Alan Wainright on thu 18 jul 96
Carrie Hawk wrote:
>This is something I've been wondering about. When I first started making
>molds, I bought plaster of paris from a local hardware store. I worked just
>fine. Then I took a workshop and was told to use Potter's Plaster.
>
>After paying out the nose for "Potter's Plaster" I am wondering, is there
>any difference? To me plaster is plaster. How unique can it be? I know there
>is such a thing as hydrocal, and I am well aware of those differences, but
>honestly I'm wondering if Potter's Plaster isn't just another marketing tool
>to separate me from my very slim dollars.
>
Dear Carrie:
There is a difference between the various kinds of plaster. While
they're all basically made of gypsum, they have different
additives to increase or decrease the setup time, and different
working characteristics.
I don't know what the characteristics of the basic Plaster of
Paris might be, but according to the literature I have, United
States Gypsum's #1 Pottery Plaster sets in 27-37 minutes, and has
a PSI strength of 2,000. Hydrocal, which you referred to, sets in
25-35 minutes and has a PSI strength of 5,000. (I assume this
refers to pressure needed to break it, once set.) Hydrostone is
the strongest plaster Georgie's carries, with a PSI of 10,000
(recommended for making press molds, because of its strength).
Our literature on the Pottery Plaster says that it makes a
stronger, more durable mold than regular plaster (which I assume
to be Plaster of Paris).
At Georgie's, we carry plaster molds made by over 30 different
companies around the country. We can tell a real difference
between the molds made with a stronger plaster and the others. In
general, the ones made with the stronger stuff last longer and
hold their detail better & longer, for a longer mold life. This
can be really important, when some of them are over $100 each.
Hope this answers your question --
Alan
georgies@worldnet.att.net
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