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!