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volcanic ash glaze...

updated mon 30 sep 96


Sue Hutchen on fri 27 sep 96

This Volcanic Ash Clear Glaze seems to have a very wide firing range. I
fire at ^6 electric, I don't know how much hirer you can push it but I
expect some.

Volcanic Ash 40
Gerstley Borate
Strontium 6
Wollastonite 10
Flint 18
Ball Clay 12
Zinc ox 2

Clear, slightly amber on white

Michael McDowell on fri 27 sep 96

Loran asked for a formula for a cone 10 glaze using volcanic ash, specifically,
ash from the 1980 eruption of Mt. Saint Helens. Usually I find that by the time
I get my digest, most questions have been answered better than I could manage.
But in this instance I have "insider knowledge" since I have been using ash from
Mt. Saint Helens in glazes ever since May of 1980, when the mountain literally
blew it's top.

The most straightforward approach to getting a sample piece of clay glazed with
that ash sample your friend collected for you would be simply to screen the dry
ash thru about 80 or 100 mesh screen and then use it alone as a glaze for cone
10. This will not work with ash from just any volcano, but it will produce good
results with Mt. Saint Helens Ash as long as you are doing reduction at some
point in your firing cycle. You might find it helpful to add 2 or 3 percent of
Bentonite, or 5 percent ball clay, to help suspend the ash in a glaze slurry as
well as strengthening the raw glaze coat. Those additions should have
imperceptible effects on the finished appearance of the glaze.

If you are doing reduction below bisque temperature, and getting effects like
celedon and copper reds then you might try using Mt. St. Helens ash as the
source of the iron in a celedon. If you have a celedon formula you are working
with, try mixing it without the iron, and then blending it with 20 to 50 percent
Mt. St. Helens ash. Or quicker and easier, just try a variety of ratios up to
50/50 between the straight ash mixture I suggested first and any clear glaze you
have already mixed up.

I have used Mt. St. Helens ash to formulate Temmoku, Kaki, Celedon, even Copper
Reds as well as many other glazes, but you have only a small amount so you
probably want to keep it simple. Just ask if you want to hear more.

Michael McDowell
Ferndale, WA, USA.