Cindy Strnad on sat 23 dec 00
Hi, Melinda.
Pumice and volcanic ash refer to the same thing. However, ash from one
volcano will differ from that of another. Probably ash from the same
volcano, spewed at different time intervals, will differ quite a lot, as
well. Suppliers don't usually specify the source of their volcanic ash, in
my experience. So, it's always an adventure when you try a recipe with this
particular ingredient.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Ron Collins on sat 23 dec 00
Can someone who has better reference material please tell me the =
difference between pumice and volcanic ash-ie: it's interchangability as =
glaze material. We have tried to look up the difference, but can't find =
what we need. Many thanks and merry Christmas...Melinda
Michael McDowell on mon 25 dec 00
Melinda wrote from Guatemala:
"Can someone who has better reference material please tell me the =
difference between pumice and volcanic ash-ie: it's interchangability as =
glaze material. We have tried to look up the difference, but can't find =
what we need. Many thanks and merry Christmas...Melinda"
Gavin Stairs has already offered a pretty good discussion of the generalities.
Let me see if I can make some suggestions specific to your situation. I know
from your earlier contributions to the list that you haven't got real easy
access to materials and supplies for higher temperature ware. It makes good
sense for you to take stock of what is available locally, and how you might
use it in both clay and glaze. Pumice and ash from the same eruption should
be expected to be nearly identical in chemical makeup. The two words describe
different physical properties resulting from just how it ejected the volcano.
The same gasses that create the pourous structure of pumice rock are what can
propel the solids thousands of feet into the air and blast them to tiny bits,
which are then called "ash" as it falls back to earth. So if you know there is
a common source for both the ash and pumice you have access to, you can regard
them as equivalent.
Just in general, Melinda, try to develop the habit of making little pinch
bowls in which you could test small samples of various materials in your glaze
fire. It might be helpfull to do this with each of the materials you already
use as well. No more than a thimble full will give you some real indications
of what it might be good for. If you find some interesting stuff, which no
doubt you will, the next project may be how to best reduce particle size to
something suitable for glaze.
Volcanic ash or ground pumice can also be used in clay bodies at both high and
low temperatures. For low temperature firing, like the pit firing we did when
you came to Vince's workshop here, you could experiment with additions of up
to 50% ground pumice / volcanic ash as a "tempering agent" to local clay, to
reduce shrinkage and give the pieces more resistance to the thermal shock of
firing, and subsequent uses. In a high fire clay body, volcanic ash or ground
pumice might serve as an alternative to feldspar, to aid in the vitrification
of the fired product.
Happy hunting!
Michael McDowell
Whatcom County, WA USA
mmpots@memes.com
http://www2.memes.com/mmpots
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