 2 is rutile 2!

Hope that helps. Good luck with your glazing. P.

RichKnobSales said...

Not sure what happened to my comment! Anyway, I was loving the turquoise tiles and would love the formula for them. Thanks in advance!


Peter said...

Hi Amy,
Sorry you had difficulty posting your comment, I did receive them both, but I have comment moderation enabled for old blog posts like this so that I can keep track of things.

If my memory serves me correctly, the two torquoise tiles in the bottom row of the photo were the basic cone 6 glaze base:

20 Wollastonite
20 Fritt 3134 (F 4108)
20 Potash Feldspar
20 Silica
20 China Clay

plus a glaze stain that I happened to have at the time, I'm guessing that I used about 7.5 percent. I'll have a hunt through my glaze samples later and see if I can add some more details to this. One good thing about this cone 6 glaze base is that it seems very forgiving, fits clay well, you can fire it a bit higher than cone 6 if your kiln is uneven, and it also behaves well when stains or other metal oxides are added.

Peter said...

Hi Amy,
just checked my cone 6 tests, the turquoise one was the base I gave plus 5 percent turquoise stain. I am assuming that you are asking about the two turquoise tiles on the bottom row of the first photo on this post?

I looked for further information about the turquoise stain on my supplier's web site, but it is just listed as turquoise 1300 + Centigrade, but no manufacturer's details given.


jim robertson said...

hi peter jr pottery here looking around to see if I whould should try some cone 6 glaxes thanks fantistic page.

Peter said...

Thanks Jim, I'm glad that you found the page helpful, P

Anonymous said...

What wonderful products. you make really beautiful things, all those patterns, figures and so pretty colors.

Peter said...

Hi "Anonymous",
Thank you for your kind words. I am glad that you enjoyed having a look at the blog. P.

Anneke Knegtmans said...

Hello Peter,
I'm an amateur potter. Stopped for 10 years due to work. Now I'm back since I'm retired from work and trying to find my way into the beautiful glazes people use these days. I love the red/blue glaze you showed and will certainly try it. I was looking for a base glaze to fire at cone 3 but don't know if that is possible as I only find low firing to cone 01. Then it jumps to cone 6 and higher. Would it be possible to lower the firing range of this glaze you are using?

Anneke Knegtmans, Napier

Anneke Knegtmans said...

Hi Peter, Anneke again here. Just forgot to ask if the base glaze you use is for oxidation or reduction firing.
Thank you.
Anneke Knegtmans, Napier

Peter said...

Hello Anneke,
Good to hear from you, welcome to my blog. Really nice to hear that you are finding your way back into potting again now that you are retired.

I have found cone 3 and cone 4 quite a good temperature range to work with. Our local earthenware clay from the Dunedin area is very happy at cone 3, which is high for a traditional earthenware (at cone 4 it is slightly over fired).

One really easy approach for a start would be to try some Abbots Clear glaze. Used really thin this seems happy enough down to cone 3 in my experience, and does fit our clay well too. From memory, I think I was using about 1.2 - 1.3 ltrs of water per 1000 gms of Abbots clear. I mostly dip my glazes. It isn't strictly necessary to add anything to Abbots Clear, but I do sometimes add a little ball clay, or bentonite. This assists suspension of the glaze in the glaze bucket, and makes it easier to apply to the pot, and more resistant to bumps before it is fired. 2 or 3 percent bentonite would be enough, or maybe 5 percent ball clay. Be a bit careful though, as adding these may slightly lift the maturing temperature, and you are towards the bottom end of that at cone 3. I don't know what suppliers you have in your part of the North Island, but, if you can't find Abbots Clear locally, just visit and you can buy it online.

As you will know, a clear glaze, such as Abbots Clear, can have other things added to it to give it colour or to make it opaque. You can experiment with iron oxide, copper carbonate, or cobalt carbonate, and maybe add about 10 percent zirconium silicate over darker clays for opacity.

Rutile is often useful in glazes to add some interest, and works particularly well with copper carbonate. I find about 2 percent rutile and 1 - 3 percent copper carbonate quite a good starting point.

I have also experimented with making my own glazes from scratch for that temperature range, and it certainly can be done. You can do it scientifically with glaze software, or do it by experimentation. Mostly it is simpler to start with a frit, such as 4124, or 3110, then add about 10 percent china clay, and probably some silica to improve glaze fit.

Something like 80 percent frit, 10 percent silica and 10 percent china clay would be a start. You would test that on a small test tile, and see if it fired glossy and clear, if it crazed, or if it looked opaque and underfired. Then you would make adjustments.

Good luck, and do report back with progress.

Best Wishes, Peter

Peter said...

Hi Anneke,
I just found your oxidation/reduction question, sorry about the delay! The base glaze is for oxidation. P.

Dorota said...

