Stephani Stephenson on sat 20 apr 02
Carol
in addition to Roger's tip regarding 'old' plaster...
how long ago did you make these 'new ' molds?
do they still feel slightly damp and heavy?
Though the plaster will cure in 24 hours
Sometimes , I have noticed, it takes a few weeks for water to evaporate
from a new mold .
after a week I can still put a fingernail scratch into a mold
after two or three weeks I cannot
Relative humidity in your area will make a difference,
(though this may not be too much of a factor for you, in an arid
climate)
Awhile back I ran out of pottery plaster #1 and substituted some
regular plaster from the hardware store for a small mold.
It was noticeably softer and inferior for my purposes.
Hydrocal will make a stronger mold, and one which 'feels' stronger and
less prone to scratching.
Pottery plaster #1 still seems soft in comparison to hydrocal, but it is
entirely durable for most press molding and slip casting . I use a
consistency of 65 with Pottery plaster and recommend Frith's book 'Mold
making for Ceramics' as a guide.
Of course I have it easy, no new unknown types of gypsum....
good luck with your experiments, but if you just made your new molds
yesterday, give them a couple of weeks. Otherwise I would suspect old
plaster (were there any hard kernels in the dry plaster or when you
added water?), or a lower grade of plaster.
best wishes
Stephani Stephenson
steph@alchemiestudio.com
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