Ernesto Burciaga on sun 21 feb 99
Here is mt take on ash glazes.
ASH GLAZES Ash glazes are usually formulated for the stoneware
temperatures (cone 8-11), because of the high silica content of the
ash. Different plants and trees will have a different ingredients;
different parts of the same plant or tree may have different
ingredients also different seasons and areas where they grow will vary
the composition of the ash. Generally, ash contains oxides of
sodium, potassium, calcium, alumina and silica. The proportions of
these ashes lend themselves to producing glazes without much
A durable flattering glaze, the typical ash glaze has about 30 to 50%
ash from vegetable, straw, wood or fireplace source The remainder of
this ash glaze is feldspar and clay. The ash being the major coloring
source for this glaze.
Preparation of the ash requires that the he plant must be totally
burned so there isn't excessive amounts of carbon remaining. The
carbon will cause undesirable defects in many glazes. To prepare the
ash for a glaze the ash is mixed with water and sieved through an 30
mesh screen. The material that is larger than the 30 mesh is
discarded and the remaining ash and water mix is allowed to settle.
After the mixture has settled allowing the ash to sink to the bottom
of the container the water is decanted of and if wanted fresh water is
mixed and the process is repeated. The final ash and water mixture is
allowed to evaporate until the ash is dry and ready for use in the
glaze. Repeated washing of the ash will remove the soluble alkalines
but too much washing will start to remove potash reducing the fluxing
action of the ash. Care must be taken to avoid skin contact because
of the caustic nature of the ash.
A different approach is to sieve the ash dry to remove the larger
particles and then use the ash directly in the glaze. This method
will allow the glaze to retain the soluble parts of the ash.
The ash can be added to a stiff stoneware glaze, increasing the
fluxing action, until the wanted result is achieved. Reduction ash
glazes can further be changed with the addition of rutile and the
other coloring oxides normally suitable for this type of firing.
When a low fire ash glaze is wanted the maximum amount of ash
recommended is 20%.