Hello from Cracov in Poland! I love artists and your pottery. You live so far away from my country but welcome to my, I've made few bowls. All the best! Dorota

Peter said...

Hi Dorota,
Good to hear from you all the way from Poland. There is quite a big Polish community here in Dunedin NZ. Best Wishes, P.

Josh said...

Hi Peter,

I'm currently a high school student in Canada and I was lucky enough to stumble across your blog! I have been looking for a cone 6 hare's fur glaze and your amazing chun glaze over tenmoku seems to be the only one that I can find that fires at cone 6. Most hare's fur and oil spot glazes that I've found require the oxidation of Fe2O3 to FeO and that doesn't really happen until cone 9 or 10. I'd be really interested to see if this works in my high school kiln (since I only have access to an electric kiln that goes to cone 6). Would you be willing to share the recipe for these two glazes with me?


Peter said...

Hi Josh,
Good to hear from you, welcome to my blog.

Regarding the chun glaze over tenmoku, that chun is really a cone 9 - 10 glaze, so I have not personally fired them at cone 6, however a few years ago someone called Mike did write in to say that he had managed to get the chun to fire at cone six by adding a small amount of lithium carbonate to the glaze and the underglaze.

In the comments that followed my blog post about chun glaze that I wrote in 2009
Mike wrote:
"i converted this to cone 6 (both the glaze and underglaze) using +.25 lithium Carbonate and +10 cluster feldspar. turned out incredible in oxidation and my 30 pieces sold out at my colleges pottery sale. ive also been adding colorants to this and it turns into a great base high gloss glaze.
May 1, 2011 at 9:23 AM"

Anyway, here are the two glazes that I have used between cone 9 and cone 11 to give a blue chun:

(The blue chun effect is best achieved by using two glazes, an almost clear one over one that is very dark.)

The top glaze part of the chun is as follows:
Potash Feldspar 46
Dolomite 6
Zinc Oxide 6
Whiting 10
China Clay 2
Silica 30
(I usually add 1-2 percent bentonite to this to make it easier to apply).

This glaze is put over an iron bearing glaze and will form a blue chun effect if put over an iron bearing glaze or slip. On its own it is a slightly milky to clear glaze, becoming more clear towards cone 9.

A suitable iron bearing glaze would be:
Silica 41
Potash Feldspar 34
Whiting 16
China Clay 9
Black Iron Oxide 10
Many other glazes with about this level of red or black iron oxide will do just as well, it is worth experimenting.

Sorry not to be able to give much advice about cone 6. I have noticed some really interesting looking cone 6 glazes on There is a glaze there called "bonemarrow glaze" that looks quite like some of the oil spot glazes
Also, on the same blog, there is a cone 6 glaze called Blue Hares Fur that looks a bit like my chun

Best Wishes, P

Ki2020 said...

Hi my name is Ki from the United States. In my ceramics class we have to start making our own glazes which is what brought me to your blog. I found it very insightful and helpful. I was wondering if you would be willing to share the two glazes that form the layered glaze with the iron bubbling through the white. Please. Thank you for all the information you have already shared!

Peter said...

Hi Ki,
Good to hear from you, I'm glad that you are finding the blog helpful. The layered glaze that you ask about can be very runny when it is fired so do take care to protect kiln shelves. It would be good to test it on the inside of a small test bowl first.

I think the dark underglaze may be one by Brian Gartside, a New Zealand potter, and the white overglaze is by John Britt. Both are for Cone 6.

The dark underglaze was as follows,
Frit 4108 28.6
China Clay 28.6
Nepheline Syenite 14.3
Silica 14.3
Talc 7.1
Whiting 7.1

+ Red Iron Oxide 10

.. and the white overglaze
Potash Feldspar 30
Gerstley Borate 30
China Clay 5
Silica 25

+ Zircon 10

(Note that Frit 4108 is somewhat similar to Ferro frit 3134, it is a low alumina high calcium boro silicate frit. You might be able to substitute 3134 for 4108 if that is what you have available).

Good Luck! P

Anonymous said...

Hi, awesome work~ I'm wondering about your use of chrome in food safe work. I've been told by teachers that chrome is highly toxic, is it really okay to use it in a bowl?


Peter said...

Hi Anonymous,
Good to hear from you. You ask a very good question.

It is always wise to question the use of any of the heavy metals that are in common use in glazes. Almost all of them are poisonous! It is possible to use most of them safely, and it is possible to make unsafe glazes too!

Chromium oxide is a highly toxic oxide, as are cobalt, nickel, copper, cadmium, manganese, and so on... Probably iron oxide is the only truly safe one!

I have a few "house rules" of my own regarding the use of such things in glazes.

I like to limit the use of oxides in glazes that may come into contact with food and drink to no more than 2 percent in a glaze, and preferably less.
It is far more likely that a small amount like this will be properly bound into the glaze and will resist leaching.
(my chromium red is less than 0.5 of a percent in the glaze for example).

