There’s another thread on this forum regarding plaster molds– https://fusedglass.wpengine.com/node/291. I can only add that it might be better to make a positive from the mold and recast the positive in clay which would then become your casting mold.
I was given boxes of plaster molds that I essentially use for molding glass using cmc as the binder or making positives and then pouring my own molds for pate de verre. The one plaster mold I used died in trying to cast glass. Kiln wash and nichrome wire didn’t help. The result looked like a cross between the creature of the black lagoon crossed with the blob.