Terra Sigillata: How to Make, Apply and Fire Terra Sig

Terra sigillata, or terra sig for short, is a very thin coating made from only the smallest particles of a clay. Brushed onto bone dry ware and buffed, it fires to a soft satin or glossy surface without any glaze. Making it takes a bucket, a scale, a deflocculant and a day or two of patience while the clay settles.

This guide is research-based. We compared published recipes and tutorials from Ceramic Arts Network, Digitalfire and The Pottery Wheel with Vanderbilt's data sheet for Darvan 7-N and Clay Art Center's product notes. We did not mix or fire terra sig ourselves. Specs checked October 2026.

What is terra sigillata?

Digitalfire describes terra sig as "a sedimentation process used by potters to remove coarse particles from a clay powder." You slurry clay in a lot of water, add a deflocculant so the particles repel each other, and let gravity sort them. Heavier particles sink, the finest stay suspended, and that fine middle layer is your terra sig. Because the particles are so small, they pack flat when you buff them, which is where the shine comes from.

It is used on low fire and alternative work: pit fired, saggar, smoke fired, raku and horsehair pots, and burnished earthenware. Digitalfire notes it can give gloss "even though no glaze (or glass development) is present."

Terra sig vs slip

Slip is clay and water with the full range of particle sizes, applied thick enough to change the color or texture of a surface. Terra sig is a refined slip: only the finest particles, applied in coats a fraction of a millimeter thick, that buff to a sheen. Ordinary slip stays matte. Our clay slip guide covers joining and decorating slips, and colored slip vs underglaze compares color options.

What you need to make terra sig

• Clay. A fine dry clay. Published recipes use OM 4 ball clay for white and Redart for red. Our ball clay guide explains that material.
• Deflocculant. Darvan 7 or sodium silicate, sometimes with soda ash.
• Water. Distilled is not required in the recipes we found; tap water is used.
• A gram scale and a 100 ml measure for specific gravity.
• A tall bucket or clear container, plus a turkey baster, syringe or siphon tube.
• A drill mixer and a NIOSH-approved respirator for handling dry clay.

Deflocculants: Darvan or sodium silicate

A deflocculant gives the clay particles a like charge so they push apart and stay in suspension, which lets the coarse grains drop out cleanly. The two common choices:

• Darvan 7. Vanderbilt Minerals describes Darvan 7-N as a sodium polymethacrylate aqueous solution with 24% to 26% active solids. Vanderbilt does not publish a dose for ceramic slips, so the amounts come from potters' recipes.
• Sodium silicate. The traditional choice, sold as a liquid. The Pottery Wheel recipe pairs it with soda ash.

Published amounts vary:

• Rhonda Willers' base: sodium silicate or Darvan 7 at 0.25% to 0.5% of the dry clay weight.
• The Pottery Wheel: 0.25% soda ash plus 0.25% sodium silicate by clay weight (1.87 g each for 750 g of ball clay).
• Pete Pinnell's recipe, as published by Mark Arnold: sodium silicate added until the mix deflocculates, "a few tablespoons" for 14 lb of clay.

How to make terra sigillata, step by step

Most published recipes use about 1 part dry clay to 2 parts water by weight: Pinnell uses 14 lb clay to 28 lb water and Kari Radasch 10 lb to 20 lb. The Pottery Wheel recipe is thinner, at about 1 to 4.

1. Weigh the water into a bucket and add the deflocculant. Ceramic Arts Network adds Darvan to the water first.
2. Add the dry clay gently to limit dust, and let it slake down into the water.
3. Mix well with a drill mixer for 5 to 10 minutes or more.
4. Let it settle undisturbed. Radasch raises the covered bucket about 6 in off the floor so it can be siphoned without moving it.
5. Siphon the middle layer. Remove the clear water on top, then siphon or ladle the middle layer into a clean container. Leave the thick sludge at the bottom.
6. Check specific gravity and adjust (next section).
7. Label and store the jar with the recipe, date and reading.

How long to let terra sig settle

Settling times in the published recipes run from hours to days. Willers settles red clays 6 to 8 hours and lighter ball clays 24 to 48 hours. Radasch settles OM 4 for 8 hours. Pinnell settles 2 hours, remixes, then settles 24 hours. Ceramic Arts Network says "several hours to several days, depending on the recipe." Digitalfire explains why: settling follows Stokes' law, so the container height and timing decide which particle sizes end up where. Stick with one recipe and one container so your results repeat.

Pinnell expects about 2 gallons per 5 gallon batch and says to discard the sludge rather than reclaim it.

