Sgraffito in Terra Sigillata
My primary decorating technique is called: "sgraffito in terra sigillata". Sgraffito means "to incise", and terra sigillata (terra sig for short) is Latin for "sealed earth". Terra sig is a very fine grained, thin slip. I color my terra sig with metallic oxides, brush it onto bone dry green ware, and then incise into the terra sig, and the surface of the pot, to create my designs. Once the design is drawn, I carve away the negative space around it with fine carving tools, leaving a texture. The piece is then bisque fired, a translucent glaze is applied, and then high fired to Cone 10 in a gas reduction kiln.
Terra Sigillata Recipe
(from Matthew Metz, Wisconsin)
4,000 grams Grolleg
2 gallons distilled water
40 grams Calgon
Empty the water into a clean 5 gallon bucket. Add Calgon by sprinkling over surface, and mixing thoroughly to dissolve into the water. Add Grolleg, and mix COMPLETELY. I use a drill mixer for this. Let the mixture stand (covered) for at least 48 hours or more to settle into three layers. A layer of clear (or slightly yellowish) water will rise to the top. The heaviest clay particles will settle to the bottom, leaving a middle layer of clay particles that are small enough to stay suspended in the water. The middle layer is the terra sigillata. (You will feel the difference between the bottom layer, and the middle layer, by its thickness; you can't see it, though). The terra sig should have the consistency of milk. Using a turkey baster, siphon off the middle layer in small batches (by weight, about 300 or 400 grams per batch). This recipe yields 12 to 15 smaller batches of the terra sig. I siphon only enough terra sig for my immediate needs; the mixture will keep indefinitely, if covered tightly.
When ready to use, I add up to 3% by weight of various oxides (red iron oxide, black iron oxide, cobalt carbonate, chromium oxide, copper carbonate, or black mason stain), and mix thoroughly. The terra sig is brushed onto BONE DRY greenware, and allowed to dry for about 30 seconds. Do not wait any longer, the sig should still be soft when you begin incising. Draw a design of your choice, and carve as desired.
Many ceramic purists would think it heresy to apply a gaze over a terra sigillata surface. Historically, terra sig was considered a final surface treatment in it's own right. Burnished or not, it forms a smooth sealed surface on wares fired to low temperatures. However, I have access to Cone 10 reduction firing at school, and my own electric kiln where I can fire to Cone 6 oxidation. At these temperatures, the terra sig loses any burnished luster and smoothness, and turns dull and boring. I've found that applying a glaze expands my color pallette, and emphasizes the textures of the carved negative space. The colored terra sig interacts with the glaze, to produce a variety of other colors, as well. My favorite glazes for the sgraffito work are celadons, shinos, copper reds, and others. Recently, I've adopted a versatile glaze colored with rutile & copper, that produces a variety of results, depending on firing conditions. All my glazes are applied (sprayed) very thinly. Any glaze that has some translucency and pools in a textured surface serves my purposes well. Contact me directly for glaze recipes.