I make sure that the glaze is fired to the correct temperature. Under firing is avoided.

I prefer to use a glaze that has a sensible amount of silica and alumina in it.

In a food container where food may be stored for some time I will usually try to use a simple, none toxic liner glaze in the part that might contain food.

A cone 6 base glaze such as this one

20 Wollastonite
20 Fritt 3134 (F 4108)
20 Potash Feldspar
20 Silica
20 China Clay

is a very safe base to use for small additions of metal oxides, as the glaze has a sensible balance of alumina to silica, and should be resistant to crazing on a clay that is mature at cone 6.

Probably the greatest time of risk from metal oxides is when the potter is mixing the glaze and applying it to the pot. Don't eat your lunch when glazing, keep free of dust, and clean up well... including your hands!

Stellaria said...

Lovely glaze tests!
Quick question - can you recall how much "too much" of the chromium oxide you put into the Chrome Tin Red to get that lovely purple from your first batch? The red is pretty, too, but......well, rather boring in comparison to the purple bloom!

Peter said...

Hi Stellaria,

Good to hear from you. Ha, Ha, regarding "too much"?? I have a confession..., I have old shop scales that are great for 2 kg amounts of feldspar, but I struggle with measuring small quantities of oxides. I am probably fairly close to a gram or 2. My best guess is that I was using close to 0.5 percent of chromium oxide in that first batch.

You can easily make a really nice range of purples by adding a little cobalt to this glaze. I sometimes make up a quantity of the base glaze, split it in half. In one half I make the regular chrome-tin red, and in the other I make up the base plus 2 percent of cobalt carbonate. This makes quite a useful strong blue. Then I make a series of mixtures between the two. That part can be done by eye.

Also note.... Glaze thickness does play some part in how this glaze looks. If really thin, it can "break" to a really pale green, if very thick it can become more purple. I suspect that the boron in the Gerstley Borate is the cause of this. In a clear borax based glaze, there is a great tendency to get a cloudy blueish area where the glaze is too thick.

So, to sum up, try about 0.5 percent chromium oxide, and apply the glaze fairly thickly to get a purple bloom.

Do let us know how you get on! P

EarthWind Stoneware said...

Thank you for the excellent post. I'm a newby to glazes (I don't understand what chemicals to use to get certain colors...yet) and was wondering if the glaze on the commissioned set of bowls (beautiful btw) was made from this base glaze? What chemicals does one add to get the yellow, blue, and green shades?

Peter said...

Hi EarthWind Stoneware,
Thank you for your question, it is good to hear from you. I think that I used the same base glaze for most of the commissioned bowls. (I think there may have been two bowls that had other bases, one being the chrome/tin red glaze, and the other being a dark green bowl where I probably added some dolomite to the regular base.)

The cone 6 base

20 Wollastonite
20 Fritt 3134 (F 4108)
20 Potash Feldspar
20 Silica
20 China Clay

is a very useful and reliable one. It will give a clear glaze at cone 6, so if you want it to make a colour such as yellow, green or blue you will need to add something to it. Really you have 2 choices, one is to add a glaze stain (and this is what many people do these days), or the other is to use metal oxides.

Glaze stains are available in nearly every colour now and are fairly easy to use. Most will be compatible with that glaze base. In the USA I think many potters use Mason Stains, but other brands are available too, just ask your pottery materials supplier. As a rough rule of thumb, you probably will need between 5 and 10 parts by weight of glaze stain for every 100 parts by dry weight of your glaze ingredients, but you will have to test this. I find that it is helpful to have a good yellow glaze stain available for making bright yellows, I did use a turquoise blue stain for one of the bowls, and a bright green stain for one of the others.

Metal oxides have a more limited range of colour (and a potter who mostly uses oxides will probably also need one or two glaze stains to give a more complete colour range). Oxides often give a more "natural" look than stains. Most metal oxides are much "stronger" than the stains, and you will find that even 0.5 percent of cobalt carbonate with give a pale blue. Chromium oxide is also very strong and less than 1 percent in a glaze will be adequate (more than that will give a rather nasty opaque paint-like mid green).

A useful selection would be cobalt carbonate (for blue), copper carbonate (for green), red iron oxide (for an earthy yellow, rust red, brown, or nearly black), manganese dioxide (brown, or sometimes brown/purple), chromium oxide (green, or blue-green with cobalt carbonate).

For pastel colours, or if you want to cover a dark clay with a pale glaze, you may need to make your base glaze opaque. You can use Zirconium silicate for this, (often known as Zircon, Zircopax or zircosil). 10-12% will usually make a transparent glaze white and opaque.

Many potters live to a good old age, however.... Most of the chemicals that are used to colour glazes are hazardous materials when they are in powder or liquid form, so use them sensibly. If using a dusty material, wear a dust mask. Clean hands well after using them, and clean up work surfaces, aprons and tools. Don't eat in the studio!!

