Terra Sigillata dates back to the Greeks and Romans. It means settled earth and is made from a watery solution of clay that is allowed to settles. There are many methods for making it. Some methods include using a ball mill. below is a simple method taught at La Meridiana in Italy. Many artists including Poala Paranetti and Rosana Antonelli have taught there use this method.
* SIMPLE Terra Sigillata recipe. Fill a plastic .591 liter bottle with 3/4 full of water. Add 125 grams of ball clay 1-10 clay to water maximum. Use a 1-liter bottle and 250 grams of clay. Add a couple of drops of Darvon 7, or Darvon 811 or sodium silicate. For a smaller bottle adjust amounts accordingly. Add more water until the bottle is almost full. Put the top on tight and shake well. Let the mix set for an hour or so..until you see three separate areas of the mix. Clear, light, and dark. If the top layer is completely clear, add more Darvon. You want some color in the top layer. Punch a hole at the bottom of the middle layer and let it squirt into a bucket. Wait until your piece is bone dry and apply to the piece with a soft brush. Burnish after each application. I use a soft sponge for burnishing. You can apply several coats as you like until it is shiny. Too much Terra sig can lead to peeling or cracking. La Meridiana http://www.lameridiana.fi.it/pottery_clay_papers.htm