What is plaster of paris?

What is the difference between drywall plaster and plaster of Paris?

• The reason I ask is I would like to try my hand at carving in plaster, but when I tried to make a block out of Plaster of Paris I couldn't get the center to dry, no matter how long I waited. So, How can I make a block of plaster than can be carved, and is there a difference between drywall plaster and Plaster of Paris? Should I try using drywall plaster?

• Answer:

  Although Drywall plaster is mostly comprised of semihydrated calcium sulphate (plaster of paris) it has a lot of fiberous binders and fire retardant chemicals mixed in as well as a "foaming agent" which makes it quite crumbly when the supporting paper is removed. Don't try to carve it, it won't work well. The reason that the center of your block did not dry is that you mixed the plaster incorrectly, it sounds like your plaster to water ratio was too low and you may have not have agitated it enough. properly mixed plaster will form a block of even hardness, no problem.