I hope that my answer isn't too confusing for you and gives you some where to start.

Best Wishes,


Pete the Potter said...

I am looking for a recipe for Celestial Blue, cone 6, oxidation. A deep dark transparent blue. Do you have anything. Thanks, Pete

Peter said...

Hi Pete the Potter,
Good to hear from you. I did a Google search for Celestial Blue and could not find a recipe, but did see a commercial glaze of that name that was very fluid and had small crystals that had formed in it. I haven't got a recipe for Celestial blue, but it is possible to make a very dark transparent blue just by adding about 2 percent cobalt oxide to a clear glaze base to a cone 6 base glaze such as this one.

20 Wollastonite
20 Fritt 3134 (F 4108)
20 Potash Feldspar
20 Silica
20 China Clay

This would give you something dark, transparent, and blue, but it may be that there are other qualities that you are seeking, such as a glaze that flows or runs, or a glaze that "breaks" to another colour where thin. You could make this glaze a little more fluid by substituting ball clay for china clay and Soda Feldspar for Potash Feldspar.

Tony Hanson's wonderful website has a recipe for a fluid cone 6 glaze
Something like this could well be an interesting base for experimentation, (you could leave out the tin and copper and add 1 or 2 percent cobalt oxide) and his explanation of how this glaze works is well worth reading.

Good Luck! Peter

Patrick Chaopricha said...

Hello Peter, this is a wonderful blog article. I'm quite fascinated by the chrome-tin red glaze and I'm planning to make a batch to try myself using your suggested recipe (or June Perry's). One question I have is the toxicity of the chrome oxide. I think you addressed some of this in your reply to somebody else, but I wondering about the firing of the glaze. In Brian Taylor and Kate Doody's GLAZE book, they mentioned that chrome oxide fume is very toxic. Is it a good idea to use this glaze and fire it in a kiln that mainly used for firing non-toxic and food safe glazes? I'm worried about chrome oxide residue and how it would contaminate the entire kiln when we fire it. I'm using the 0.15% as suggested in the recipe so that's not a lot of it in the entire glaze content. Nevertheless, based on your experience, is it safe enough to fire at Cone 6? We work with a lot of young kids in our pottery program (age 4-12) so safety is our utmost concern. Your insight would be greatly appreciated.


Patrick Chaopricha said...

Additionally, have you sent out a piece of the Chrome-Tin red to a test lab for leaching and toxicity test at places like BSC lab?

I'm thinking about sending it out for an analysis so that I can feel safe enough to use it in our studio.


Peter said...

Hello Patrick,
Good to hear from you. The question of toxicity of glaze materials is an important one, especially if you are dealing with children in your pottery programme. Without being paranoid, I still treat most glaze materials as if they were potentially hazardous. One of the most insidious hazards in a clay workshop is actually the dust from dry clay itself, and aprons and work areas should be kept clean and spills mopped up. Regarding Chromium Oxide, as a raw material I would not let young children work with it. Some Teenagers, maybe, but only if sensible and under supervision, and with disposable dust masks.

Chromium oxide as part of a fired glaze. I am confident that, if this particular glaze is fired correctly, the tiny chromium oxide content will present no hazard. I also would be confident that, at cone 6, this glaze is unlikely to contaminate other work in the kiln, or the kiln itself.

Regarding testing, I have not sent a sample of this glaze to a lab for testing.

As a "rule of thumb" I like to keep any hazardous metal oxides at below 2 percent in any glaze that I make that might be used on domestic ware. I would normally use a safe "liner" glaze inside a mug,cup, teapot or food container. A simple and reliable tenmoku glaze is what I often use for that, and if any metal was released from the glaze, it could only be iron oxide... which might even be of benefit if it were ingested!

I know that many potters, including those running children's classes, use glaze stains. And some manufacturers advertise them as being "safe". I still would treat such things with caution, and the "safe" claim with skepticism! I would try to keep to a relatively low percentage of them as colouring in a glaze to make quite sure that they were well incorporated into the glaze matrix.

Hope this of is of some help.

Patrick Chaopricha said...

Thank you so much, Peter. I do feel much better after reading your comments. I'll be the only one handling the chormium oxide in the studio as far as dry ingredients go. We actually have a separate area where I do all the mixing outside near the dry chemical storage with a fine particle filter dust mask and safety goggles. I also always wear rubber gloves when working with any of the ingredients. Are there some potential hazard for kids to use the liquid glaze if they want to do the dipping method? The 0.15% of Chromium oxide along with other ingredients mixed with water should be quite diluted I imagine. Maybe I should still have them wear gloves when using this glaze. Perhaps I should just make that a practice for any children using the glaze anyway so that they never have to get their skin in contact with the chemical. OK, I'm excited to see how this Red Tim Chrome glaze comes out. Thanks again.

