Terra Sigillata is a very thin slip made from clay, water and a deflocculant. The purpose of the deflocculant is to separate the particles of clay into larger and smaller particles. The smaller particles are the terra sigillata.
Although not necessary, using a glaze hydrometer makes it much easier to mix the terra sigillata to the correct consistency (a hydrometer costs about $15 at Capital Ceramics).
Easy Step by Step Recipe for Terra Sigillata
- 10 lbs. dry powdered ball clay (although you can use any dry clay).
- approximately 14 quarts water. Ideally use distilled or filtered water but water out of the tap is fine.
- 1/2 oz. soda ash and 1/2 oz. sodium silicate. Both of these ingredients are very inexpensive but you can substitute 1 oz. Calgon. Sodium silicate comes as a liquid but you weigh the liquid the same as you would a powder.
Use a 10 gallon bucket for mixing. Dissolve the deflocculants (soda ash and sodium silicate) in one cup on hot water. Once dissolved, mix into 14 quarts water and then mix in the clay. Mix using your hand or some type of mixer. If you have a hydrometer it should read about 1.15 specific gravity after it is mixed. Once mixed, put the mixture on a table and leave UNDISTURBED for 20 hours.
After the 20 hours the larger heavier particles have settled to the bottom of the bucket. The terra sigillata is the uppermost, thinnest liquid in the bucket. Use a flexible clear plastic hose to siphon off the thin liquid into another bucket. Make sure you don't agitate the mixture or move the container at any time during the process. Siphon from the top of the bucket and not the bottom. Try to keep the siphon hose tip barely immersed in the mixture - this makes it easier to tell when you begin to reach the thicker material. As soon as you start reaching the thicker material, STOP siphoning. You should have approximately 11-12 quarts of terra sigillata. The thicker material left remaining in the bucket should be thrown away.
The terra sigillata will be too thin of a mixture to use immediately. To thicken it, you can set it aside and let it evaporate for 2-3 weeks. A quicker option is to let it sit undisturbed for a few days and the mixture will continue to settle. The top of the mixture starts to look like slightly cloudy water. Carefully siphon off 4-5 quarts of this "water" (save the "water" you siphon off as it still has very fine particles of clay in it and enough water will eventually evaporate so that it also becomes a useable mixture). Thoroughly stir the remaining mixture and it should be ready to use. Note: If you have a hydrometer, the specific gravity should be between 1.13 and 1.17.
For best results when using the terra sigillata, apply when the pot is bone dry. It also helps to lightly sand the surface with fine steel wool. Apply the terra sigillata with a very soft, wide brush. Apply several coats (2-5) and try to avoid drips from running over the surface. The terra sigillata does not need to dry between coats. Before the terra sigillata drys, while still dark but not wet, polish with a soft clean cloth until the surface shines. There is no need for hard rubbing provided you catch the terra sigillata at the right stage.
For a more thorough discussion on making and using terra sigillata go to Vince Pitelka's site: http://iweb.tntech.edu/wpitelka/professional/terra_sig.htm
- Submitted by Chuck Parsons