Specific gravity: check and adjust

Specific gravity tells you how much clay is in the liquid. The common target is 1.15, with 1.1 to 1.2 as the usable range. To check it, weigh 100 ml of terra sig: at 1.15 it weighs 115 g. Pinnell's method is to mark the level of 100 g of water in a cup, then weigh the same volume of terra sig and divide by 100.

• Too thin (under 1.1): leave it uncovered to evaporate, or remix and settle for less time.
• Too thick (over 1.2): add a little water, or remix and settle longer.
• Read it cool. The Pottery Wheel notes warm mixtures read lower.

Vince Pitelka's figures, quoted by The Pottery Wheel, are 1.15 for ball clay and 1.20 for Redart. Thin terra sig soaks into the pot; thick terra sig peels.

Applying terra sig

Terra sig goes on bone dry or nearly bone dry ware with a smooth, dust-free surface. Ceramic Arts Network suggests smoothing handbuilt pieces with a rib at leather hard so there is less to fix later.

1. Stir the jar and use a soft, wide brush.
2. Brush one thin coat and let it lose its wet look.
3. Repeat. Pinnell applies two to three thin coats; Ceramic Arts Network uses at least three for maximum sheen, buffing after each.
4. Keep it thin. The final layer should be a fraction of a millimeter. Thick terra sig cracks and flakes.
5. Coat just inside the lip so the edge looks finished.

For color, Pinnell mixes 1 cup of terra sig with 1 to 3 tsp of Mason stain, and Radasch caps stain at 3 tablespoons per cup. Test each color, since every stain saturates differently.

Buffing terra sig to a shine

Buff when the surface looks waxy and gray but no longer wet, per Ceramic Arts Network. Use your fingertips first, then a chamois, soft cloth or polishing mitt, with light, even strokes. Buffing presses the tiny particles flat, and the shine you see before firing is a good guide to the fired result at low temperature. Polishing bare clay with stones and spoons is a separate technique covered in our burnishing pottery guide.

Firing range for terra sigillata

Terra sig can go to almost any temperature, but its shine drops as heat rises:

• Clay Art Center says its terra sig "may be fired to any temp," but the polished surface starts to lose its shine over cone 04.
• The Pottery Wheel says most potters fire it low, at cone 02 or below, which is about 2,014°F (1,100°C), and some recommend no more than about 1,450°F (788°C).
• Higher firing makes the pot harder and less porous but usually satin or matte. Grolleg kaolin terra sig is reported to keep most of its shine up to cone 10.
• Published recipes: Radasch's OM 4 base is listed at cone 03; Pinnell's recipe at cone 04 to 6 in oxidation.

Let pieces dry at least 24 hours and avoid a fast early climb, as Ceramic Arts Network advises. Terra sig is a favorite surface for pit firing and horsehair raku, where the smooth coat takes smoke marks well.

Troubleshooting terra sig

• Peeling or flaking: usually too thick, either in coats or in specific gravity. Digitalfire adds that the terra sig's shrinkage must fit the body; bentonite can help drying flakes, and feldspar or frit can help firing flakes.
• Streaky or burned away at higher heat: Rhonda Willers is quoted saying SG 1.10 tends to streak at mid and high fire, and 1.15 to 1.20 works better there.
• Dull surface: fired too high, too few coats, or buffed while still wet.
• Bare patches: over-sponging the pot before coating, which strips the fine clay.
• Persistent problems: Ceramic Arts Network suggests a tiny amount of CMC gum, which can reduce sheen, or a different clay.

Common mistakes

• Stirring or bumping the bucket before siphoning.
• Scooping up the sludge with the middle layer.
• Skipping the specific gravity check.
• Brushing on one heavy coat instead of several thin ones.
• Expecting high gloss after a cone 6 firing.
• Using terra sig as a food surface.

Dust and food safety

Our clay dust safety guide covers studio cleaning in more detail.

What you need

A few pounds of OM 4 ball clay or Redart, a bottle of Darvan 7 or sodium silicate, a scale and a tall bucket are enough for a first batch. Our powdered clay guide explains where to buy dry clay and how to mix it safely.

Sources: Ceramic Arts Network, "Terra Sigillata 101: How to Make, Apply and Troubleshoot Terra Sig"; Ceramic Arts Network recipe pages for Pete Pinnell's terra sigillata (Mark Arnold, Ceramics Monthly), Kari Radasch's OM 4 Base Terra Sigillata and Rhonda Willers' Terra Sigillata Base; The Pottery Wheel, "How to Make Terra Sigillata" and "What Temperature to Fire Terra Sigillata"; Digitalfire glossary, terra sigillata; Vanderbilt Minerals Darvan 7-N data sheet; Clay Art Center terra sigillata product notes. Specs checked October 2026.