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Terra sigillata or terra sig, which literally translates to “sealed earth,” is a special type of decorative slip made from fine clay minerals. We sell quarts of it from New Mexico Clay in a White and Red.
You may be familiar with the red and black vessels dating back to ancient Greek and Roman times. Although, this was a historically used technique that fell out of fashion, it has made a return in the world of pottery with functional and conceptual work alike. This watery slip is best applied to bone dry ware in a thin application on a fine particle clay body, which lends to the final outcome of a smooth, semi-shiny quality. Traditionally, this type of slip is either terra cotta or white, although colorants can be added once the sedimentation process is completed and the final product has been siphoned off.
To make terra sigillata is relatively simple. You want to choose a clay or clay body that has fine kaolinite particles and combine it with water as well as deflocculant. The amount of water depends on what type of clay you are using for your recipe. For instance, highly plastic clays with finer particles need more water. As far as deflocculants are concerned, there are quite a few choices. Sodium silicate, sodium metaphosphate (found in Calgon) and sodium carbonate will all work, but Darvan 7 or 811 usually yield the best results.
Once combined and stirred up, this mixture will sit for at least 2-3 hours and not more than 24 hours. This is the part of the process where separation through sedimentation is occuring. All of the large particles and settling to the bottom, whereas the finer particles are staying suspended. After a period of time the fine particles will begin to settle as well, so this is why you want to avoid letting it sit for too long. Once the non-plastic particles have settled, you will siphon off the desired part of the slip. It is best to let stand at least a few days to mature because the longer you let it sit the glossier the final fired results will be. The clay minerals are allowed time to split into smaller pieces, thus reflecting more light.
If you don’t want the hassle of making your own terra sig, there are commercial products available that we carry at Stone Leaf. New Mexico Clay makes beautiful red as well as white versions that we keep well stocked for only $12.95 per quart. Skip waiting for things to settle and jump right into decorating.
Terra sigillata can be used in various applications within pottery. You can achieve lovely results with pit and sagger firing, or apply to your piece then etch away designs. Try creating new colors with mason stains or oxides. See what the results are like on different clay bodies at different temperatures. If you are interested in this process there are lots of books focusing on the technique and many recipes online.
Enjoy the endless experimentation,