Peter said...

Hi Patrick,
I'm impressed by your care with glaze preparation, you sound like you are doing a great job.

I think that it would be good practice for the children to wear gloves when glazing, particularly young children. The hazard whilst actually glazing is probably quite minimal, but the real difficulty with children (and adults!!) is to make sure that the hands are well washed afterward and there isn't glaze trapped under fingernails and so on. If gloves were worn when glazing, and taken off afterwards, it would reduce the risk of a child walking away with glaze on the hands and sucking a thumb, or eating lolly or lunch!

Do let me know how the chrome/tin glaze works out for you. I find it really helpful to make sure that chrome/tin reds are sieved through at least an 80 mesh sieve 2 or 3 times to make certain that the chrome is well dispersed through the glaze. 0.15 is a tiny amount, but should work well. If you wanted to you could experiment with a little more chrome oxide, but 0.5 is probably the absolute maximum. Additions of very small amounts of cobalt carbonate can shift these glazes from red through a useful range of purples to blue.

Best Wishes,

Patrick Chaopricha said...

Hi Peter,

Looks like I messed up the recipe for my first batch I made this weekend. I have not tried firing the glaze on my test tile yet to see what would happen, but I will. I accidentally substituted China Clay with Ball Clay. Please correct me if my information is wrong. Technically Ball Clay is a Kaolinite mixture and it's a secondary clay so it's not as pure like China Clay (Grolleg Kaolin). Bally Clay is also more plastic than pure Kaolin. I didn't have Grolleg on hand but I'm getting some soon so I'll mix up a correct batch to test also.

Do you have any ideas of what the result might be with Ball Clay substitution in place of China Clay? I imagine it's going to be a lot duller in color since Ball Clay is not as pure and white. The contamination of various minerals in Ball Clay would probably affect the end result as well. I looked up the chemical make-ups of Ball Clay and it seems similar to Grolleg Kaolin but it has about 10% lower content of Al2O3 but 10% higher content of SiO2. The other chemical (CaO, MgO, K2O, Na2O, TiO2, Fe2O3) are similar. Shrinkage (LOI) is also about the same at 12%.

I guess I'll just try it anyway and see since I have seen Ball Clay and China Clay in the same recipe for other glazes but usually a lot higher percentage of China Clay compared to Ball Clay. Hopefully the glaze won't be crazing since having a high plasticity clay might make it not stable.

Peter said...

Hello Patrick,
Fear not! The clay content of this glaze is not all that high, at 11 percent, so the "contamination" by trace elements from ball clay will probably not be very significant if it is a white ball clay, so I would not be surprised if the colour was unaffected. Definitely give it a try. We occasionally used to be able to get a local ball clay here that was very rich in iron oxide, and was an earthy yellow colour, and something like that may have been a problem, but the usual commercial varieties seem fairly low on iron oxide. It is possible that the glaze will mature at a very slightly lower temperature than if it were made with China Clay, but this should not be a problem as I know from my own experiments that this glaze will cope with being fired to above cone 7 and a little below cone 6.

Regards China Clay, I use what ever China Clay I have on hand at the time, and don't make any attempt to use a really pure one such as Grolleg. Grolleg Kaolin would be useful if I was making a blue celadon for reduction firing, or making a porcelain body, but most of the time is not really needed.

Ball clay shrinks more than china clay as it dries, so you may find that you have minor shrinkage cracks in the freshly applied glaze. The modest amount of clay in this glaze should mean that this isn't really a problem. You could lightly smooth over any cracks in the dry glaze with your finger, but these will very likely heal when fired.

Good Luck!

Patrick Chaopricha said...

Hi Peter,
We just did a cone 6 fire of the Tin/chrome glaze tonight and the result came out somewhat disappointing. It was green!!!

I think I added too much Chrome Oxide. My scale was not able to get a sub decimal places for grams. I was making a small match using a the recipe to get to about 1000 mg dry weight. With the recipe, I need 1.5 grams of Chrome Oxide and 50 grams of tin oxide. I think the tin oxide was correct, but I think chrome oxide was probably close to 8 grams by mistake (0.8% instead of 0.15%). I also accidentally substituted China Clay with Ball Clay. I also found that we used Gillespie borate instead of Gertley Borate because that's all we had and it was supposed to be a 1:1 substitute.

I think the Gillespie Borate and Ball Clay substitutes might have been ok, but the too high percentage of Chromium Oxide is what turned the glaze green instead of red/purple I was expecting. Do you think that's a correct assumption?

I'll mix up another batch but this time I'll get a more precise measurement of the chrome oxide with my new scale that has a 0.01 gram precision.

