In woodfiring, wood is used as the heat source, and the kiln gets up to above 2400F. The wood also creates the glaze on the surface of the pots. Over the course of a tree's life, it sucks minerals up from the ground, many of them are the same minerals we use in glaze formulation. When we burn the wood in the kiln, the flame flows through the kiln and deposits ash on the pots. The ash contains these trace metals, and when the kiln gets hot enough, they melt, stick to the pottery, and form a glaze on the surface. The finished pieces show the path the flame took through the kiln.