Sil Horwitz (silh@iag.net)
Fri, 12 Mar 1999 15:51:58 -0500
At 99/03/12 12:00 PM -0700, Dick Sullivan wrote:
>My first guess is that plaster is not the medium as it is chemically
>reactive and is going to cause problems. It is an alkaline surface and as
>such is going to play hell with any chemically active emulsion that you put
>on it.
Depends on the "plaster." You are thinking of the calcium carbonate/cement
mixture used for walls and stucco, etc. Plaster used for molding is "Plaster of
Paris" which is calcium sulfate; it is not alkaline, though the impure mixture
sold in art and hardware stores may have something in it that "who knows?"
Calcined calcium sulfate (Plaster of Paris) is a very inert substance which has
a great affinity for water; the resulting plastic mass soon hardens to fit the
mold. Since the sulfate ion is more reactive than the calcium ion, the
substance would be acidic if it were soluble, which it is not.
The problem might lie in the unknown nature of the commercial plaster of Paris.
It is possible that it contains some lime (which is alkaline) to provide
greater strength, as pure Plaster of Paris is not known for its strength. It
might be possible to give the "plaster" an acid bath (say in vinegar or
hardware store muriatic acid sold for pool use) to neutralize the alkalinity.
Don't blame me if it blows up during this treatment (it won't - might fizz a
bit strongly) as I have no idea what each manufacturer puts in his commercial
Plaster of Paris. When you use materials outside of what they are designed for,
you must find out what's in them and then experiment, experiment!
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net
Visit http://www.psa-photo.org/
Personal page: http://www.iag.net/~silh/
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