Published Mar 22, 2023
Terra sigillata is an ultrarefined clay slip that can give a soft sheen when applied to bone-dry wares and, if polished or burnished while still damp, may give a high gloss. The ancient Greeks and Romans used this technique in lieu of glaze. I love the buttery surfaces that can be created with terra sigillata and I have been meaning to try it for a long time.
If you've been intrigued by terra sig, today's post is for you. Sumi von Dassow gives the low-down on terra sigillata, from mixing, to applying, to burnishing, and, of course, troubleshooting. She also shares a number of terra sig recipes. - Jennifer Poellot Harnetty, editor
The term terra sigillata, which means ‘sealed earth’, comes from the name of a type of Roman pottery mass-produced around the first century AD. This pottery was decorated with impressed or stamped decoration, which is what the word ‘sigillata’ refers to. (Think of the kind of stamp, or ‘seal’, which would have been used to seal wax on a paper document.) These pots were coated with the same kind of very fine slip which Greek potters had been using for hundreds of years to create their famous black and red pottery. Though many books incorrectly refer to this slip as a ‘glaze’, it was not actually a glaze but the material we now call terra sigillata.
Making Terra Sigillata
Terra sigillata, or ‘terra sig’ for short, is made by mixing a suitable clay with water and a deflocculant and leaving it to stand until the heavier particles of clay settle out. (Deflocculant weakens the electrical attraction between particles of clay, thus breaking up small clumps of clay and allowing the individual particles to float freely.) The deflocculant causes the finer particles to float in the water, which can then be decanted for use. In general it is not possible to buy terra sigillata, so if you want to use it, you must make your own.
To make terra sigillata, you will need a clear glass or plastic jar with a wide mouth, an accurate gram scale and a length of clear plastic tubing for siphoning. The only ingredients are water, dry clay and deflocculant. Many kinds and colors of clay can be used, including ball clay, kaolin, local clay or scraps of whatever clay body you usually work with. There are also many possible deflocculants, the most commonly used being sodium silicate, soda ash, Darvan 7 and Darvan 811. You might find recipes calling for Calgon water softener, but don’t try those – Calgon doesn’t work since it was reformulated to eliminate phosphates. Lye can also be used as a deflocculant, and I have even experimented with using the waste water from washing wood ash.
Not all clays are equally suitable to make terra sig, and the proportions of water to clay to deflocculant will be different depending on what clay you use. It is a question of experimenting with different types until you find something suitable. You can try substituting any dry clay, including scraps from your clay body, for the clays called for in the recipes which follow. The process is simple, but a bit time-consuming. First, measure your water, and stir in the deflocculant. Weigh out your clay and add it to the water. For best results, be sure to weigh these materials precisely. If you have a ball mill, you can ball mill the mixture, otherwise, shake or stir it vigorously. Then place the jar, loosely covered, somewhere where it won’t be disturbed for several hours to several days, depending on the recipe.
After the appropriate settling period, you will see a layer of dark sludge on the bottom of your jar, and if it has been a long settling period you may see clear (or possibly dark-colored) water on the top (this varies depending on the type of clay and length of settling). It’s the part in the middle – hopefully, about half the mixture – which you need. Use a syringe to remove carefully as much of the water from the top as you can without taking any of the fine clay particles along.
When clay starts getting into your syringe, it’s time to siphon off the middle layer into a clean container, using the clear plastic tubing – or for a small batch, simply use the syringe. Don’t be greedy. If you get some of the heavy sludge into your terra sig, it may never settle back out and you’ll have gritty terra sigillata which won’t shine up as well. It’s a good idea when you get close to the layer of sludge to switch to a new container, so if some of the sludge gets in you don’t contaminate the whole batch. You now have a batch of rather thin terra sigillata (along with a lot of sludge which can be discarded or used for some other purpose). It can be used as is, or allowed to settle and evaporate for a few days before using.
Applying Terra Sigillata
Terra sigillata can be applied in two ways, by brushing or spraying. Brushing is easier, but may leave noticeable brush-marks. On the other hand, spraying requires more equipment, and may leave a bit of a pebbly texture where the droplets land on the pot.
Before you apply terra sigillata, your pot must be smooth and dust-free. Terra sigillata is so fine that even if you cover a textured surface with several coats, the texture still shows. This is wonderful if you have a deliberately textured surface, and in fact, the only way to burnish a textured surface is with terra sig. However, if you have sanded the pot, the surface is likely to be covered with little scratches from the sandpaper, which will not be covered up by the terra sig.
Even more important, if you have sanded your pot, you must carefully sponge off any dust. Dust will cause the terra sigillata to peel off after firing. Therefore, if you want to achieve a really smooth burnished surface using terra sig, it is most effective to apply it to a wheel-thrown pot which has been ribbed smooth after throwing or trimming, or if handbuilding, to smooth the entire surface at leather-hard stage with a rib.
Terra sigillata should be applied to a bone-dry or almost bone-dry pot. If you are brushing it on, you need to apply at least three coats. If you are putting white terra sigillata on white clay, three coats is probably plenty. The terra sigillata needs to soak into the clay, but should not be allowed to dry completely between coats. Once you have applied several coats, the surface should be buffed with your fingers, a cloth or chamois-leather before it dries completely. The pot is ready to buff when the surface looks waxy and grey but is no longer wet-looking. If it has lightened in color, it has dried too much and another coat of terra sigillata must be applied. For the greatest degree of sheen, apply three thin coats and buff after each coat.
