From what is plaster made of?

On Tue, 29 Jul 2003 11:45:17 +0300, "A." wrote:

Hello, people!

What actually is that material called plaster, from what moulds are made? I
have heard somebody doing moulds from gypsum, but is it the same material?
( I' am from the other edge of the world , Estonia, Europe ). What else
except water do I have to add to that material if making a mould?
And about cones - probably there is no ceramic suppliers selling them in our
country. Is it possible to make cones by itself or are they strictly
industrial merchandise?

Have a nice summer,
greetings


Plaster is gypsum that has been calcined to remove some of the
"chemical" (bound) water. There are different states of hydration,
and the one you need for plaster is obtained by roasting the
gypsum for many hours at low temperatures, around 300F or so.
If the temperature is too high, over 350F or so, the gypsum could
go to another hydration state such that it wouldn't be set up
when you tried to use it as plaster.

Water is the only thing you need to add to plaster to make a mold.
For best results with pottery or molding plaster, you would add
70 grams of water for every 100 grams of plaster. Use cold water;
hot water may speed up the set too much. Sprinkle the plaster
into the water slowly. You can get decent results without weighing
if you just sprinkle the plaster until it fills the container to the
surface of the water, and then starts to stay on the surface, but
weighing is better. Many instructions recommend that you then
let it sit (slake) for a couple of minutes before you stir it, but I
don't think it's really needed. They also recommend letting the
stirred plaster sit until it's just about ready to set up before you
pour it, but again I don't think that's needed either. Try to avoid
trapping air bubbles when you pour, and after you pour you should
hit the container sides to dislodge any trapped bubbles. Then just
let it sit. It will get warm as the reaction takes place. You can
test the surface by scraping with a nail or something to see how
the setting is coming along. Be patient!

Hope this helps.








Bob Masta
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