Scott North on fri 25 feb 00
Hello,
A student here is concerned about the safety (possible leaching of
manganese) of the following glaze that he would like to use on functional
ware. He is aware of the hazards of working with manganese dioxide, but
here is interested in the safety of the ultimate user of the pottery. The
following glaze is fired to cone 10, reduction.
Dolomite 24.3
Magnesium carbonate 2.9
Soft wood ash 48.5
EPK 14.6
Silica 9.7
______
100.0
Manganese diioxide 4.0
Any helpful information about the safety of this glaze would be greatly
appreciated. Thanks. Scott
Scott C. North
Geology Department
Humboldt State University
Arcata, CA 95521
phone: (707) 826-3210
FAX: (707) 826-5241
Craig Martell on sat 26 feb 00
Scott sez:
> A student here is concerned about the safety (possible leaching of
>manganese) of the following glaze that he would like to use on functional
>ware.
>
>Dolomite 24.3
>Magnesium carbonate 2.9
>Soft wood ash 48.5
>EPK 14.6
>Silica 9.7
> ______
> 100.0
>Manganese diioxide 4.0
Hi:
No real way to tell if this glaze is OK without an analysis of the wood
ash. This glaze is probably too low in silica to be durable and hard for
functional use. The absense of feldspar reduces the amount of silica in
the glaze. Wood ash is usually very high in CaO and low in silica unless
it is a bush ash, which will sometimes be more silica rich. The only sure
sources of silica in this glaze are the epk and silica which are too
low. Even with an ash analysis so one could calculate the seger formula of
this glaze, it should be tested in a credible lab such as Alfred for acid
leaching. I'd put my money on failure for this one and if it were me, I
wouldn't use it for domestic ware.
regards, Craig Martell in Oregon
John Hesselberth on sat 26 feb 00
Scott North wrote:
>----------------------------Original message----------------------------
>Hello,
> A student here is concerned about the safety (possible leaching of
>manganese) of the following glaze that he would like to use on functional
>ware. He is aware of the hazards of working with manganese dioxide, but
>here is interested in the safety of the ultimate user of the pottery. The
>following glaze is fired to cone 10, reduction.
>
>Dolomite 24.3
>Magnesium carbonate 2.9
>Soft wood ash 48.5
>EPK 14.6
>Silica 9.7
> ______
> 100.0
>Manganese diioxide 4.0
>
>Any helpful information about the safety of this glaze would be greatly
>appreciated. Thanks. Scott
>
>Scott C. North
>Geology Department
>Humboldt State University
>Arcata, CA 95521
>phone: (707) 826-3210
>FAX: (707) 826-5241
Hello Scott,
While the only way to know how much manganese will leach from this glaze
is to have it professionally tested, I would predict this glaze is awful!
Testing can be done for $20--it's not very expensive. Specific
instructions are given on muy web site at:
http://www.frogpondpottery.com/glazetest.html
Let me expand on my prediction that this glaze would leach terribly. It
has almost no silica or alumina unless more is contained in the wood ash
than my analysis would predict. The glaze is way outside traditional
limit formulas where "good glass" is formed. I'd bet this glaze is so
bad that a simple overnight soak in vinegar will take most of the color
(manganese) right out of it. Please don't use this anywhere near food
without testing it. On the other hand, if you do test it please share
the results with this group. We are trying to learn how to predict
leaching and we are at the very early stages of understanding it.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
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