I only glazed my test tile and a couple of small bowls with this glaze. Should I quarantine them since the chrome oxide level is a bit too high. Not sure how safe it is being at 0.8% instead of 0.15% of chrome oxide. What's the rule of thumb in terms of percentage vs. toxicity for chrome oxide? I know that copper carbonate and cobalt carbonate should be under 2%. I'll have to try a leaching test to see.

You make glazing look easy, Peter. I do have a lot of learning to do.

Peter said...

Hi Patrick,
Hey... you've just invented a green glaze! :-)

I suspect that 0.8 would be too much for the red.. so more accurate measuring is essential. One trick I used in the bad old days before I finally gave in and bought some nice accurate electric scales, was to weigh out 10 grams of chromium oxide and carefully make a line of it on a clean piece of paper. The line would be about 1 cm wide, a couple of millimeters thick and as long as 10 grams needed. I would then carefully divide this line into 10 with a knife, so that each piece now weighed a gram. Then I would cut a one gram piece in half, giving me 2 half gram pieces, and half again, giving me 0.25 pieces. It was possible to be accurate enough to get chrome reds reasonably well that way although good electric scales make things so much easier!

I often have used Gillespie borate instead of Gerstley, and that has worked well.

Regards "quarantine", 0.8% is still a very low amount of chrome oxide in the glaze and my own feeling is that it would be very safe and unlikely to do harm to anyone.

Regards "a lot of learning to do", one of the really great things about making pots is that the learning never needs to stop! This means that going into the studio to make pots can be a daily adventure, rather than a boring chore. One of my favorite books is "The Potter's Dictionary of Materials and Techniques" by Frank Hamer. I suspect that it would be available through Amazon or other book sellers. The Potter's Dictionary is nicely written and makes wonderful "bedtime" reading!

Good luck with the Chrome/Tin glaze, I'm sure you'll start to get results when you measure more accurately.


Patrick Chaopricha said...


Wow, I love the line measurement technique you have. Very clever! I didn't even think about how I can measure 10 grams easily with the scale and all I had to do was divide it into 10 units to get 1 gram precisely! I did get a very precise electronic gram scale and just measured out 0.75 g today to make a new batch. I decided to cut the batch size in half so I don't waste the ingredients if this recipe didn't work out.

I decided to not keep the green glaze since it was extremely runny and the test tiles were stuck to the kiln. My suspicion was that the Ball Clay of unknown origin might have contain too little silica and alumina compared to China Clay. We bought Ball Clay from a bankrupt studio for cheap but it didn't have any information on it other than "Ball Clay" on the 5-gallon bucket. So my new batch of Chrome Tin Red uses Edgar Plastic Kaolin (EPK) instead of Ball Clay that I have. I was reading about how to fix running glaze and many people suggested adding more Silica or Alumina. I didn't try changing Silica percent since Ball clay is already high in Silica so maybe what I need more of is the Alumina, I figure try using EPK first since it's higher in Alumina content than Ball Clay. EPK = 37% Alumina, 46% Silica. Ball Clay = 25% Alumina, 59% Silica (according to I also think EPK is closer to China Clay content (Alumina: 36.3% Silica: 47.8%)

I'm crossing my finger to see how this new batch turns out. We are doing a Cone 6 firing tomorrow. I'm more cautious with the test tile this time and have a catching bowl underneath in case it runs again. I'll let you know how it comes out.

Thank you for the book suggestion. I have ordered it and it's going to be a great read. I also want to thank you for all your guidance. It's quite amazing that I have gotten to talk to a very experienced and responsive potter at the other side of the world.

Peter said...

Hi Patrick,
Good to hear from you. I'm just heading off to bed, but thought I'd check the emails and so on before I did, and found your comment there. Glad you are trying the glaze again. EPK should work well I think. I was a bit perplexed by the green glaze being extremely runny and it makes me a bit suspicious about the Ball clay that you used, have you made other glazes with it? The glaze has never been particularly runny, and I am surprised that the switch from china clay to ball clay would make so much of a difference. I might try that myself sometime and see if I get a similar result.

I find that I can make quite small amounts of glaze for testing these days, now that I have electronic scales and can measure accurately! If the glaze is a simple one, I might just make 100 g of dry ingredients, but if the glaze has very small amounts of an oxide, as in the case of this one, I will often make 200 g of the dry ingredients up instead of 100 g.

These days I also always weigh the water that I add. As a "rule of thumb" most glazes need roughly an equal weight of water to the dry ingredients.... in other words, 1kg of materials = 1kg of water (which is 1 ltr). I would usually start by adding 900 g of water to 1000 g of dry ingredients then top it up with additional water once I have sieved the glaze and have got more idea of how thick or thin the glaze is.

Very pleased you have ordered the book, it is one that you should find useful for many years.

Any help I can be is my pleasure :-)

Patrick Chaopricha said...

Hi Peter,

The book is excellent. It's really is a potter's dictionary explaining so much essential information.

