Does Plaster of Paris shrink?

Does Plaster of Paris shrink as it dries? I would like to make a mask out of it but I don't wan't it to shrink and crack.
Gratias agimus tibi,
josh1324

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Burf6 years ago
No, it doesn't shrink. That's why it is used for things like paster cast impressions by police departments, and molds for casting gold and silver jewelry, etc.
Well, it does shrink a bit - try doing thick sections.

Steve
Its been a long time since I studied this, but as I recall, basic plaster of paris, without additives, actually expands slightly as it is curing. In the neighborhood of 0.3% to 0.5 %, nominal, if memory is correct. I don't recall the figures, but thermal expansion and contraction is also relatively small.
True, buts it shrinks very slightly in storage, so that dental impressions can be problematic
I am not sure, in the sense of very very fine tolerances.

Without going all engineering on the subject, chemistry, hydration, molecular combinations, free water etc......

I would assume that as another has alluded too, dental pattern castings in plaster, will dry out and shrink - and almost absolute accuracy in dentistry is necessary...

But for general purposes, and "regular objects" like statues, masks etc., and other things, the general impression is that the shrinkage is so small, that it's not worth considering.

How about you cast up an exact ONE meter length of it, from a precisely measured mix, and measure it as it dries over say one year...in an atmosphere of 10% moisture at 25*C - and then come back here and tell us all exactly what you have found out.

Or as it's easy to actually cast, line a 5 meter length of PVC pipe with some kind of wax paper, and cast up a 5 meter rod of plaster and measure that?

Cut the pipe in half - length ways and then stick it together with tape and rubber bands and then fill it with plaster of Paris and then open up the pipe once it has set.

A 3% shrinkage rate would come to 9mm...
rickharris6 years ago
It will do what you want - DO NOT totally encapsulate any body past in Plaster of Paris.

you may find making a mould with alginate works better than a direct plaster cast,
Plaster will not shrink as it dries, therefore, it can be used to create a court-admissible cast as well as a reference which can be used in an investigation.