Dawn Z
The plaster you are using might be low-grade- Try using plaster of paris. Also, when you are mixing the plaster?; Start with a bucket of water, then add plaster till you get a nice size island. Mix well, even using your hand as a mixer helps. Then tap your bucket around untill all of the air bubbles are released. When you are ready to pour, make sure the object you are casting is well supported, and has plenty of room. After you pour, tap the sides to get any air pockets out. When your object has set for 24 hours, pull out the clay. (Thow the plaster covered clay away! Or it will explode if you fire it.) Finally use a knife to bevel any sharp edges, and file down any undercuts.

tigris
there is nothing really which is better for the main reason that the plaster absorbs water from your clay which greatly speeds up the time it takes for the clay slip to become firm enough to remove from your mold. You can try using hydrocal instead of plaster, it's a harder and higher quality version of plaster.

plainrsc
I am using silicone from silicone caulking to do press molds for clay. I put thin layers on and let each layer set before putting on a new layer. There are disadvantages of using silicone, The silicone mold will not absorb any moisture from the clay; the plaster molds will absorb moisture from the clay and release the clay. When using silicone the moisture is only released from the exposed side. If using to large a silicone mold the clay will crack when it shrinks.

dragonlady
plaster is your best bet because it absorbs the water from the clay. you may be making the walls of your mold too thin. Also, instead of using POP, use Hydrocal 30 (from a builders supply house) or put some strips of cheesecloth or plaster bandage in the backsides of your molds.

overglazed
You can use regular clay instead of plaster to make molds. The drawback is there is shrinkage after you bisque fire your mold - so it's not an exact sized mold. Bisque used as a mold will also absorb moisture from the clay - but if you fire the bisque to vitrification hardness there will be hardly any absorbtion. I have made many molds this way because they are lighter than plaster and I don't like to run the risk of getting plaster around my clay - accidents do happen. If you can calculate the shrinkage rate into your original item you make (it's about 6% on my clay), you can come very close to the end results you want. Just be sure to use good mold release - I use Murphy's Oil Soap with good results. Good luck!

mike1942f
Contact the nearest pottery/ceramics supply place, like Trinity Ceramics near Dallas or Seattle Pottery Supply and ask about the plasters they can provide - there a several variations which are harder, set slower/faster, etc. Trinity's catalog, in front of me lists 9 plasters and refers to the maker, U.S.Gypsum and www.gypsumsolutions.com