Plaster Mold Mixing Calculator

How it works

Slurry volume per 100 g plaster ≈ 38.5 cm³ (plaster at 2.6 g/cm³) + consistency (mL water); plaster (g) = volume ÷ (38.5 + consistency) × 100; water = plaster × consistency ÷ 100

Plaster is mixed by weight to a “consistency” — the parts of water per 100 parts of dry plaster — and that ratio, not the volume, determines the mold’s hardness and absorbency: 70 consistency pottery plaster is soft and thirsty, ideal for slip casting, while Hydrocal at 45 is dense and hard for master molds. Guessing the amount wastes plaster or, worse, leaves a mold short and stiffening in the bucket, so the volume to fill is calculated from the cottle and the model inside it, and the slurry volume per 100 g of plaster (about 38.5 cm³ of solids plus the water) converts that to weights. Add 10% for what stays on the bucket and stick. Always add plaster to water, never the reverse, let it slake, and mix gently to avoid air bubbles that become pinholes on the casting surface.

Worked example

A 25 × 20 cm cottle filled 8 cm deep is 4,000 cm³; subtracting an 800 cm³ model and adding 10% gives 3,520 cm³ to fill. At 70 consistency that needs about 3,240 g of No. 1 Pottery Plaster and 2,270 mL of water — 5.5 kg of slurry in an 8 L bucket, setting into a mold of roughly 4.8 kg.

Frequently asked questions