I have just started sculpting with an amazing nylon fiber clay. It fires to cone 6. The application of additional clay is the opposite of what potters are used to doing. Apply wet clay dipped in water to totally dry clay....then, rub aggressively . There really is no need for slip as the water works just fine.
The only supplier of this clay that I have found is New Mexico Clay works. Shipping is prompt.http://www.nmclay.com/ProductDesc.aspx?code=MagicClay&type=4&eq=&key=it&desc=MagicFiberClayCone6
Please google Lorri Acott-Fowler. This is her clay body and her work is amazing! http://www.lorriacottfowler.com/page/page/4112062.htm