Terra Sigillata 101: How to Make, Apply, and Troubleshoot Terra Sig

saggar-695Terra 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 sig 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 Harnetty, editor.


Everything you’ve ever wanted to know about terra sigilatta

by Sumi von Dassow

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.

Naked Raku Orb by Charles and Linda Riggs, 2003. 7 in. (18 cm) in width. Stoneware painted with white terra sigillata and polished with a soft cloth, bisque fired to cone 010, covered in resist slip and glaze. Sgraffito through glaze before raku firing to 1400ºF (760°C).

Naked Raku Orb by Charles and Linda Riggs, 2003. 7 in. (18 cm) in width. Stoneware painted with white terra sigillata and polished with a soft cloth, bisque fired to cone 010, covered in resist slip and glaze. Sgraffito through glaze before raku firing to 1400ºF (760°C).

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.

Saggar-fired Orb by Charles and Linda Riggs, 2003. 9 in. (23 cm) in width. White stoneware sprayed with white terra sigillata, polished with a soft cloth, bisque fired to cone 010, saggar-fired in a raku kiln with wood shavings, steel wool, copper, and salt.

Saggar-fired Orb by Charles and Linda Riggs, 2003. 9 in. (23 cm) in width. White stoneware sprayed with white terra sigillata, polished with a soft cloth, bisque fired to cone 010, saggar-fired in a raku kiln with wood shavings, steel wool, copper, and salt.

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.

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.

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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.

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Troubleshooting

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.

Recipes:

Some of these recipes suggest checking specific gravity with a hydrometer. 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.


**First published in October 2011
Comments
  • Thanks a very informative article, I will try some of your appliction techniques. I’ve used terra sigillata for years but still learned a lot from your article.

  • Have tried it years ago and made my own from found clay. I’d like to try it again after reading this article

  • FYI – Calgone in it’s original form may be gone but the main ingredient which potter’s used it for isn’t. SODIUM HEXAMETAPHOSPHATE and some perfume were the two things which made it what it used to be and the former is still available and has been called for in European Terra Sigillata recipes for years.

  • My question…is the deflocculant essential, or will “patience” work as well? What I mean is, liquid clay will settle out, leaving a watery layer on top. Is the “middle” part useable as terra sig even though you haven’t deflocculated it? I’ve always wondered…

  • TWO BASIC (Easy) TERRA SIGILLATA RECIPES

    RED ART
    400 GRAMS RED ART CLAY
    1600 GRAMS HOT WATER (7 CUPS)
    40 GRAMS SODIUM HEXAMETAPHOSPHATE (SHP)

    WHITE
    400 GRAMS WHITE BALL CLAY (KYOM4, TENN10*)
    1600 GRAMS HOT WATER (7 cups)
    20 GRAMS SODIUM HEXAMETAPHOSPHATE (SHP)

    Measure your hot water from the tap into a tall clear glass or plastic container. Take about 2 cups of this water, place it in a Pyrex measuring cup and add the SHP, whisking to dissolve. (If you are having trouble dissolving the SHP, place the measuring cup in the microwave for 45 seconds, remove and whisk again to blend.) Add the SHP water from the measuring cup to the rest of the measured out hot water in the tall glass or plastic container.
    Slowly add the pre-measured bag of dry clay (Red Art, KYOM4, Grolleg or Tenn10*) to the prepared water causing as little clay dust as possible. If you are really concerned about the dust, wear a dust mask. Once all the dry clay has been added, whisk the mixture for a few minutes to make sure it is blended then place it where it can rest undisturbed. (If there is a chance that something may fall into the container, cover it loosely with a lid or piece of paper.) Over a few days to two weeks, the mixture will separate into 3 distinct layers – the top is water, the middle is your terra sigillata and the bottom is clay sludge. Siphon the middle layer of sig into a clean lidded container then siphon off the water (and get rid of it). Dispose of the bottom layer of sludge in a responsible manner.
    The SIG will continue to ‘separate’ and it’s up to you if you want to remove more of the water to get a thicker consistency SIG…. it should really be the consistency of half ‘n half to light cream, no denser. Sometimes all you have to do is leave your containers of ‘sig’ sit for a few weeks uncovered – water evaporates and the consistency becomes creamier.

    *These are all different types of clays ranging from fire clay to china clay to kaolin to ball clay. Each makes their own unique type of Terra Sigillata.

    Adding COLORANTS to your TERRA SIGILLATA

    WHITE
    Simply use the Tenn10, Grolleg or KYOM4 Terra Sigillata straight from the jar.
    BLACK
    There are different shades of black and several ways to make them. The blackest black seems to come from adding 6600 Mason stain to the Red Art Terra Sigillata. Start by adding ¼ tsp to 2 oz of the Red Art Sig and if it tends to fire a little brown, add more 6600 stain. I also like the grey black I get from adding the 6600 to the KYOM4 Sig base.
    CHROME GREEN
    Chrome Oxide added alone to Tenn10 or Grolleg Terra Sigillata can be a little intense so I usually tone it down with a pinch of the 6600 black Mason stain. Again start out with the ¼ tsp amount added to 2 oz of the whitest Sig you’ve got and go on from there. If you want a very pale green, start with a pinch…

    Mason Stains
    Light colors such as pinks, yellows, peaches and blues are best added to a white terra sigillata base. The white base will allow you to see just how concentrated the color you are making has become. KYOM4 Terra Sigillata works just as well as a base but the colorant you add is affected by the grey tone of the basic terra sigillata so it is harder to tell how strong the color really is.
    Darker stain colors such as forest green, browns and rusts work well with KYOM4, Tenn10 or Grolleg Terra Sigillata. Stains can also be added to Red Art Sig but the red iron oxide which gives it its name will affect the stain you are adding, so be prepared for some surprises.

    RUTILE
    Rutile is a weak red iron oxide which can be used to color terra sigillata. It is good for producing a tan color or for toning down some shades of blue which can be harsh.
    >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    I can supply SHP to anyone who needs it – 250 grams for $8.50 or 500 grams for $12.50 including US shipping.
    BHClaysmith@gmail.com

  • Thank you for sharing this beautiful surface which was a huge reason why I made a career out of clay over 30 years ago. I have some sodium silicate that has dried to a hard jell like consistency. Can I re hydrate it? Did add some warm water and it does seem to be dissolving it but what is the “normal” consistency for use.

  • Thanks for your informative article. I feel like I can get started after reading just it. Your work is beautiful!

  • Thank you for this article.

    My question… what is temperature need for firing?and how long needs for firing?

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