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.