Clay Times Back Issues Vol. 2 Issue 7 • Nov/Dec 1996 | Page 17

Volcanic Ash Glazes
BY MEL JACOBSON

I

taste.” It is possible to measure
cones, it was more controllable. By
n seeking a simple yet beautismall batches of test glaze materials
changing the formula to 70/30, we
ful glaze that I could use in a fullfired it to cone 10 reduction. Several using tablespoon, cup, or gram
time high school clay program, I
measures, as long as the basic
students tried 50/50 combinations
stumbled over a small article by
premise of 60/40 is followed. We
to fire at cone 06, but we did not
Richard Behrens about the use of
had almost zero failure due to
explore this area beyond a few
pumice as a glaze material. This
glaze, but as always (with
was in the early ’70s and
students) we had failure
colemanite had changed and
due to technique.
gerstley borate was becoming the mid-range flux of
The idea or concept of
choice for many potters.
this simple, low-tech glaze
Behrens suggested using
is ideal for home potters,
combinations of volcanic ash
beginners, schools, and
with gerstley borate to create
commercial potters. It is
a variety of temperatures for
inexpensive, easy to make,
a simple yet effective glaze.
does not settle in storage,
This began a long study to
and dries on the pots like
find a solution to our glaze
cement. Being able to hanneeds. Then with the explodle the dried, glazed pots
sion of Mount Saint Helen’s,
without chipping or powand the publicity related to
dering is great for schools
the abundance of volcanic
and commercial potters.
ash, we decided that we had
our glaze. High school stuThis glaze has endless
dents loved the idea of using
possibilities. It can be
a material that nature blew
Plate with stand by Larron Lerdall. Decorated with commercial
dipped, sprayed, brushed,
out of a massive rupture in
stains, mixed colors. Glazed with 60% volcanic ash,
thinned, thickthe earth. It helped them
40% gerstley borate and fired to cone 6 in an electric kiln.
ened, and colunderstand how the whole
ored to endless
process of nature melting earth was experiments, as we needed
hues. It can be
what they could do in a studio with a low-temperature clay
used clear or
body.
a kiln and their pots.
opaque. (Larron
Lerdall adds 10%
Several chemist/potters
We began with a formula of
zircopax and
have run an analysis on
60% volcanic ash and 40% gerstley
mason stain at 5“Insight” and suggested
borate. In the early stages, we fired
➤ Larron Lerdall is a full-time
7% for colors.) It
only that a small percentto cone 4 and used the clear glaze
potter from Mesa, Arizona. He started
also works well
age of silica be added for
to cover other commercial glazes
using 60/40 as his base glaze 10 years
over other glazes
and stains. Soon, however, we were strength if it is used for
ago. “I liken my glazing technique to a
reverse batik process. I tape off a
or in combinaadding iron, copper, and cobalt and dense commercial ware.
bisque piece using various sizes of
tion with slips.
with the addition of zircopax, we
masking tape, pricing stickers, mailing
Students at
Over the last 10 years,
began to achieve opacity. As a
labels, etc. I first spray a base coat (usuHopkins High
we have made the glaze in
result of a mistaken cone placeally a dark color—sometimes several
School in
large batches, using 60
ment, we found that we could fire
colors), then remove the tape and spray
colors onto the raw bisque. This gives
Minnesota have
pounds of ash with 40
to cone 6 without running the
true colors and combinations of colors
tried just about
pounds of gerstley in a 50glaze. Our