Cone 8 Glazes, Change and New Clay

"Where do you get your clay from?"
Late last month I heard the sad and troubling news that Southern Clays, a Dunedin based clay manufacturing business, had gone into liquidation. I am very sorry about this as it means that, short of arming ourselves with picks and shovels, local potters can no longer obtain clay that is from the Otago - Southland area. I am also sad, because I always enjoyed a visit to the factory that was in the wharf area of the city, and found Barry, the owner, very helpful. I will particularly miss the earthenware clay that was produced there, a rich red firing brick clay, that made good strong planters and domestic ware. This would have been much the same clay that made the bricks that many of the older Dunedin buildings are constructed of, including the one that we live in.

Waikouaiti Post Office Building (where we live and work), from an old post card that was given to us.

Potters in the South Island are left with three choices; pick up a shovel and dig, or buy their clay from Nelson at the top of the South Island, or from the North Island. Options 2 and 3 incur significant freight charges.

Southern Clays stopped making a white clay body several years ago, so those of us needing a white earthenware or stoneware had to look elsewhere back then. The porcelain that I have used for the last 2 or 3 years comes from Decopot in Palmerston North, and I recently ordered a stoneware clay from them and some very interesting samples of other clay bodies that they produce. Much as I like this clay, I will feel a little sad every time someone asks me, "Where do you get your clay from?" And I cannot proudly say that it is from our area and point in the direction of Dunedin!

Dropping to Cone 8.
With the change in clay, I have decided to come down a little from Cone 10  (1280 - 1300 C or 2336 - 2372 F) to Cone 8, say to around 1250 - 1260C (2282 - 2300F). This will save some power and wear and tear on the kiln, but should give me most of what I look for from a high temperature glaze.

The change to new clay and a different firing temperature means adjusting and testing glazes that I already use, and coming up with some new ones, so my studio has looked like a laboratory, with small glaze test tiles, test bowls, plastic bottles, sieves, mixing bowls, note book, and electric scales crowded together in my small work space. I did quite a bit of reading about Cone 8 glazes, and re-read "Oriental Glazes" (first published 1978) by Nigel Wood, a most interesting book that discusses in great detail Chinese glazes from around AD 900 to more recent times. The glaze materials are refreshing in their simplicity; limestone, clay, wood ash, and maybe a little feldspar are the main ingredients, and it is good to remember that the most sublime Sung Dynasty glazes were made with materials that a potter could dig out of a hillside, find at a quarry, or obtain from a bonfire. I realised that many of the glazes that Nigel Wood discusses are in an ideal temperature range for cone 8.

I also consulted "Colour in Glazes", by Linda Bloomfield (Published 2012). This has a useful looking collection of earthenware and stoneware glazes, and lots of rather yummy colour photos of them. Many of the stoneware glazes are for Cone 8 or Cone 9, and they are arranged according to colour. This makes for a very pretty book. If you read it with close attention you will discover that the range of glaze base recipes is a lot more limited than it first appears, and the same keep appearing with colouring oxides changed to give the different colour options. It is nice to see glazes grouped according to colour, and they look rather stunning like that but, when I came to use the book, I kept wishing that it was arranged differently so that each base recipe was dealt with one at a time.

I also discovered a very useful collection of Cone 8 glazes on Ontario potter Steve Irvine's website some months ago (sadly the link to his Cone 8 glaze page seems to have been removed when I checked a few minutes ago, and I may email him to see if he would consider putting it back again).

I copied a selection of likely glazes into a graph paper notebook, then spent a few happy hours examining all of them with the help of Tony Hanson's Desktop Insight glaze software.

I find that having a peep at how a glaze is put together chemically is an enormously helpful first step in understanding how a glaze is going to look and behave when fired, and how it compares chemically with other glazes that are known to work.