Watch out for two things when you are brushing on terra sigillata: don’t let it drip down your pot, because the drips will show; and don’t allow your brush to lose hairs, as the hairs will make a permanent mark. Be sure to use a good quality soft brush – a watercolor mop brush works well. If you are brushing terra sigillata onto a wheel-thrown pot, the simplest way to apply a nice even coat is to put the pot on the wheel and let the wheel do the work for you while you move the brush up and down. Once you have enough coats on part of the pot, you can start burnishing with the fingertips of one hand while you are still brushing the terra sigillata onto another part of the pot with the other hand. If you have a large pot you are almost required to do this to get a really good polish, or the terra sigillata may dry out too much before you finish brushing it on. Don’t touch the surface until it has soaked in, though if the terra sigillata comes off on your fingers, it isn’t ready to burnish yet, and you will mar the surface by touching it. After you have applied enough terra sigillata to the whole pot, and there are no wet patches, then you can start using a chamois-leather or a soft cloth, or even a thin plastic shopping bag, to bring the surface to a high gloss. Check out this post in the archives for more on burnishing ceramics!
If you are applying terra sigillata to a handbuilt or sculptural piece, you may find it impossible to use the wheel to help with the job. In that case, you can still brush it on, but be careful not to touch any wet spots. You may also want to experiment with pouring, or even dipping if you have a large enough batch of terra sigillata and a way to safely hold a delicate piece of greenware.
Step 1: Weigh out Darvan 7 or 811 and add it to the measured amount of water.
Helpful hint: If you always mix your terra sigillata in the same container, place a permanent water-line on the side of the container so you don’t have to measure the water every time.
Step 2: Weigh out the dry clay, add it to the water solution, and mix thoroughly.
Step 3: Stripes become noticeable down the sides of the container as the heavier particles begin to settle.
Step 4: After settling (in this case, approximately 3 hours, but could be as long as several days), siphon off the middle layer of liquid from above the layer of sludge. Be careful not to get any of the settled sludge in your siphon.
Step 5: Center a pot on the wheel upside-down, and while the wheel spins begin brushing on the terra sigillata from the foot. Apply two or three coats, until you cannot see the underlying clay color clearly.
Step 6: Once you have applied enough terra sigillata to the lower portion of the pot, and it has soaked in so it is not glossy wet, use your fingertips to begin polishing the surface as you continue coating the pot with the other hand.
Step 7: Turn the pot right-side up and finish coating it with terra sigillata. Apply just inside the lip; there is no need to coat the entire inside of the pot.
Step 8: Using the polishing material of your choice-a car-polishing mitt works well -bring the entire surface to a high sheen.
Terra sigillata occasionally suffers from problems adhering to the clay it is applied to. Terra sigillata is not integral to the material the pot is made from, as is the surface of a stone-burnished pot; at the same time it doesn’t adhere by melting like a glaze does. Some potters prefer to burnish with a stone for this reason, but you can keep this problem under control by attention to some rules of thumb in applying terra sigillata and firing your ware. Most important is to make sure the terra sigillata is quite thin and watery. It should take several coats to cover your pot, and the final depth of applied terra sigillata should be a fraction of a millimeter. The pot must be dry but not dusty. If necessary, use a damp sponge to remove dust from the surface before applying terra sigillata. However, beware of sponging the surface of your pot too much: every time you rub it with a wet sponge you are removing particles of plastic clay and leaving behind the heavier particles of clay and coarse grog, which don’t hold on to terra sigillata as well. After coating your pot, allow it to dry completely before firing; it is a good idea to wait 24 hours.
Avoid a too-rapid increase in temperature during any firing – a pot which comes out of a bisque firing perfectly smooth may peel in a hot and fast pit-firing. If you experience a great deal of peeling despite following all these rules, you may have to change the clay you use or the terra sigillata recipe. One last thing to try is adding a little CMC gum to your terra sigillata. This is a gum which is often used as a binder. It comes as a powder which has to be dissolved in hot water before it can be added to your terra sigillata. Add only a very small amount at a time and test as you go. CMC may reduce the level of sheen you get from your terra sigillata, and it will slow the drying time.
Many potters use terra sigillata as a canvas for the fire to paint on, either pit firing or saggar firing their burnished pots. However, other potters like the satiny surface of terra sigillata as is, without any special firing. Ricky Maldonado is a potter who uses terra sigillata as a base for painting intricate patterns, rendering them with tiny dots of low-fire glaze covering the burnished surface of his pots.
You can also try using terra sigillata to partially cover a surface which has been burnished with a stone. In this way you can have a multi-colored burnished pot with a high level of sheen. I frequently use terra sigillata made from Cedar Heights Redart clay to paint intricate patterns of fine lines on burnished pots made from red clay. I then smoke-fire these pots to achieve a black-on-black effect. I adopted this process because I found that plain clay slip often rubs off a burnished surface, while the process of buffing terra sigillata to shine it up seats it even on an already burnished surface.
Some of these recipes suggest checking specific gravity with a hydrometer (here's a great hydrometer DIY!). You can also pour 3½fl oz (100 ml) of terra sigillata into a graduated cylinder and weigh it – if it weighs 115 grams it has a specific gravity of 1.15. This is the recommended consistency for terra sigillata. If it is much thinner it is hard to apply enough without over-saturating your pot with water. If it is much thicker, it may peel off after firing. In practice you will get to know just how you like your terra sigillata and you won’t have to check the specific gravity of every batch you make.
Check out this video in the archives on Marcia Selsor's super quick terra sig!