Pigmentation principles: why powdered pigment doesn't work
This text was written after a Wednesday get-together where a discussion about the pigmentation of plaster developed. Since I wasn't able to explain everything on the spot, I decided to put it all down on paper — and the principles in question apply more broadly than plaster itself anyway.
1. What is a pigment — and what is not
A pigment is a substance that imparts color by selectively absorbing certain wavelengths of visible light and reflecting others. A black pigment absorbs almost all wavelengths; a red pigment absorbs blue and green and reflects red. It is important to distinguish a pigment from a filler. Fillers are white or neutral substances that are added to paints, coatings, and masses to increase volume, improve texture, or reduce cost—but do not provide strong color or good hiding power on their own. Typical fillers include calcium carbonate (chalk, CaCO₃), calcium sulfate (gypsum, CaSO₄), barium sulfate, and similar materials. The difference between a true pigment and a filler is not just in color—it lies in a physical property called the index of refraction.
2. Refractive index and opacity
The refractive index (IR) describes how much light is refracted and scattered when it passes through a substance or encounters its surface. The higher it is, the more the particle scatters light — and thus appears more opaque, "opaque". A few comparative values: Titanium dioxide (TiO₂) — IR around 2.7 → excellent coverage, strongest white pigment Zinc oxide (ZnO) — IR around 2.0 → good coverage Calcium carbonate (CaCO₃) — IR around 1.59 → poor coverage, filler Calcium sulfate / gypsum (CaSO₄) — IR around 1.52 → almost transparent in a binder, filler This means that gypsum and chalk, although white as a powder, become almost transparent in the formulation of a coating or mass. They do not compete with the pigment — they are subordinate to it. That's why a small amount of pigment easily and evenly colors a gypsum or chalk-based mass, while the same amount of pigment in a mass containing TiO₂ would be barely visible.
3. Agglomeration — the enemy of even color
Pigment powders do not exist as perfectly separate particles. Due to electrostatic attractive forces and surface tension, the particles spontaneously group into clusters called agglomerates or aggregates. Agglomeration is particularly pronounced in: fine particles (the smaller the particle, the higher the surface area to volume ratio, so the attractive forces are relatively stronger) pigments with a high specific surface area, such as carbon black high temperature or humidity conditions When pigment powder is mixed with filler or binder powder, the agglomerates do not break down—they remain as compact clusters. The visual result is an uneven color: dark spots where pigment particles have accumulated, and pale areas where they are absent. The user then concludes that "more pigment is needed"—but this is not true. The problem is not a lack of pigment, but its poor distribution.
4. Dispersion — meaning properly dispersed pigment
Dispersion is the process of breaking up agglomerates and evenly distributing individual pigment particles throughout a medium (water, oil, binder). A well-dispersed pigment means that the particles are as evenly distributed as possible — each filler or binder particle "sees" the pigment, not just the neighborhood of the agglomerate. Dispersion is achieved by mechanical and chemical means: Mechanical: mixing with high shear forces (mixers, mill aggregates, ultrasound). Mixing with a spoon or spatula is not sufficient to break up agglomerates. Chemical: the use of dispersants and surfactants that adsorb to the surface of the particle and prevent it from re-adhering to neighboring particles.
5. Why liquid colorant works better than powdered pigment
Liquid colorants are not just pigment dissolved in water. They are ready-made systems that contain: Pigment — already dispersed to the level of individual particles or very small clusters Dispersants and surfactants — which keep the particles separated and prevent re-agglomeration Liquid medium — which allows the pigment to be evenly distributed throughout the material being colored before that material begins to set or dry When a liquid colorant is added to the mixing water (e.g. in gypsum, concrete, mortar), the pigment is already in an ideal state of dispersion. The same amount of pigment is evenly delivered to each part of the mixture. The color effect is therefore much more intense than with dry-mixed pigment — with a significantly lower total amount of pigment. The same logic applies to paints and varnishes: pigment pastes and dispersed pigments provide better coverage and color uniformity than pigments that have not undergone the dispersion process.
6. Practical application — gypsum example
Gypsum is a good example because it illustrates all the above principles at once. Because it has a low refractive index (~1.52), it is not a true white pigment — it does not resist staining when mixed with a binder. This means that a small amount of black pigment can easily and evenly color the gypsum mass. Why then does it happen to many people that they have to add a large proportion of pigment in relation to the mass of plaster? Because they mix the pigment in powder form directly into the gypsum powder. Pigment agglomerates (especially Fe₃O₄ or carbon black) remain intact, the distribution is uneven, and the result is disappointing. The conclusion "we need more" is wrong — we need better.
Correct procedure:
Add the colorant (or pigment dispersed in water) to the mixing water Mix the water with the colorant well Only then add the gypsum and mix until a homogeneous mixture This way, the pigment is distributed throughout the entire mass before the gypsum begins to set. The result is an even, intense color with a much smaller amount of pigment than with dry mixing. For those who do not have access to professional colorants, a good alternative are liquid pigment additives available in building paint stores — usually in the form of small bottles intended for tinting wall paints. It is the same principle: the pigment is already dispersed in a liquid medium with additives that prevent agglomeration. Added to the mixing water, they give a more even result than powdered pigment with a significantly smaller amount.
Conclusion
The intensity and uniformity of color in a mass depend not only on the amount of pigment — they depend on how well the pigment is dispersed. A pigment powder mixed with a powder of another material almost always gives worse results than a pigment that has been previously dispersed in a liquid medium, in the presence of dispersants. When you encounter the problem of "the pigment does not color enough," it is worth asking yourself: is the problem not in the way it was added — and not in the amount.
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