"Terra sig" , as most potters say, is made from clay , water, and a deflocculant such as Darvan. The mixture settles, and is siphoned to separate out only the finest particles to be used as terra sigillata. When applied to unfired clay surfaces, "terra sig" can be 'polished' or burnished with a piece of plastic bag. The polished surface can be retained when fired at a low temperature. I have found it to flake and peel when fired at cone 6.
You can add mason stains to the terra sig to have a variety of colors. In recent years, you can purchase pint jars of premixed terra sigillatta. I have purchased it from Clay King.... it comes in two colors.http://www.clay-king.com/glazes_ceramic_pottery/terra_sigillata.htm
Here is a picture of the Old Mine #4 on red earthenware clay....fired to cone 05. This is used on the bottom of a dish....brushed on, rubbed with a sponge and burnished with plastic wrap.