CASTAWAY WORKSHOP TOOL

Jesmonite Calculator

Plan your cast — from mould cavity to mix

Mode

Plain is a single-mix pour. Terrazzo adds pre-cured Jesmonite chips for a speckled aesthetic — the calculator works out chip weight and reduces the base mix to account for displacement.

Cavity

Cavity volume

ml

From Fusion 360, or water-fill the cured mould once demoulded. This is the volume of the finished cast piece.

Waste margin 10%

Covers pot residue, mixing losses and a clean pour. 8–12% is typical for Jesmonite — higher than silicone because the mix thickens fast.

Material Cost (Optional)

Your Jesmonite cost per kilogram

£ / kg

Enter the price you pay per kg of Jesmonite (combined kit weight including base and liquid). Leave blank if you only need the mix quantities. Cost works out at total kit weight × your price per kg.

Mix schedule

Total wet mix —g

Cavity volume —ml

Base mix needed —ml

Waste allowance —ml

Base mix — 2.5 : 1 by weight

Base powder —g

Liquid —g

How to. Mix base and liquid at 2.5:1 by weight. Add pigment (up to 2% of total mix weight) into the liquid before blending if you want colour. Pour within 8–12 minutes — working time shortens at warmer temperatures. Demould from around 30 minutes once firm; full cure in 24–48 hours.

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Jesmonite Calculator

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