Plaster for a mold: exact plaster and water quantities
Mix too little and the pour stops mid-mold; mix too much and it sets in the bottom of a bucket, a lost block. This tool converts your cottle’s volume into grams of plaster and litres of water, at your chosen consistency — one mix, the right amount.
Plaster heats as it sets: never cast plaster around a body part — serious burns are documented. Wear a dust mask while mixing (silica dust) and work in a ventilated space.
Plaster to weigh
10.02lb
- Water to measure
- 0.88gal
- Volume to fill
- 1.32gal
Plaster heats as it sets: never cast plaster around a body part — serious burns are documented. Wear a dust mask while mixing (silica dust) and work in a ventilated space.
The method
The volume to fill is the cottle’s minus the model’s. The question then is: how much material fills that volume?
The answer rests on a physical constant: 100 g of plaster powder occupies about 37 cm³ once wetted (gypsum density ≈ 2.7). Add the mixing water — c grams per 100 g of plaster, the “consistency” — and the mix makes (37 + c) cm³ per 100 g of powder. All that remains is a division.
At the pottery standard consistency (73), one litre of mold therefore takes about 910 g of plaster and 0.66 L of water. A stiffer mix (65) needs more powder; a softer one (80) needs less but yields a weaker, more absorbent mold.
In the formula: V the volume to fill (L), c the consistency (g of water / 100 g of plaster).
plaster (kg) = V × 100 / (37 + c) · water (L) = plaster × c / 100
Good to know
- Allow 5–10% more plaster than calculated: bucket-wall losses, pouring generosity — better to discard a little slurry than to re-pour a mold.
- Always sift the plaster ONTO the water (never the reverse), let it slake 2–3 minutes, then mix: that is the key to a lump-free, bubble-free batch.
- NEVER pour leftover plaster down a sink: it sets underwater and blocks pipes for good. Let it harden in a flexible bucket, then pop it into the bin.
- Water at 20–25 °C (68–77 °F) gives an even set: warmer, the plaster sets too fast; ice-cold, it takes forever.
- The firing cost of your pieces is calculated in this same category.
FAQ
- Which plaster should I use for pottery molds?
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A molding plaster sold as “pottery plaster” (hard α-β type): it absorbs evenly and withstands casting wear. DIY patching plaster sets too fast and crumbles; very soft modelling plaster wears out within a few casts. The recommended consistency is printed on the bag — pick that one above.
- How do I measure an irregular model’s volume?
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By water displacement: submerge the model (wrapped in film if it dislikes water) in a graduated container and read the difference. If it floats, hold it under with a thin rod. Otherwise approximate it with simple volumes (cylinders, boxes) and err generous: the surplus goes into your margin.
- Why is my mold not absorbing the slip?
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Three classic causes: too stiff a mix (not enough water — less porosity), a mold not fully dry (allow 1–3 weeks of air drying depending on thickness), or a surface saturated with mold soap. Consistency 73–80 gives the most absorbent casting molds.
- How long before the mix sets?
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Pottery plaster typically gives 15–20 minutes between the end of mixing and thickening — enough to pour unhurried. The countdown accelerates with warm water, prolonged or vigorous mixing, and old set plaster left in the bucket (it seeds the reaction): work with a clean bucket.
- Can I pour the mold in several batches?
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Avoid it for a casting mold: the junction between batches creates a less absorbent seam that will mark your pieces. If the volume exceeds one mixable batch, prepare two buckets in parallel and pour back-to-back, or design the mold in parts — which is exactly why you calculate first.