Michael McDowell on tue 23 may 00
I've been preoccupied with my gardening of late, so I've not chipped in on
this thread before now. Marcia, I would be suspicious that your Mt. Saint
Helens Ash sample has somehow become contaminated if it is coming out with a
dark blue tinge. In twenty years, things do happen. I do concur with Paul
Lewing's information on the geographical variation in samples tested around
the Northwest at the time of the erruption. My sources were somewhat
different, but they also showed that the material was pretty consistent
throughout the range where it was deposited. There was just a small dropoff in
iron content with distance from the volcano as the most significant systematic
geographic variation.
Jeff Lawrence's work with volcanic deposits from the Jemez Mountains in New
Mexico, and the analysis he has shared here, shows how different a material
can come from such a similar (also volcanic) source. We potters should be
careful then to be specific as to source when offering or requesting
information regarding volcanic ash.
In this vein, I'd like to ask Frank Martin to revise the glaze formulas he
just posted for "volcanic ash" glazes by specifying which erruption from which
volcano, if he knows it. If not, then at least the spot from which it was
recovered, or the supplier from whom it was purchased would help clarify his
meaning. To say "Volcanic Ash" in a glaze formula is only slightly more
informative than saying "Powdered Rock".
Michael McDowell
Whatcom County, WA USA
mmpots@memes.com
http://www2.memes.com/mmpots
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