Richard mahaffey on fri 27 sep 96
Dear Loran,
Mt ST. Helens Volcanic ash usually makes a passable glaze all by itself.
Interestingly it is almost identical in chemical composition to the local
clay in the Puget Sound region that is the product of glacial till. This
clay is an earthenware clay and as such it melts at cone 10.
Don Johnson did a Masters thesis at the University of Puget Sound in 1973
on the local clay and found that with 5 to 40 % Kingman feldspar (use
custer) (Ie. 100 volcanic ash, 5 feldspar) to
produce a brown thru brown-purple to a blue purple with the maturing point
rising with the percentage of feldspar as well as increasing spot size
increasing with the percentage of spar.
I would be inclined to try Nepheline syenite in place of the custer. also
the addition of cobalt can produce some interesting results.
About 15 parts Neph. Syen. and 100 parts volcanic ash may
be optimum. As I recall it will produce a nice medium brown with a
metallic-like sheen.
I hope this helps.
All usual disclamers apply: This advice is worth what
you paid for it. The results you get may vary. Test all formulas before
commiting to a pot. This is a ground up mountain of about one cubic mile
in volume so who knows what part you got. This stuff comes out of the
ground (or sky in this case) so it can (will ) vary incomposition. And no
waranty is expressed or implied. ;}
Best of luck, and test, test, test.
Rick Mahaffey, Tacoma Community College, Tacoma, Washington, USA.
Hluch - Kevin A. on sun 29 sep 96
Rick notes that volcanic ash can be a passable glaze all by itself. This
is most certainly true.
While in Kenya I found a mineral from a local desposit that when fired to
cone 10 was a faultless greeenish tinted clear glaze. A little bit of
screening and application to bisque was all it took. This mineral became
the base for subsequent testing by Kenyatta University students and we
produced very excellent results including matt glazes, opaques, and textured
glazes with rather straight forward combinations of other minerals.
Since the material was found half way to Lake Magadi (a soda ash lake) in
the Rift Valley it was most probably pumice or volcanic ash.
Test it, use it, vary it, adulterate it, and keep good notes. I'm sure
you'll find something.
Where I teach we use a glaze at cone 6 that has a high
percentage a of volcanic ash and it is a popular glaze. Unfortunately,
considering the millions of tons of ejecta from volcanoes (e or no e, all
you Quaylemeisters?) pumice is surprisingly expensive.
Kevin A. Hluch
On Fri, 27 Sep 1996, Richard mahaffey wrote:
> ----------------------------Original message----------------------------
>
> Dear Loran,
>
> Mt ST. Helens Volcanic ash usually makes a passable glaze all by itself.
> Interestingly it is almost identical in chemical composition to the local
> clay in the Puget Sound region that is the product of glacial till. This
> clay is an earthenware clay and as such it melts at cone 10.
>
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