search  current discussion  categories  materials - plaster 

plaster won't cure properly

updated sun 21 apr 02

 

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