I take particular notice of the Thermal Expansion of the glaze, as this number will help me correct any tendency for the glaze to craze. As a starting point I look for a figure that is around 6.8. If the TE is well in excess of 7, I know that the glaze is very likely to craze on the clay that I use. This number is only a starting point, but it is a useful reference to have when adjusting a glaze.

I also check the numbers for silica and alumina to see that they are at levels that look "sensible" for a good, strong glaze that will cope with domestic use. The glaze software has reference guides in it that indicate a range of likely figures for workable glazes. These guides do not tell you what to do in a dogmatic way, but they do help you steer a course that makes a successful glaze far more likely!

I take notice of what the fluxes are, these can have a substantial affect on the oxides that are added to a glaze for colour. A glaze high in magnesium will have a lovely silky feel, but a cobalt blue may end up purple. Zinc will brighten copper or cobalt and may assist with glaze fit and durability, but it will also turn chromium oxide or chrome based stains brown. High levels of calcium will assist with making chrome reds or pinks, and will help make a physically strong glaze, but may "bleach" iron and give yellow rather than brown. And so on!

Calculating glaze formulae is useful, and glaze software helps a lot, but the interaction of all the components of a glaze is so complex that glazes will always have to be physically tested. "The proof of the pudding is in the eating!"

Firing and Transforming!

I used tiles for the glaze tests, but also glazed some small bowls with samples of the glaze bases.

I made some test tile holders out of some insulating fire brick offcuts. I used an old hand saw to cut the grooves in the fire brick.

I used Cones on this kiln shelf and the one below it to confirm the temperature of this test firing. This is a small kiln, and there is almost 1 Cone different in temperature between the bottom shelf (cooler) and this middle shelf. I can even this out somewhat if I need to by having the lower bank of elements on full and the top at about 3/4 power for the final part of the firing, but a temperature difference between shelves can be useful when testing glazes as I can test them at Cone 7 and Cone 8 simultaneously, and obtain lots more information from one firing.

The test tiles came out well. Most of the new glazes look useful, and one delightful discovery was that some of my Cone 10 glazes will fire happily at Cone 8, and a slightly modified Cone 6 glaze that I had made up a lot of some years ago will fire at cone 8.

I will put up some recipes as time permits over the next few weeks.

You might be interested to see the lower shelf of small test bowls, and their transformation when fired. If you look carefully, you may see that I have placed all the test bowls on little pads made of slices of insulating fire brick. This is to help protect the kiln shelf if any of these untested glazes misbehave and run.

I have done many hundreds of firings over the last decade or so, but the transformation of a glaze in a firing is still magic!

 Time Marches on,  
So I had better head back to the Studio!

Progress is continuing with the potting. I'm not quite back to full production as yet, but the shoulder is considerably stronger than when I posted last. 

Almost autumn here now, with chilly nights and a flush of gold on the willows that I see out of the window as I write. There is a gathering of swallows on the wires!


Anonymous said…
Hi Peter, wonderful post! It took me years to gather up the courage to start learning about glaze chemistry but with the very generous help from Ron Roy, (who is 81 and still going strong!) over the past five years I have come a very long way. I must say that making a dinner set for my daughter and son in law's wedding gift last summer, using glazes I formulated myself, was immensely pleasing and I felt so rewarded for all my efforts and the hundreds upon hundreds of tests I had done up to that point. Now I can't do a firing without putting as many tests as I have time to make, it is so addicting. I'm very sorry to hear that your clay supplier will no longer be in business, that is a major inconvenience especially as you had such confidence in their clay. Best wishes as you move forward! Owen in Oregon, I fire cone 10 in reduction.
Peter said…
Hi Owen,
Thank you so much for writing in, and for your encouragement. How nice that you have been assisted in learning glaze chemistry by Ron Roy. I am sure that he will have enjoyed passing on his knowledge too. That dinner set with your own glazes that you made as a wedding gift was a real achievement!