The test tile came out after my recipe as followed:

21 Gillespie Borate
16 Nepheline Syenite
11 EPK
20 Whiting
32 Silica

Add: 5 Tin Oxide
Add: 0.15 Chromium Oxide

The tile came out white with some red in some spot. Looks almost like a red peppermint candy cane with more white than red. Or maybe like a tub of yogurt with some red strawberry sauce. When using 2 coats, it's definitely more white with little red streaks compared to one coat. I don't know whether the glaze was too thick. I measured it to be at 1.53 specific gravity. Maybe this should be more liquid? I'm going to try to mix it down to 1.3 specific gravity so it's a bit thinner and see what happen. Using EPK makes it not runny like Ball Clay did so that's good, but I was a little disappointed that it's not entirely red. Do you happen to know what your specific gravity was for your recipe?

I also did some leaching tests on my accident green glaze using the lemon and vinegar leaching method. The leaching didn't happen so the glaze is pretty safe. I didn't keep the glaze though since it was way too runny. I'll do the test on this red one after I figure out how to get it to be more red than white.

I might try a new batch using Gerstley Borate instead of Gillespie Borate to see whether that would make a difference. We just got some Laguna Gerstley Borate to try. I also just got some Georgia Kaolin to try instead of EPK but maybe that's another experiment.

This is quite fun actually. I'm going to try to make 100 g batch like you suggested so that I can make several combinations to try out. Maybe one will finally work.


Peter said...

Hi Patrick,
Sorry you didn't get the red, but I am glad there were some spots of red there... that is a start!

I don't think that the specific gravity will make much difference with this. My own experience of this glaze is that it is red unless very thin, such as over rims of cups, when there is sometimes a "break" to white with just a hint of very pale green. Where very thick I get a more purple version of the red.

I rechecked the recipe, and the amounts of everything are correct (I had wondered if I had made a typo like getting the decimal point in the wrong place for the Chromium Oxide!)

It is interesting that your glaze has red in some spots, but not in others, and that it is worse when the glaze is thicker. I realize that you have probably done this very well, so please forgive me for asking this, .... did you sieve the glaze through at least an 80 mesh sieve? I would do this 2 or 3 times. (It might be worth trying 100 mesh). It sounds from your description like the chromium may be unevenly distributed in the glaze batch.

Another cause for this would be if your weighing of 0.15 was out for some reason and the amount you put in was a lot less, 0.015?? (I have been known to make a power of 10 error in an oxide in the past with one glaze that I made up, on that occasion I added 10 times too much cobalt... Decimal points are frightening things, and such an error is horribly easy to do!).

My other thought is that, if you have any of your glaze batch left over, you could try adding an extra bit more chromium oxide (add only a tiny bit), and sieve really well to disperse it.

The Gerstley vs Gillespie is interesting. I think you should definitely try the Gerstley Borate, as I know that the chemistry of it and its various substitutes can vary quite a bit.

Your EPK should be OK and no need to substitute for another Kaolin.

Good luck with the testing. Sorry this is being difficult, but I am glaze you are quite enjoying it. I will think of you when I am next stirring glazes.

Best Wishes,


Patrick Chaopricha said...

Hi Peter,

I calculated the recipe out by weight before I started mixing the dry chemical together. I started the calculation out at 1000 g so I could just easily multiply the recipe you have just by 10 since your recipe is for 100 g. I then divide everything by 2 to get to my 500 g batch. The 500 g doesn't count the additives (Tin and chromium). I just did the same thing with them (multiply 10 then divide by 2). I guess I can alternatively just multiply everything by 5. My latest batch I just made 100 g as you suggested in the last post. I learned quickly that 500 g is a bit much to waste when the glaze doesn't work. In any case, here's my 500 g batch:

Gillespie Borate 105g
Neph Syenite 80g
EPK 55g
Whiting 100g
Silica 160 g
Tin Oxide 25 g
Chromium Oxide 0.75g

If I take 0.75g / 500 g, I get 0.0015 or 0.15% for Chromium Oxide. So I think the math is correct for the weight of the ingredients.

I mixed the dry chemical together first and then slowly added water until I get to about 450 g of water weight in. I measured out the water ahead of time to get about 45:50 ration water:dry ingredients. I guess I'm treating like how I mix stuff when I do baking. I was reading in some books that it's better to have a bucket of water and add dry chemical to it so that the powder has time to settle in the water. I actually didn't wait overnight to mix the wet and dry glaze together as I discovered later suggested in this book. I just started mixing the ingredients with my electric drill and a mixing head. I ran the mixture through a 100 mesh sieve twice. I wasn't sure if I have lost too much stuff from the mixture since I notice that not all the dry stuff got through. Do you normally wait a day to mix the glaze and then run it through the sieve? I just purchased an 80 mesh sieve since the book suggested using an 80 mesh.

