Pigment is not paint. Paint sits on a surface; pigment goes inside the material, and every grain of gypsum that sets around it holds that colour for the life of the object. Get the quantity and the mixing order right and a cast comes out even, deep and predictable. Get either wrong and you get streaks, a chalky surface, or a piece that is suddenly lighter than the bottle promised.
This is the working method: what to add, how much, in what order, and how to know before you commit a full mould.
Why gypsum behaves differently from paint
Gypsum sets by hydration, not by drying. The water you pour in becomes part of the crystal structure, and whatever is dissolved in that water gets locked into the matrix as it forms. That has three consequences.
First, colour has to go into the water — not onto the powder, not onto the finished piece. Second, the surface you see is not a film; it is the material, so the tone runs all the way through and a chip never reveals a white core. Third, the cast dries lighter than the slurry looks. A batch that reads as deep terracotta in the bucket can dry to dusty rose. Judge nothing wet.
Three ways to colour a cast
- In the mix — pigment stirred into the water before the powder. Even colour through the whole thickness; sanding or a chipped corner stays the same tone. This is the default, and it is what most of our pieces use.
- Surface wash — a thin pigmented slurry brushed over a cured cast, then wiped back. Colour settles into pores and edges, which reads as age or stone. Good for texture; it will not survive heavy sanding.
- Mica on top — mica powder is not a tint. It is a pearlescent layer that sits on the surface and shifts with the light. Use it over a coloured cast, not instead of pigment.
Ratios that hold
Dose by weight of the mixing water, not by eye. Liquid pigment is concentrated; small differences change the result a lot.
| Result | Pigment by water weight | Per 100 g of water |
|---|---|---|
| Pastel / tint | 0.5–1% | 0.5–1 g (10–20 drops) |
| Mid tone | 2–3% | 2–3 g (40–60 drops) |
| Deep / saturated | 4–5% | 4–5 g (80–100 drops) |
| Beyond 6% | — | weakens the set; avoid |
Two hundred drops is roughly one 10 ml bottle. Past 6% the pigment starts interfering with the crystal growth — the cast sets slower, sands softer, and can bloom white at the edges as it cures. If you need a darker result than 5% gives, switch to a darker pigment rather than adding more of a light one.
The mixing order that prevents streaks
- Water first. Measure the water into a clean container.
- Pigment into the water. Add the dose and stir until the water is uniformly coloured — no ribbons, no sediment on the bottom. This is the step people skip, and it is the single cause of most streaky casts.
- Powder into the coloured water. Sift or sprinkle the gypsum in and let it slake for 30–60 seconds before mixing. Adding water to powder traps air and sets unevenly.
- Mix to a consistent slurry. Scrape the sides once. If the colour looks uneven in the bucket, it will look uneven in the mould.
- Pour in one pass. Pigment settles. A pour that stops and restarts can leave a visible line where the heavier slurry landed.
Why the colour changes
- It dries lighter. Expect 20–30% lighter than the wet slurry. Always judge on a cured chip, never in the bucket.
- Mica shifts, pigment does not. Mica reads differently from every angle; that is the point of it. Do not use mica to fix a tone — it will not hold still.
- White gypsum is not neutral. The base is warm-white, so every colour lands slightly warmer than it would in resin. Blues and greens shift most.
- Water ratio changes tone. More water means more pores and a paler, chalkier surface. Keep the water-to-powder ratio fixed when you are comparing colours.
Test before you commit the mould
A full test costs ten minutes and 50 g of gypsum. Mix one small batch of plain slurry, split it into three cups, and add pigment at 1%, 3% and 5% of the water weight. Pour three chips, label them, and leave them 24 hours. You now have a physical colour chart for that pigment in that gypsum — and a reference you can hold against the next batch instead of guessing from a screen.
Frequently asked
How much pigment per 100 g of gypsum?
Dose against the water, not the powder. At a normal 100:35 water-to-gypsum ratio, 3% of the water weight is about 1 g of pigment per 100 g of powder. Start there and adjust on a chip.
Can I use acrylic paint instead of pigment?
Not in the mix. Acrylic is a binder that films over particles; in a cast it interferes with hydration and leaves a weaker, patchy result. Acrylic is fine on the surface after curing, but it is a coating, not a colour.
Why is my cast lighter than the bottle?
Three usual reasons: you judged the colour wet, the water ratio was higher than the test batch, or the pigment settled because the slurry was not stirred just before pouring.
Doing it on real pieces
The 24-colour pigment set is dosed by dropper, which makes the 1 / 3 / 5% test easy to repeat. If you want the pearlescent layer, mica powder goes on after the cast has cured. The pigment system page covers where colour sits in the build order, and the mixed texture system shows what happens when pigment, crushed stone and glass are used in the same pour.