Bobbruch1@AOL.COM on mon 18 nov 02
<<recipes, to help suppress the effect. (People who mix their own clay using
water high in soluble minerals also use this method.) This is probably the
most common means of combating scummimg. It works pretty well (though I'm not
sure of the actual chemical process), but it's not 100% effective.
-Snail
Are you suggesting that clay manufacturers add barium to stoneware clays, or
only to earthenware clays? Is the addition of barium in a clay body is
listed anywhere on the box? Anyone know if there are laws requiring the
labeling of barium or any other toxic agents? I think that some clays contain
manganese which people ought to know about in order to assure proper handling
of the materials.
Bob Bruch
Snail Scott on mon 18 nov 02
At 06:27 AM 11/18/02 EST, Bob wrote:
><<>recipes, to help suppress the effect...
>
>Are you suggesting that clay manufacturers add barium to stoneware clays, or
>only to earthenware clays? Is the addition of barium in a clay body is
>listed anywhere on the box? Anyone know if there are laws requiring the
>labeling of barium or any other toxic agents? I think that some clays contain
>manganese which people ought to know about in order to assure proper handling
>of the materials.
>
Typically, it's only added to red earthenware, since
the effects of scumming are most visible on such
clays. The amount added is tiny, though, less than 1%.
Not enough to have any toxic effects, even if you did
ingest some of it. I don't worry about handling or
firing it. (Though if you were inclined to try one of
those clay-eating diet regimens, you might be best to
'mix your own'.) ;)
As for the manganese-bearing bodies: the toxicity
from manganese is primarily due to fumes, and NO
kiln fumes are good, so I don't really feel that
such clays need special labeling.
-Snail
Ron Roy on fri 22 nov 02
Hi Bob,
Far be it from me to be avocating a soft attitude to Barium Carb. but - the
amounts are small - 2lb in 1000 lb. dry mix - and during the process of
tying up the salts it become insoluble. The danger could be that too much
is used.
I would recommend anyone using Barium to stop scumming simply use as little
as is needed. Start with the miniumum and add a bit if the scumming is
still a problem. Unfortunately it is the only material I know of that
works.
It is a little more complicated than I have outlined - if you need to know
more Hamer has a more complete explanation. He recommends 1 to 2% Barium
Carb. but much depends on how much soluble calcium is present in the clay.
None of the other Barium materials works by the way - I have never seen
anything else recommended.
RR
><<>recipes, to help suppress the effect. (People who mix their own clay using
>water high in soluble minerals also use this method.) This is probably the
>most common means of combating scummimg. It works pretty well (though I'm not
>sure of the actual chemical process), but it's not 100% effective.
> -Snail
>
>Are you suggesting that clay manufacturers add barium to stoneware clays, or
>only to earthenware clays? Is the addition of barium in a clay body is
>listed anywhere on the box? Anyone know if there are laws requiring the
>labeling of barium or any other toxic agents? I think that some clays contain
>manganese which people ought to know about in order to assure proper handling
>of the materials.
>
>Bob Bruch
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Jim Murphy on fri 22 nov 02
I don't know how Hamer explains it, however, the action of Barium Carbonate
in casting slip is explained by Robert Tichane in his book "Clay Bodies"
(Pub. 1990) as follows:
"The addition of BaCO3 will solve the problem of too much CaSO4 (Calcium
Sulfate) in H2O because of the fact that BaSO4 is only 1/10 as soluble as
BaCO3. When the amount of CaSO4 is known, the addition of an equivalent
amount of powdered BaCO3 will convert soluble CaSO4 to insoluble BaSO4 and
insoluble CaCO3.
CaSO4 + BaCO3 = BaSO4 + CaCO3
Molecular weight of CaSO4 is 136 and BaCO3, 197, hence about 10 parts BaCO3
will neutralize the effect of about 7 parts CaSO4. For example, at 200 ppm
of CaSO4 in water, one liter would contain 0.2 grams. Thus 0.3 grams BaCO3
per liter or 1.2 grams per gallon would neutralize the effect of this amount
of CaSO4."
Hope this helps.
Jim Murphy
Ron Roy on tue 26 nov 02
Hi Jim,
I think this does help but there are also salts associated with many clays
- sometimes it is constant and sometimes it comes and goes.
Another reason to test raw clays before using them.
It is sometimes quite spectacular - one set of bars heavy with bloom or
yellow stain and the next set clear.
I'm thinking the salts in water vary from season to season as well.
How do we find out how much CaSo2 is present? I suspect there are other
salts present as well because the colours vary quite a bit - disolved iron
will come to the surface - seems like another good reason to use a variety
of clays in a body to cut down the chances of salts being a problem from
time to time.
RR
>I don't know how Hamer explains it, however, the action of Barium Carbonate
>in casting slip is explained by Robert Tichane in his book "Clay Bodies"
>(Pub. 1990) as follows:
>
>"The addition of BaCO3 will solve the problem of too much CaSO4 (Calcium
>Sulfate) in H2O because of the fact that BaSO4 is only 1/10 as soluble as
>BaCO3. When the amount of CaSO4 is known, the addition of an equivalent
>amount of powdered BaCO3 will convert soluble CaSO4 to insoluble BaSO4 and
>insoluble CaCO3.
>
> CaSO4 + BaCO3 = BaSO4 + CaCO3
>
>Molecular weight of CaSO4 is 136 and BaCO3, 197, hence about 10 parts BaCO3
>will neutralize the effect of about 7 parts CaSO4. For example, at 200 ppm
>of CaSO4 in water, one liter would contain 0.2 grams. Thus 0.3 grams BaCO3
>per liter or 1.2 grams per gallon would neutralize the effect of this amount
>of CaSO4."
>Hope this helps.
>Jim Murphy
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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