Plaster Mixing Calculator

It depends on the plaster. Plaster of Paris uses roughly 70 parts water to 100 parts plaster (0.70:1 by weight). Harder gypsum plasters use less water: Crystacal R is 0.35:1 and Herculite No.2 is 0.42:1 (Saint-Gobain Formula datasheets), and Type IV dental stone is around 0.20:1. Less water gives a harder, denser, less porous cast; more water gives a softer, more porous one.

No. Jesmonite AC100 is a two-part acrylic-modified composite, not a water-gauged plaster. You mix 2.5 parts Base powder to 1 part AC100 Liquid (acrylic) by weight — never with plain water, which would give a weak, mis-cured cast. Add the powder to the liquid and mix until smooth. This calculator switches to the AC100 Base/Liquid split automatically when you select Jesmonite.

For gypsum plasters, always add plaster to water — sift the plaster into the water and let it soak (slake) for 1-2 minutes before mixing. This reduces air bubbles and gives a smoother, stronger cast. (Jesmonite is the reverse: powder is added into the liquid.)

Mix gently — avoid whipping or fast stirring. After mixing, tap the container on the table to release bubbles. When pouring, pour in a thin stream from one corner and let it flow across the mould. Vibrate the mould gently after pouring.

After mixing, plaster goes through stages: thin (like cream), then thicker (like yoghurt), then sets. Pour at the cream stage for detail work, or at the yoghurt stage for quicker setting. If it starts to feel warm, it is about to set — pour immediately.