Plaster mix calculator
Measure your mould, pick your plaster and get the plaster and water to weigh out, from the mixing ratio the maker publishes for it.
Never cast a hand in plaster
- Never make a cast that encloses a hand, foot or any other part of the body. Plaster gets hot as it sets and can cause severe burns that may need surgery.
- Plaster dust can irritate the eyes, skin, nose and throat: mix in a well-ventilated place and wear eye protection.
- Keep plaster and wet mix away from children.
- If plaster gets in your eyes, rinse them with water for 15 minutes; wash it off skin with plenty of water.
From the cautions on USG's plaster data sheets and DAP's safety data sheet for its hobby and craft plaster of Paris; see Sources below.
Mixing and setting
Weigh both the plaster and the water: the ratio decides how strong and absorbent the cast will be. USG asks for drinking-quality water between 21 and 38 °C (70 and 100 °F). Sift or scatter the plaster onto the water slowly and evenly, let it soak until it is wet through, then mix.
USG gives a set time of 14 to 24 minutes for both plasters here: machine-mixed for Pottery Plaster, which takes longer mixed by hand, and hand-mixed for Casting Plaster – 20 Minute. Dry finished casts to a constant weight; USG says pottery plaster moulds are safe from damage up to 49 °C (120 °F).
How the amounts are worked out
USG publishes a wet density of 1,567 kg/m³ for its Pottery Plaster mixed at 65 parts water. From that, 100 g of plaster takes up about 40.3 ml of the slurry and each gram of water 1 ml, so every consistency gives its own split of plaster and water.
Questions
What does a consistency of 65 mean?
65 parts of water to 100 parts of plaster, by weight: 650 g of water for every kilogram of plaster.
Why did my plaster set too fast?
Warm water and long mixing both shorten the set time. Use cooler water within USG's range and mix only until smooth.
Can I measure the plaster in cups?
USG asks for both to be weighed for every mix, because the ratio of water to plaster decides how strong and absorbent the cast will be.
Sources
- USG, USG Pottery Plaster submittal sheet (IG1365): normal consistency 65; machine-mix Vicat set 14–24 minutes; wet density 1,567 kg/m³; water at 21–38 °C; weighing, soaking, mixing and drying; safe from calcination up to 49 °C; caution against casting any part of the body.
- USG, USG Casting Plaster – 20 Minute submittal sheet (IG2004): consistency 63 to 73; hand-mix Vicat set 14–24 minutes; the same caution.
- DAP Products Inc., Plaster of Paris (hobby and craft) safety data sheet (revised 8 May 2015): do not cast any part of the body; failure may cause severe burns that may require surgical removal of affected tissue.
- National Institute of Standards and Technology, Guide to the SI, Appendix B.9: conversion factors for inches, US fluid ounces and ounces.