Terra sigillata is an unrefined suspension of fine clay particles in water. The term is Greek, and potters will tell you it means something like earth seal or settled earth. Terra sigillata has been used in pottery since early Greek and Roman times, or before. HandbuildersMonthly brings you an easy terra sigillata recipe. No need for mills, strainers, electric mixers or days of your time. We’ve added tips for using terra sig. It’s always fun to expand your arsenal in the pottery studio.
Easy Terra Sigillata Recipe for Any Potter’s Studio
- A transparent plastic or glass one-liter bottle — water bottle is perfect
- About a liter of hot water — some say use distilled to avoid mineral issues
- About an eighth teaspoon of a deflocculant like Darvon 7, Darvon 811, sodium silicate, sodium silicate with soda ash — your choice. All are available online at clay supply sites. I buy a pint at a time and split it with a friend since a little goes a long way.
- About 9 ounces (by weight) of powdered white ball clay. Our expert, Liz Summerfield uses XXSagger. Various potters have success with various clay powders. Experiment as you develop your terra sig process.
- Optional colorants — Mason stains, coloring oxides like cobalt or iron. NOTE: Liz, uses 1 teaspoon of stain to 1/2 cup of terra sig. She reminds us that the color and sheen will be impacted by the end firing temperature. Grind the particles as finely as possible so the mix ends up homogeneous.
- Turkey baster or similar tool and a sharp knife for hole punching if you use a plastic bottle
Renowned ceramicist Liz Zlot Summerfield coached us on refining our terra sig mixing and application methods. If you don’t have at least one of her videos, you are depriving yourself of invaluable information.
Anyway, fill the bottle about ¾ with water. Add your ball clay, cap the bottle, and shake it like you’re a Spanish dancer on a caffeine high. Add seven or eight drops of your deflocculant to the bottle (a bit more than an eighth tsp) and recap the bottle. Shake the mix again. Let it sit. Sources differ on how long to let it stew. Some say as little as half an hour will do it. Experts who use this process often find that more like 24 hours is better, if you can spare the time. Sometimes, the mix won’t separate properly in half an hour.
Don’t disturb the bottle while the layers settle out. When you see at least two, but preferably
three distinct layers of liquid (varying tones of the clay color), siphon off the almost clear top layer with a turkey baster or similar tool. If you use a plastic bottle, hold the bottle firmly to the table top, and punch or cut a small hole above the dark layer. Drain the terra sig above that layer out.
Glass bottle? Use the turkey baster to move the layers out of the bottle. The terra sigillata is the layer that looks thin like light skim milk—it is not a clear liquid. Discard the sludge from the bottom and the thin liquid from the top.
As you work with the terr sig, you can thin it with a little water if needed, or evaporate it to thicken it. You’ll find a consistency you like. Note that porcelain can be used to make terra sigillata, but the yield per volume will be less than earthy clays give. Colorants get stirred in before you use the terra sig.
Brush Terra Sigillata Onto Bone Dry Greenware
In our studio, we use terra sigillata when we’re going to do a wood fire. We sand and refine our surfaces, then follow the terra sig application tips below to prepare the pots for other woodfire enhancements.
We also use the terra sig, tinted or plain, to create a velvety surface on finished pieces that will be ultimately fired in our electric kiln. You can get lovely textural and color effects.
To apply terra sigillata, you can put the pot on a banding wheel or work on small sections as you turn the piece in your hand. Rotate slowly. With a soft, wide brush like a hake brush, trail on a thinish layer of terra sig. Run the brush smoothly around as the pot revolves.
You can also paint onto small sections as you revolve a piece in your hand. Try to avoid drips, as they will show in the final surface. When the damp sheen goes away, rub the surface briskly with a soft cloth like a chamois, velvet, tee shirt, soft flannel, soft brush, or plastic bag scraps. Repeat for another two coats. You’ll be amazed at how quickly this process develops a gorgeous, smooth gloss.
You can hear potters exclaim over the magical transformation the first time they try it. Here, again, is a place to experiment. Try burnishing with your warm bare hand—or a hand coated with oil or lard. Try various fabrics. Try a paper bag, wadded up and kneaded until it’s limp and soft.
“For a soft sheen,” Liz advises, “use a veggie bag to burnish the sig before it dries completely. Burnishing too late will cause a powdery surface that I am not looking for.”
By the way, we hear that grocery bag plastic is harder and denser than other plastic bags — might produce a glassier shine. If you don’t like the shine you get, brush on another layer and rub it again.
Liz Summerfield uses a lot of terra sig on her decorative surfaces for color and texture variations. She told us, “The surfaces I use are all about a push and pull. Some are waxy (terra sig) , some are velvety (underglaze) and some are shiny (glaze). I am interested in how they work together side-by-side visually and tactually. I use very little glaze in my work—just as a liner inside functional pieces and as an accent gloss on the outside. I mostly work with terra sig and underglaze.”
Liz advises two to three coats max. She says if the terra sig cracks off before going into bisque, it is too thick. She always applies at bone dry, so the coats can seep into the surface and dry quickly. Liz will burnish (rub with cloth or plastic) between coat two and three and after three. She believes coat three adheres better to a burnished surface in which the clay particles are laying down.
Finishing Up with Your Terra Sig
It is possible to apply terra sigillata to bisqued clay, a stage where the work is not as fragile. On bisque, some artists find they can apply the slip by dipping, spraying, or brushing. However, much depends on the compatibility of the slip and the fired clay. Do a lot of testing to decide whether your slip and body work together. Often, the terra sig slip will peel off the bisqueware. If that happens, your option is to stay with applying it to greenware.
When you fire a terra sigillata piece, you can fire to almost any temp you want, though most artists feel lower temps preserve the shine. Just make sure you’re at least at vitrification for the clay you used to make the slip. Vitrification is the point at which the clay molecules reform and bond, kind of creating stone. It’s the recommended cone temp for your clay body or the ball clay you used for the terra sig.
Bottom line, you can burnish almost any clay body to a nice shine. Use a spoon back, a rock, or spend a bunch of money on a burnishing tool. You can burnish with oil or without. Burnishing like that takes a lot of time and more effort than I’m comfortable expending. You can save a ton of elbow grease and time by mixing up some terra sigillata and burnishing with a soft, sensuous piece of velvet. Your call.
You can hook up with Liz on Facebook and ask about her workshops!
Underglaze, glaze, and terra sigillata provide matte, bright gloss, and soft shine over slip trailing. Beautiful.