Lewis on sat 18 jul 98
The following is paraphrased from The Potter`s Book, B. Leach:
Old slipware glazes containing lead sulfide were prone to sulfuration--the
sulfur in the glaze would turn into sulfuric acid, which in turn combined
with
the soluble salts present and formed sulfates. This was especially likely
if the fuel, kiln, or ware was damp, and if reduction occured before the
glaze melted. Sulfuration leaves the glaze matt, rough, yellowish green,
and non-transparent.
Leach goes on to write that they eventually switched to litharge, which is
free of sulfur.
At earthenware temperatures, sulfates can be left suspended in the glaze,
appearing as opaque whitish spots. As I understand it, sulfuric acid fumes
can combine with calcium in the clay and cause scumming or spotting in
oxidation firings at any temperature.
You could try washing the tailings in the same manner that wood ash is
washed, which could possibly reduce the sulfur content. If it were up to
me, however, I would avoid this material. I`m not sure what sort of ware
you`re making, but for high fire glazes there are many other rocks, clays,
and ashes available that are free from sulfur and heavy metals. Let me know
if you want more info on homemade glazes.
Good luck,
Colin Lewis
mc.lewis@swipnet.se
>Date: Fri, 17 Jul 1998 13:43:40 EDT
>From: Earl Brunner
>Subject: Manganese processing byproduct
>
>----------------------------Original message----------------------------
>We have access to the tailings of a production facility that extracts
>manganese dioxide from ore that is shipped from Africa. The tailings have the
>following rough composition:
>SiO2 15-40%
>CaSo4-@H2O 15-40%
>CaCO3 15-40%
>FeSO4 5-20%
>
>lesser elements:
>Al 8%
>Ba .7%
>Co (cobalt) .8
>Cu .2
>K 2%
>Mg .1%
>
>and on down the list of Ceramic glaze chemicals. My question is, There is a
>fair amount of sulfur in this mix, but almost everthing else is desirable.
>What is the problem with sulfur? I have looked in all my books, and with the
>exception of a couple of negative, but not specific, references nothing is
>said. Is this stuff usable?
>Specifically, what is the problem with sulfur in glaze composition? (there
>are tons of this stuff) Earl Brunner
Monona Rossol on tue 21 jul 98
----------------------------------------
Colin Lewis wrote:
The following is paraphrased from The Potter`s Book, B. Leach:
> SNIP Sulfuration leaves the glaze matt, rough, yellowish green,
> and non-transparent. <
This is because they sulfur compounds/acids at high temps can dissolve and
etch the glaze.
> Leach goes on to write that they eventually switched to litharge, which is
> free of sulfur. <
Cute. I'll take the sulfur.
> At earthenware temperatures, sulfates can be left suspended in the glaze,
> appearing as opaque whitish spots. As I understand it, sulfuric acid fumes
> can combine with calcium in the clay and cause scumming or spotting in
> oxidation firings at any temperature. <
That, too.
> You could try washing the tailings in the same manner that wood ash is
> washed, which could possibly reduce the sulfur content. <
Most of the sulfates are pretty insoluble and wont respond to washing. One
method of removing sulfates from minerals is to fire them to the point that
the sulfates decompose and then repulverize the material.
> If it were up to me, however, I would avoid this material. <
Good thinking.
Monona Rossol, industrial hygienist
Arts, Crafts and Theater Safety
181 Thompson St., # 23
New York NY 10012-2586 212/777-0062
http://www.caseweb.com/acts/
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