Most of the time, cratering is achieved with silicon carbide (SiC), which works at a wide range of temperatures due to the fact that it doesn't break down and generate carbon dioxide until it comes in contact with molten glaze. Sometimes the SiC is added to glazes, and sometimes it is added to a slip that is applied to greenware before bisquing. Creating a "foaming slip" that only causes cratering where it is applied, seems to be the best way to control exactly what happens with the pottery. (It also helps to avoid glazes that foam up in the kiln and hit neighboring pieces! In a shared studio kiln, this would be a disaster.)
A few weeks ago we picked up 5lbs of 400-mesh silicon carbide so that we could use it for various experiments. I made up two slips with the SiC, each with a 10% addition (by weight) of SiC. One of the slips was B-Mix, and the other was Dark Brown (both cone-10 clays). The only real difference between the two slips is that I added a lot less water to the B-Mix slip, so it was extremely thick and therefore would hold a lot more SiC.
Then I made up a handful of flat test tiles to give the slips a try. Most of the test tiles have six indentations in them. The first two indentations have no SiC as a control; the second two have one coat of foaming slip; and the final two have two coats. (Initially I was going to try two glazes on each test tile, but I later realized it was more important to apply the glaze uniformly, so I dipped each tile in its own glaze. For subsequent experiments I would just make tiles with three indentations.)
|The B-Mix test tiles. The bottommost test tile was an experiment with textures.|
|Top: Matte Black. Bottom: Long Beach Blue.|
|Top: Eggshell Matte. Bottom: Green to Black.|
In contrast, the matte glazes (especially Matte Black) had large blisters and bubbles that would easily crack and flake away when handled.
In general, I would expect glossy glazes to produce more durable results when used with cratering. Glossy glazes also seem to fare better with a large concentration of SiC than matte glazes do, since they won't trap gases nearly as much.
The other interesting result from the above tests is the difference between the Dark Brown tests (less SiC) and the B-Mix tests (more SiC). The B-Mix tests have very large craters, and had a lot of blistering and flaking that made the results less durable. The Dark Brown tests have much smaller craters, and the difference between the middle and right indentations is pretty visible. This just goes to show: A little silicon carbide goes a long way! Heavy application will result in a piece that isn't very durable. (Even if it's not functional ware, it will still need to be handled from time to time.)
Finally, it is obvious that different glazes respond differently to the silicon carbide. The Green to Black already foams a bit when applied heavily; it produced significantly larger craters than the Eggshell glaze did. It's clear that individual glazes will react differently to the cratering effect, so experimentation is necessary when trying this with new glazes.
Next time I work with the crater glazes, I will start experimenting with some vertical test tiles, and see how the cratering effect is affected by vertical surfaces.