Richard Aerni, an upstate New York clay artist, is known for his ash glazes. He is a self trained potter who is well known throughout North America for his clay work.
Ash glaze, as the name suggests, is a glaze derived from ashes. The earliest such glazes can be traced back to the Shang period in China (c. 1500 B.C.), and it is thought they were produced accidentally, the result of white-hot wood ash being carried through the kiln with the draft of the fire and settling onto the pots, where the searing white heat melted it to a glass. Three thousand years later, wood ash remains an important and immensely popular feature of pottery glaze making.
For the modern potter, the satisfaction of working with ash glazes comes from following an ancient tradition as well as from using materials that occur naturally. The results of different wood ashes often vary dramatically, making it possible to achieve a wide range of unique finishes. Even wood from the same species of tree garnered just miles apart can produce subtly different results. (excerpt from Ash Glazes by Phil Rogers.)