Thursday, March 7, 2013

Clay Tests

It's been a while since I posted.... been busy! One thing that always gets me about doing pottery is how much waiting is involved. Of course I kept busy by interleaving other tasks, but just making some tiles, firing them and then testing them for crazing, etc., seemed to take forever.

First I visited my trusty clay retailer. I was surprised to find that my favourite clay maker* had 5 or 6 white, cone 6, clays available. I had thought there were only 2 or 3. So I got a box of each of the un-grogged ones.

Then I made some 1/2" thick tiles, about 5" square of each clay, including the one I usually use. Then they had to dry. Then I fired them to cone 6, with witness cones right beside them to make sure of the temperature reached. Then I quickly weighed them while they were still warm.

Then I boiled the suckers. Over 2 hours in a huge pot of boiling water.

Then I weighed them all again.

Then I calculated how much moisture each had absorbed, and expressed it in a percentage. (For those who are interested, subtract dry weight from wet, then multiply the result by 100, then divide by the dry weight.)

Shock! Awe!

My 'usual' clay, the one I have been using for years now, had an absorbency of around 2.25%. This was almost the highest of the ones I tested! No wonder I am having trouble with delayed thermal shock. Luckily, one of the others came out to about 0.9 %.

Then I took all the tiles and put them in the freezer overnight and the next day dropped them into boiling water. This was scary, I almost felt like I should 'suit up' in some kind of protective gear, but in actual fact nothing happened.

None of them crazed. Of course, to test for delayed thermal shock, I'm going to have to keep doing it. Since I only like one of the new clays I tested, I'm only going to test it and my 'usual' one. Freezing and boiling every day or so for a month or two should separate the women from the kiddies....

All the glazes worked well with my two production glazes, in fact the clay that had the least absorbency also looks the best with my white glaze. Here's a picture of it with my clear glaze and some brush strokes of the main colours I use. You can see they all work pretty well. BTW, these brush strokes are on top of the unfired glaze, because that is how I work.

Can it really be this easy????

* I put a little 'star' here to explain that I am not going to mention which company's clays I am using. They are a terrific company and I am most grateful to them for keeping us supplied with excellent and dependable clays. They do not claim oven-proof-ness for any of their clays, and they point out (quite correctly) that it is up to us potters to regularly test our products. And no, I do not have any other connection with them than as a customer!