While this finish can be applied to naked clay I prefer to apply a colorant to the bisque piece prior to the raku firing, I find this adds an organic touch to the work and adds several layers of depth to the finished peace which can simulate natural stone.
Red iron is my current choice of colorant.
Colored with iron oxide prior to raku firing, the piece is placed in a raku kiln and rapidly fired to 1800 deg. F. The piece is pulled out of the kiln while red hot and horse hair applied to the surface. The horse hair will crinkle and curl because of the hot surface it then carbonizes and infuses into the body of the piece.
Sometimes the horse hair vaporizes creating a smoke which is then captured in the matrix of the hot ceramic and becomes a feature in the finished piece.