Jeff Lawrence on tue 9 dec 97
Hello all,
The slippery topic of plaster mold release incited a private exchange which
might be of interest to some. Following are a few pointers on working with
plaster that I've learned. I'm sure other ways work, and I'm also sure that
much of this has been said by the more experienced mold-makers in our
group. These work best for me.
> I have discovered a lot
>of ways not to do things; I stewed about them all the time I spend redoing!
>So I hesitate to sermonize (simonize?) to the group on a topic where I still
>have so much to learn.
>
>Between us, though, there's not much too it. Apply 2-3 coats of mold soap to
>your model in the following manner:
>Brush or sponge it on, let it dry for a few minutes until it is starting to
>dry in places, then wipe it all off with a paper towel, until there are no
>gobs or wet streaks remaining. The drying seems to leave a deposit, which
>you want. Let the coat dry a few (maybe 5-10) minutes, then apply another
>coat, repeating the wiping. The purpose here is to build up a thin even coat
>of the soapy residue that still holds a little moisture.
>
>After 2-3 coats, you should be able to polish the surface with your paper
>towel. If it doesn't get shiny, let it dry for a few minutes and buff it
>again. High rpm, low pressure works best for me. You can test your surface
>with a spray bottle of water -- a spritz will bead up and run off if you've
>got it right. Wipe the water off right away after the test, of course.
>
>The soap will seal the surface for as long as it has some moisture (the mold
>soap I use comes off in big dry curly flakes after a few months). However,
>you can (and should) pour your plaster as soon as you get that shine,
>because as it dries it becomes less effective.
>
>There is a useful comparison between this soap sheen and the shine of
>burnished clay. I'm sure you've noticed how you can get a mirror finish on
>fresh slip, which dulls as the surface drys. Similarly here, except that the
>soap layer takes a shine easier and takes longer to lose moisture (and
shine).
>
>About when to unmold, I only have my experience to fall back on. When I've
>tried to unmold while the plaster is still warm, two problems arise. For
>one, the plaster is still weak and will often break. The other factor is
>that it is in a state of expansion and will key on to the model more
>tenaciously. So the luckless moldmaker in a hurry is trying to remove a weak
>part that holds on tight. Bad combo!
>
>The best time I've found to unmold is right after the fresh plaster has
>cooled. At this point, it seems strong enough to stay together even under
>pressure (some grunting always seems necessary). Waiting longer should give
>the plaster more strenght, but I've found it best to proceed anyway. More
>important than the slight added strength is that at this point there is a
>water slick between the model and the fresh plaster piece. This water has no
>attachment to either side, and lets them apart easily.
>
>Whenever I let the plaster sit over night, small pieces of the new part will
>react with the underlying plaster of the model, even through the mold soap.
>I attribute this to the "sweat" of the plaster dissolving the shiny seal.
>The water dissolves the soap and both plasters drink of the water. Thrown
>into intimate contact, the young plaster commingles with the old. A tender
>analogy, perhaps, but not so hot for the final mold result.
>
>If you think the above is of general interest, let me know and I'll post it.
>
>BTW A key is the right soap. Next time you have a mold project, try to get
>a hold of Crystal Soap Co's soap.
Call them at 800-525-3842 x 222 to talk to Anna Marie. Unfortunately, they
only sell the concentrate in 40lb pails at $1.35 per pound (UPS C.O.D.) or
325 lb drums at $1.23/lb. (prepay, frt collect). Both fob Lansdale PA, USA.
Get plastered!
Jeff
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