Tiles, Glaze Testing, Glaze Problems, Chun & Zinc

Testing the shrinkage of clay using a carefully measured series of tiles.

At the same time as shed building, fixing leaky gutters, having trees felled, and commencing some extra building.... I have made a start on testing glazes and clays for a tile commission. What I will be making will be glazed tiles for a hearth, approximately 4 feet square.

I tested 3 clay bodies. One is my usual stoneware clay, another is 4 parts of the stoneware clay to one of earthenware (this gives a warmer, almost toasty colour and helps the clay vitrify at a lower temperature), and the other is a white porcelain-like clay that I use for my crystalline glazed pots.

Testing Times....
The first test to do was to roll out a small slab of each clay and accurately mark each out like a ruler, using either centimetres or inches. After firing the clay it can be measured against a real ruler, and shrinkage of the clay determined. Clay can shrink a lot when it is drying and more again when it is fired. Some clay may shrink 15 percent or so from when the tile or pot is made, to how it is when it is finally unpacked from the kiln. When making tiles, it is important for me to know what the shrinkage is, so that I can work out the best size of tile for the job, and how many I need to make. I also made several tiles of different sizes that I accurately measured just after making them, and recorded the measurement on the back for reference.

The other thing to test was slips and glazes.

4 test tiles with the same glaze over different clays and slips with 1% and 2% cobalt.
The recipe for this cone 9 - 10 glaze is
Potash Feldspar 49.4, Whiting 14.6, China Clay 12, Silica 23.6, Magnesium Carbonate 0.3

Slip-sliding away....!
Slips are usually made clay that has had water added to it to make it runny like thick paint or cream. Sometimes colouring is added, and sometimes a little feldspar, glaze frit or other flux to help the slip attach itself strongly to the pot. Slips can be used under a glaze to modify the colour, or to add texture. A good time to add slip to a pot or to a tile is when the clay is still soft and slightly bendable. Soft leather hard. You can add a layer of slip by painting or pouring it.

The basic slip I used was 90% ball clay to 10% frit 4124. I find that this fit my clays well, and the slip has a very slight satin sheen if fired to cone 9 (1260C or 2300 F). I mix this to a creamy consistency with water. Alone, this slip will make a buff stoneware almost white. I added yellow ochre to the slip, by mixing yellow ochre and water to a similar consistency as the slip and doing a 50/50 blend of it. I also made a pale cobalt slip, a schist slip, and a black slip.

About the Glazes...
The glazes that I tested were mostly ones I use a lot. I won't give you details of all of them here, as an already technical blog post, will get even more boring for you, but for those of you who are still awake ... here are two of them;

One was a chün glaze (by Emanuel Cooper) that I almost always find reliable, and I have published this one in the comments after an earlier post (March 10, 2009), but I will do so again here:

"Reliable" chün in the good old days when it worked....over a saturated iron glaze.

Potash Feldspar 46
Dolomite 6
Zinc Oxide 6
Whiting 10
China Clay 2
Silica 30
To this I usually add 2-3 percent bentonite to help it stay suspended in the glaze bucket, and to make it easy to apply to the pot.

Another glaze that I find very useful is this one that started as a recipe by Janet De Boos.

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

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

De Boos glaze with 3% copper carbonate and 2% red iron oxide over different slips and clays. Top left is my stoneware/earthenware mix with schist slip. Top right is the white stoneware clay. Bottom Left is stoneware/earthenware mix with yellow ochre slip. Bottom right is stoneware/earthenware mix with a pale cobalt slip. The "Z" pattern over each tile is a brushload of the "reliable" chün ..... Hummm, should have been a splash of blue!

In these tests I wanted to also try it with a little cobalt carbonate, as my client really wanted blue tiles.

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

To Work....
Well, I applied various slips to most of the test tiles when they were freshly made. I managed to dry them rapidly with the assistance of the pot belly stove. I bisque fired them, then applied the glazes, and fired them again. I fired them accurately to cone 9 (no point at all in testing to an inaccurate top temperature when testing glazes). I had only a short time to do all this, as I wanted to have some test tiles ready for my client who was due to visit Dunedin early in the week. In fact, time was so tight that she was able to attend the unloading of the kiln and handle the tiles when they were still slightly warm!

A Good Firing... And Something Odd!
Most tests worked well, and gave me very useful information. Two or three of the tiles were just the sort of colour that my client wanted, so I will be able to go ahead with the commission now.

The De Boos glaze with rutile does give a chün type effect.

The really odd thing about this test firing was the reliable chun glaze..... For the first time ever... it did not produce any colour at all. In stead of the usual delightful optical blues, there was just a clear and boring glaze.

Umm..... my "reliable" chün. All these should be blue and beautiful! All we have is a boring clear glaze over various slips.

The Big Bag of Zinc!
When I was working on crystalline glazed pots form my most recent exhibition, I bought a 25 kg bag of zinc oxide. Prior to that, I had purchased my zinc in 1 and 2 kg quantities. I used some of the new zinc when making up glazes for some of the crystalline pots... and had difficulties. My fairly reliable crystalline glaze suddenly produced only few crystals, and some pots had to have a further firing, and a coat of zinc and silica added to help promote crystal growth.

Using this same zinc oxide, my reliable chün ... no longer is reliable!

What Happened???
I think that the probably cause may be how finely ground the zinc is. I suspect, but don't know, that my 25 kg zinc is much finer ground than the other zinc that I have used has been. This would mean that it would go into suspension in the glaze much more easily, and also act more powerfully as a flux. I could probably get more crystals to form in a crystalline glaze that was made with this zinc by, either adding more zinc, or slightly lowering the top temperature that I fire to.

How to Solve it?
With the chün glaze, I am not so sure... I may get it to work by firing to a slightly lower temperature, or it may still refuse. Chün glazes get their colour (usually a beautiful blue) by scattering light within the glaze. Probably this is done by having some minute undissolved particles of silica, or other material within the glaze. In this glaze I do not know if the zinc is simply working as a flux that helps control the maturing temperature of the glaze, or if the glaze relies on tiny undissolved particles of zinc in the glaze to scatter the light. If zinc is what scatters the light, then... maybe the finer ground zinc simply will not work!

More tests are needed to determine this.

Hope this is Helpful!
Anyway, I have written all the above in the hope that it might assist someone. Several people have asked me about the chün glazes that I have shown on my blog, and I have tried to be helpful with recipes and other advice. I know that some have struggled to make the chün glaze work, so..., maybe I am now experiencing something similar.

The good thing, from the point of view of this commission, was that I did test several different glaze bases so there was something there for my client. It would have been disastrous if I had only produced variations on my "reliable" chün!