Terra Sigillata is Latin for “sealed earth.” Its usage dates back to ancient Roman times. It was developed as a method of sealing the surface of pottery forms, and for decoration (red and black slip). In today’s ceramics, terra sigillata is a generic term for a thin slip made by separating the tiniest, finest particles of clay from the larger particles. Often called “Terra Sig” or even just “Sig” for short, it can be made from almost any clay body.
A simple search on the Internet will pull up lots of information on ways to make terra sigillata. I prefer a simple method that lets me get to the fun part quickly and easily. My recipe is based directly on one I learned from Wisconsin potter Matthew Metz. The only modification I made was to change the original 28 cups of distilled water to an even 2 gallons (28 cups is just a few cups short of 2 gallons).
Sgraffito = fancy way of saying “to scratch or incise into the surface.”
I use powered oxides or Mason stains, between 3% and 5%, to color my sig, by weight. I brush the wet slip onto bone dry green ware, wait about 30 seconds, and then draw my design through the layers of the slip. I use a dull pin tool for this. A pencil or a dried up ball point pen would work, too. Once the design is drawn, I then scratch off the slip from the negative space around the outside of the design. My carving tools are tiny, sharp, and leave a mark in the clay surface, creating a distinctive texture. Once the carving is finished, I bisque fire it, apply a glaze to the entire piece, and fire it up to maximum temperature to produce the final product.
So, you put the glaze OVER the Terra Sigillata? I thought you couldn’t do that?
Traditionally, Terra Sigillata was a final surface treatment, with no glaze on top of it. Terra Sig, when burnished and fired to low temperatures, can have a lustrous, smooth surface. It worked well as a durable surface, for ancient civilizations, before they knew what glaze was. However, I choose to use Terra Sig for reasons other than the final surface. I like the way it absorbs into the dry clay surface, and the way I can carve through it, to produce sharp lines, and strong contrast with the background white clay. I put a lot a work into my carving, and I want that to show. But I also like the shine and color that glazes can add to the work. So, I spray my glazes lightly over the entire vessel, over both the terra sigillata slip, AND the exposed clay body. I choose translucent, “celadon style” glazes, so that the carving and textures will show through, as well.
It’s not a traditional way of using terra sig, but it is a method that works for me. It gets me the results that I want. Besides, there is no such thing as the Pottery Police. No one is going to come and take away my brushes and carving tools, just because I’m doing something unconventional. The only rules I have to follow are the laws of physics: if it’s fired too hot, it will melt. If it’s dropped on the ground, it will break.
Lisa’s Recipe for Terra Sigillata (adapted from Matt Metz)
4,000 grams dry Grolleg clay
2 gallons distilled water
40 grams Calgon
Empty water into a clean, 5-gallon bucket. Measure Calgon, add to bucket, and mix to dissolve. Add Grolleg clay in 500 gram batches, mixing after each addition. Set mixture aside to settle for at least 2 days, minimum. Use a turkey baster to siphon off the middle layer in small batches as needed. Leave the water layer on top, to keep it from drying out prematurely. The sig should be the consistency of whole milk.