Any clay will work, since we are firing so low. Clay with grog in it will be more thermal shock resistant, however clay with none will burnish most easily. It is all an experiment. White firing clay will show more color. Try adding some sawdust to your clay for handbuilding.
I use a Amherst Potters Supply low fire white ball clay talc body.

Burnishing is any means of smoothing the surface to a sheen. Use a spoon or smooth stone or rubber rib on leather hard clay. As the clay stiffens, the burnish will increase. Some potters burnish wetted bone dry pots. It is okay to fire work that is not burnished as well, though people often prefer the shine and enjoy the smoothing process..

Terra sigillata (earth seal) is a slip made from clay and water and a small amount of deflocculant, settled and separated to harvest the finest particles of clay, which, when applied to the clay piece are easily polished. It is not difficult to make. Here is a starter formula: 1 liter water, 4 grams soda ash, 400 grams of clay. Allow the clay to settle and wet thoroughly before mixing. Let settle for 20 hours. Siphon off the top 1/3 including the water. This is the TS. For the clay you can use ball clay for white, redart clay for orange brown, a mix for pink. Try any other clay or add colorants in the form of oxides or carbonates or mason stains to experiment.
Read Vince Pitelka's Super-refined Terra Sigillata

We will be nesting and wrapping the pieces combustible materials and resists. Play with soaking firing materials( try fabrics, straw, etc) in salt water, copper solution, iron or ochre and dry them for firing. Bring these along as well as sawdust, straw, hay, scrap wood, firewood. The fire will be as hot and long as we are able to feed it. I will be preparing materials, you can gather seaweed and we have lots of down wood to burn.

Everyone does it his/her own way.
Pit firing Links http://www.claystation.com/technical/firing/pit.html