Jonathan Kaplan on fri 19 jul 96
There are some basics here that are important.
For ceramic work, there are basically two types of plasters: obviously
Plaster, and gypsum(plaster) cements.
For general mold work, working molds, wastes, spares, etc. USG Pottery #1
is the all around first rate material. Mix at a 70 consistency. Jiggering
requires a plaster with perhaps a bit more surface hardening additives, so
use USG's "Puritan" plaster. If you carve or otherwise shape plaster, their
"Moulding Plaster" is fine. All plasters work at a determined consistency,
say 70, 0r 37, etc..... means 100 pounds of plaster is set up to the proper
consistency by xx amount of water. Since most plasters come in 100 pound
bags, the ratio is by pounds....eg for pottery#1 plaster from USG, 70
pounds of water are necessary to set up 100 pounds of plaster. While it is
true that any amound of water can set up a given quantity of plaster, the
optimum hardness, absorbency, etc. are best achieved with the proper water
to plaster ratio. Weigh the plaster, weigh the water, add the plaster to
the water (always) soak for 2-3 minutes, mix for 2-3 minutes, pour. These
times vary.
Gypsum cements are another material altogether. They are designed for
optimum surface hardening, dimensional stability, and very low expansion.
Ultracal 30 (consistency 37) is my all around choice for case molds.
Hydrostone is also a wonderful material for this. Ceramical is the only
material for press dies.
Contact US Gypsum in Chicago 125 South Frankiln St Chicago, Illinois
60606 (800) 487 4431 or FAX (312) 606-4519. More materials then you
could possibly use, but their volume mix calculator is the de facto
measuring device in my mold room.
WHile I think most potters have a "fear of plaster" once the proper
education is provided for this material, it is indeed one of the potters
best friends.
Jonathan
Jonathan Kaplan
Ceramic Design GroupLtd./Production Services
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139 voice and fax
call before faxing
"Show up, pay attention, tell the truth, don't be attatched to the results!"
Katy Sheridan on sat 20 jul 96
I had the pleasure of mixing #1 potters plaster using the calculator
supplied by U S Gypsum, for filling some metal trays to be used in our damp
room. The amts came out exactly right each time filling the mole right to
the rim. I will never be without it! There was no waste.
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