Aside from mixing, pugging, wedging, throwing, trimming, handling and drying pots before they ever go through that first bisque firing, we frequently add yet one more technique: we add layers of sprayed on engobe to the bone dry work. Here are just a few pictures of Vic Duffhues doing just that.
In some cases, the pots are sprayed with what we might call the main engobe, and in case you’re wondering what that is, I’ll explain. There are multiple techniques used to add color and depth to pottery. Slip, possibly most commonly used, is a wetter version of the clay itself, sometimes with an oxide added for color. It is generally applied to pottery that is considered leather-hard, well before a pot is dry enough to be fired. Engobes generally have a much lower clay content, and they can be applied to a wider range of pottery in various stages. Like slips, they can have colorants added, including commercial stains or oxides.
We like spraying layers of color onto our work and usually do this when the pots are bone dry. We generally use an engobe for the first application, allowing it to dry before adding another different engobe, or sometimes a smoother and finer Terra Sigillata. There are many recipes for engobes, slips and terra sigillata available and each will work differently depending upon time of application, clay body, firing temperatures and so forth. As with all underglaze applications, potters have to experiment and fine-tune before coming up with a version that works best for them.
When these pots are once again bone dry, they are fired in the kilns to somewhere in the neighborhood of about 1000 ° C or approximate 1950°F, all depending on how hard or soft we want the pots to be before we apply final glazes. If you’re curious about firing temperatures, you can consult this chart from Bellevue College.
Many of our pots actually go through three firings before we reach that final finish we’re seeking. During our second firing, which we also do at a lower bisque temperature, we can fire on glazes so that they will be stiff enough to allow handling, but soft enough to allow for further glazing. We do this to add depth and richness and texture, a step that ensures that glazes will marry in the final firing. One of our final glazes is actually frequently called lizard skin or alligator glaze. It has to be applied to a pre-fired underglaze because it is actually a reticulated glaze, meaning that it shrinks more than the glaze surface underneath. The shrinkage during drying and firing leaves the surface beneath this glaze visible. This technique would once have been considered a glaze flaw and named a crawl glaze, but it has become rather popular in the last 15 or so years being deliberately applied to decorative pottery. Our studio is unique in the way the technique is applied to functional stoneware pottery.