Saturday 22 February 2014

Searching for a good crackle

With ceramics, it's exciting when serendipity leads you down new and interesting paths. When I tried formulating a matte ash glaze using mainly china clay and wood ash (based on a recipe by Carlos Versluys, which I read about in the 2nd edition of  Phil Rogers' excellent book, Ash Glazes), I discovered that when applied thickly, it crawled and split, creating interesting crackle patterns. The high proportion of raw ash in the glaze gives them an unpredictable element, which I like, but it also means that they are extremely sensitive to firing temperature, thickness of application and the type of clay body or slips underneath.

In my latest electric firings, I've tried adding a little magnesium carbonate to encourage crawling. This works up to a point, but at higher temperatures, it begins to act as a flux and the crackle effect is lost as the glaze begins to melt and fuse back together. Peeling is also an issue where the glaze is very thick..although fairly robust, curls of glaze can crack off if enough pressure is applied. Okay for sculptural work perhaps, but would not be suitable for most functional ware.

Here are some of the recent test tiles and experimental pieces:

Tea caddy,
matte copper ash glaze over black slip

Sculpture, "King for a day",
matte copper ash glaze

Decorative bowl,
matte ash glaze with magnesium carbonate

Vase,
matte ash glaze with magnesium carbonate
Vase, 15 inches tall
Matte copper ash glaze

Vase close-up

Test tiles
The white test pieces are a matte ash glaze with very high clay content over black slip decoration. Develops some nice cracking, but again, thickness of application over the bisque is critical to avoid peeling. In an electric firing, only black slips containing manganese and cobalt show through clearly .. the test on the left in the photo below has a dark slip containing red clay and iron oxide only, and it's barely visible through the glaze:

Test tiles
I particularly liked the test tile second from the right, which is a matte ash glaze with red iron oxide added .. works nicely over the Potclays grogged pink body. I'll be trying this again with a higher clay content to try to achieve a more pronounced crackle.

Thanks for reading.

6 comments:

  1. love your sculpture, good luck with your crackle experiments, time seems to bring solutions but they sometimes seem so slow to me.

    ReplyDelete
    Replies
    1. Thank you, Linda. I was thinking exactly that the other day and feeling impatient. As Hamada said, "clay and wheel, they teach us."

      Delete
  2. So nice to see your tests are coming along nicely. I love the glaze on the tea caddy. I would love to use it for a tea bowl for matcha.

    I can't do glaze making right now as I haven't got space, but like to start again sometime. You are inspiring, as always! : )

    ReplyDelete
  3. Thanks for the kind words, Midori. :) I have bowls and jars everywhere, need to clear some space, but it's hard for potters to throw anything away, isn't it.

    ReplyDelete
  4. Bravo for your experiments Mark. You have a lot of patience. Not me, I buy my glazes...
    Love your sculpture "King of a day", really beautiful work. And bowl, and very nice crackle tests - gorgeous textures. Keep going, good luck :)

    ReplyDelete
    Replies
    1. Hi Ilona, thanks so much for your encouragement :) Fortunately, I enjoy learning about glazes and the properties of materials..it's more challenging to achieve interesting results in an electric kiln, no doubt about that!

      Delete