After everything went through the sieve twice, I measured the specific gravity by putting the glaze in a cup and then divide that weight by water weight of the same volume. I ended up with 1.7 at first and I added more water to get to 1.53. I just picked that because I was mixing a commercial dry glaze just before doing my own. That particular glaze called for a 1.53 specific gravity, but it was a black Temoku glaze so maybe it needed to be thicker. The other commercial glaze I use have varying suggested specific gravity from 1.3 to 1.65 so I just used that as the ball park value.

One thing I didn't mention last time was that my test tiles were bisque brown clay rather than white clay. I wonder whether that could contribute to the problem. I should try making test tiles from white clay as well. Both brown and white clay types were for cone 6 vitrification so that the glaze doesn't craze on the clay body (which it didn't). The fired glaze texture was smooth and shiny with no bubbles, cracks, or crazing. It's just the streak of red and mostly white/light grey being the result.

I'm anxious to see the result of the new test tiles since I mixed the glaze up very thoroughly and diluted it down to sp.g = 1.45 as I mentioned in the last post. The glaze did apply thinner so I'm curious to find out whether it works better. I also mixed up another batch but using Kaolin Georgia Tile #6 instead of EPK. This batch I also mix all the main ingredients first thoroughly and then added Tin Oxide and mixed thoroughly and then finally added Chrome oxide. I just tried to make sure that I really get everything mixed together. I didn't wait 24 hours but I did wait 30 minutes before running the glaze through the same 100 mesh sieve. I only did it once this time since I only had a 100 g batch and it went through the sieve pretty easily with almost not clumps left over on the sieve.

I'm crossing my finger and hoping that either of the batch would work this time. I'll let you know once the tiles come out.


Peter said...

Hi Patrick,
Well done! I wish I could employ you to make up my glazes for me you are doing a great job. You are right in your calculations, although I must say that x 5 seems one less step than x 10/2! You put more effort into glaze preparation than I do. I don't do much of a dry mix, just a light stir, careful not to create dust. I put a measured amount of water into a mixing bowl and add the dry ingredients to water, and generally just let it sit for a while without stirring. I usually don't have the time to leave glazes for hours, but ideally leave a glaze to sit over lunchtime. When I resume work I then stir things up before sieving. A paint stirring attachment for an electric drill is a great invention and works really well for larger quantities, or I sometimes use a small kitchen blender for small amounts. If I make 2 liters or so of glaze I often put a bucket of glaze on my electric potter's wheel and get the wheel to spin the container of glaze whilst I hold a stirring paddle steady! Works well and isn't as noisy as a drill so I can listen to the radio whilst the wheel does the work!

The brown clay is probably not the ideal body for this, but I would have thought that the magic should still happen and red appear!

Best of luck with the new test tiles, I do hope something works!


Patrick Chaopricha said...

Hi Peter,

The last test tiles also didn't turn red. I have now confirmed that if the glaze is on too thin then it stays white/grey. I'm running another round of test but this time I'm doing 3 and 4 coats. I'm going to have these tiles sitting on a pot lifter so it doesn't stick to the kiln shelf in case it runs. I'm also putting it on a glossy black glaze as the 2nd layer just to see how it interacts with other glazes.

The Kaolin (Tile #6) didn't produce much of a different result than the EPK one. So I can rule that out. I found out the origin of my ball clay and it turns out to be Kentucky OM#4 Ball Clay which is very popular and prevalent in many recipes. What I noticed though is that when OM4 is used in a recipe, it's usually in conjunction with a Frit 3134. Usually it 14% Frit 3134 and 18% OM4 Ball clay.

I'm wondering whether my Tin Oxide is low quality or something. We bought it from somebody else that was closing their pottery studio and I have no idea how good it is. I'm getting a different Tin Oxide from a reputable vendor so that I know that I can trust the chemical. I bought Chrome Oxide from an actual pottery supply place so I know it's good. It's just so weird that getting red can be this hard when I read in many books that Tin Chrome red is quite easy to produce. It's interesting because it seems as if the Tin didn't react with the chrome much at all.

I also made a Floating Blue glaze which is quite popular and the recipe is consistent. That one actually turn out OK on the test tiles. I'm running one more test over a glossy black glaze to see how it looks. I'll do another run with a small bowl and then do a leaching test to check for food safety. Once that's done, I'm going to scale it up to a 5 gallon bucket. It'll be my first made-from-scratch glaze! Once I got more reliable food safe working glaze, we can cut down on the commercial glaze we buy and that should save us a lot of money!

Thank you so much for getting me hooked on making glaze. Your blog is what really inspired me.

Peter said...

Hi Patrick,
How sad that the tiles didn't turn red, that is very strange and frustrating. The thought about Tin oxide was interesting, and it certainly would be a good idea to try some from a different source, just in case the one you are using is not all it claims to be!

Best Wishes,