#1 Pottery Plaster is the standard for making molds, sculptures or bats. Hydrocal White & Hydrastone are good for sculpture, Hydrastone is suitable for outdoors.
Always mix plaster into water. Sets in 20-30 min. Use the plaster mixing ratio, below.
100 lbs #1 plaster: 70 lbs. water
100 lbs Hydracal White: 45 lbs. water
100 lbs Hydrastone: 32 lbs. water