I was really excited when I first used Insight glaze software to help me make a series of cone 3 alkaline copper glazes. It was so interesting to start with an idea, do the calculations, and have a lovely looking series of glazes as a result. Prior to that point, most of my glazing had been starting with someone else's recipe, using it, and occasionally modifying it.

I hope to be firing my wood fired kiln again fairly soon (it has been out of action for quite a long time), that will give me a chance to do some reduction firing and it will be fascinating to try some of the cone 8 glazes in it.
cookingwithgas said…
We have been testing like mad as well as we work into a small gas kiln and go back to cone 6. I plan to read this in depth later. Cheers!
Peter's Dad said…
What a magnificent post!
It gives a real insight into some of the qualities needed to make a good potter; being able to blend art, science and intuition - to say nothing of immense patience!
I don't have a hat on at the moment, but if I had, I'd raise it and bow.
Peter said…
Hi Meredith,
Good to hear from you. Agggh, testing! A lot of work for you, especially when you are so busy. Quite a change from cone 10 to cone 6, but a fascinating one.

Hello Dad!
Thank you for the lovely comment. I try to keep my hat on these days, it is too chilly without it!
Anonymous said…
A change of clay and glaze temperature - a leap into the unknown indeed. I've been at cone 6 for a while. Our American colleagues have done more work in that area than is common in the UK. The trouble is they are all addicted to the dreadful Gerstly Borate so most of their recipes are useless to me. I used to experiment like crazy but I've been a bit lax of late. I've been playing with commercial stain powders in slips under the glaze instead. Just back from Thailand (massage training) and I've loaded the kiln with the pots I made before leaving ready for a bisc tomorrow. Good luck with your experiments.
Peter said…
Hi Mike,
Nice to hear from you. I enjoyed reading about your trip to Thailand, it would have been a really colourful and fascinating change of scene for you being over there. I haven't had trouble with "the dreadful" Gerstley Borate, but colemanite is very badly behaved, and the few tests I have done have always resulted in a great deal of spitting of glaze over the kiln shelf, which is most annoying (to put it mildly!!). I like that cone 8 glazes have less need to resort to frits or such things as Gerstley Borate than their cone 6 cousins, it is remarkable how 2 cones different in temperature can make for much simpler glazes. Using coloured slips under glaze is an interesting way to go, and stains certainly make sense for that.
Anna said…
what a great post Peter, I'll be sharing this one. Australian potters lost their main white stoneware mine not so long ago and the clay makers have been doing lots of testing with mixed results. Fingers crossed I haven't been caught out yet. I look forward to seeing your new glazes and hope the wood firing happens soon for you. Love that image of the swallows too. All the best to you, Laura and Stopit.
Peter said…
Hi Anna,
Thank you for your comment, sorry to take so long to respond! Interesting that Australian potters lost white stoneware too, I guess we will have to all show our pioneering spirit and adapt to change and/or start digging and refining our own clay! It is a time consuming process changing to a new clay though and, can seem a bit overwhelming on top of the other challenges we face with making and marketing work. I sometimes wish I was the cat..., Stopit spends much of the day in a blissful state in front of the heater!
Unknown said…
Ive just been reading about your cone 8 tests with great interest and admiration of glazes achieved. Do you have a preferred firing cycle for approx cone 8?
Peter said…
Hello "Unknown"
Sorry to take a few days to reply to you, thank you for writing in. Regarding preferred firing cycle for cone 8, my electric kiln was mostly fired manually, without a controller, until quite recently. My usual practice with glaze firings is to fire fairly slowly for the last hour, at no more than 60 degrees C per hour, and I do like to try to hold the top temperature for 20 minutes or so until the final cone is down. I do "fine tune" this depending on the results of previous firings. The clay body, the glazes, the freshness of the kiln elements, how heavily loaded the kiln is, all affect results.

I find cones essential when glaze firing, even if I use a controller.

Sorry not to be more helpful with this, but kilns all fire differently and you will have to experiment to find what works for you and your circumstances.

Best Wishes,


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