History of Ash Glazes
The potters in China developed downdraft kilns earlier than anyone else. These kilns trapped more of the heat produced by the fire, which allowed the potters to fire into the higher temperatures. In large part, these kiln designs also brought about the discovery that wood ash, when heated high enough, melts into a glaze all by itself.
During the Shang period (approximately 1751 – 1111 BCE) it appears potters began mixing ash with lime or earth to produce applied glazes. These were usually a greenish yellow with spots of deeper green. Even today, a satisfying glaze can be made by mixing roughly equal parts of wood ash, feldspar, and clay. (Testing is still required, however.)
Ashes in the Fire
As the flame moves through a wood fired kiln, it can deposit ashes. As the ashes melt on the surfaces of the pots they land on, they form a shiny, mottled green to amber glaze called "kiln glost". This kiln glost would mainly develop on the rims and shoulders of the pottery since those were the surfaces most likely for the ash to settle.
Kiln design and the firing methods really impact how much ash is deposited onto ware and where on the pots it is deposited. Fly ash (ashes that are deposited onto the ware during a wood firing) will result in glazes on pot rims, shoulders, and possibly on the side of the pot facing the draft as the flame moves through the kiln.
Composition of Ashes
Wood ash is an alkaline containing widely variable amounts of silica, alumina, potash, sodium, magnesium, phosphorus, iron, and other trace elements. Ash from faster growing plants such as reeds or weeds is similar, but has more silica than slower growing plants like trees. Roughly speaking, ashes from softwoods are higher in fluxing agents (ergo more fluid), and oaks and willow ashes contain more phosphorus, which effects translucence.
Natural wood ash is never exact. I have fired pots using wood ash from two different Sheepshead apple trees which had grown up next to each other, and had the same glaze recipe give noticeably different results.
Use of Glazes Formulated with Ash
Once a pot has been dipped into an ash-based glaze (or any other low-clay content glaze) care must be taken to prevent any damage to the raw glaze coat. Before firing, the raw coat is very powdery and fragile and will scratch and rub off easily.
If desired, you can add 2% bentonite to the glaze recipe to help adhesion. Another option is to lightly spray an adhesive onto the raw glaze coat to act as an outer protective shell until firing. Equal mixtures of water and liquid laundry starch in a misting bottle works well.
If firing at to cone 10, especially in reduction, you can also add interesting glaze effects by sprinkling ashes onto the rims and shoulders of the glazed pots just prior to firing.
A Caution about Using Ashes
As I’ve mentioned, ashes are very alkaline. Raw ashes and raw ash glazes are highly caustic and will damage unprotected skin. Ashes can be washed before using, which reduces their alkalinity, but also makes them even more refractory. Another solution is to be sure to use rubber gloves (which is a normal safety precaution anyway).