Back in the summer of 1985, I took a workshop at Pigeon Lake Field Station in northern Wisconsin on Raku and Primitive Firing. We made pots, burnished pots, covered them in terra sigillata, and pit-fired them in a variety of situations, from actual holes in the ground to brick boxes to nested steel drums. The resulting pieces weren't functional by any means, but they were gorgeous.
I've played with the techniques from time to time over the years: wrote an instructional manual in grad school, taught a course a few times at the Craft Center, and most recently, unearthed the skills and tools to make small sculptural animal masks.Terra sigillata,
Latin for painted earth,
is a very fine clay slip used decoratively on the surfaces of low-temperature pots. Terra sig is made from a very fine particle clay like ball clay or earthenware, mixed to a very thin consistency and deflocculated to keep the particles suspended. Over time, all but the very finest particles settle on the bottom, at which point the remaining liquid is carefully decanted or siphoned off and kept, and the thicker stuff on the bottom is discarded.
I apply four or five coats of terra sig by brush to bone dry ware, waiting for the wet shine to disappear between applications. After the last coat, while it still looks leather-hard, I buff the surface with a soft cloth--old t-shirts or worn-out flannel jammies work well. The result is a waxy shine that can be enhanced if the pot is burnished beforehand. Clay particles are very tiny plates, and by burnishing the surface of a pot with a polished stone or metal spoon, you can make them line up with one another resulting in quite a glossy surface.
Greek red-and-black ware uses terra sig; so does Native American blackware and colored Pueblo pottery. Low-temperature firing in sawdust smokers or pit kilns gives the best shine. Even at the comparatively low temperature of a bisque firing, clay particles start moving out of alignment, negatively affecting the shine.