Short answer
Particle size sets how a pigment scatters light: particles far smaller than a wavelength scatter weakly, so the pigment looks transparent and strong; particles near half a wavelength scatter most, which is what a white needs; large particles or flakes reflect like tiny mirrors. Dispersion is the process of breaking pigment clusters down to their working size and keeping them apart — poor dispersion wastes colour strength and shifts hue.
Light interacts with a particle according to how big it is compared with the wavelength (roughly 400–700 nm). Much smaller particles, a few tens of nanometres, are in the Rayleigh regime: they scatter weakly and preferentially at short wavelengths, so a film of them is nearly transparent and colour comes almost entirely from absorption. Particles comparable to the wavelength, a few hundred nanometres, scatter most efficiently per unit volume — the target for white and opaque pigments. Particles much larger than the wavelength, including the platelet flakes of effect pigments, behave more like small surfaces that reflect and refract as bulk material would, so they give sparkle, lustre and angle-dependent effects rather than even colour.
For absorbing pigments, grinding finer generally raises tinting strength and transparency, because more of the pigment's molecules face the light and less scattering dilutes the colour with white. That is why transparent grades of iron oxide and phthalocyanine exist for glazes and metallic car basecoats. The trade-off is surface area: a finer pigment exposes more of its molecules to oxygen, moisture and light, so a fine grade of an organic pigment can be less lightfast than a coarser grade of the same chemistry, and fine pigments are harder to disperse and more prone to flocculate. Pigment makers therefore offer the same Colour Index pigment in several particle-size grades for different jobs.
Pigments rarely arrive as separate primary particles. Carbon black, for example, forms primary particles of roughly 10–300 nm fused into aggregates that act as one unit, and these cluster further into agglomerates of about 100–1000 nm held by van der Waals forces; the industry association notes that agglomerates only break down under sufficient shear. Dispersion is the manufacturing step that applies that shear — in bead mills, three-roll mills or high-speed dispersers — while the binder and dispersing additives wet the new surfaces so particles stay apart. A pigment that is dispersed but later flocculates, drifting back into loose clusters in the can or the drying film, behaves like a coarser pigment: weaker, often duller and sometimes shifted in hue.
Flocculation reveals itself because clusters break up again under mechanical force. Rubbing a finger over a partly dry film of a tinted paint shears the flocs, and if the rubbed patch comes out stronger or a different hue from the surrounding brushed or sprayed area, the paint was flocculated — the basis of the rub-out check used in paint laboratories. Poor dispersion shows as specks, streaks, lower gloss and batch-to-batch strength differences. In mixed colours, a pigment that floats to the surface or separates from another produces a patchy or streaky finish. None of these are visible in the can, which is one reason paint colour can differ between two tins of the same shade.
| Particle | Typical size | Optical consequence |
|---|---|---|
| Carbon black primary particles | 10–300 nm | Absorb strongly, scatter very little |
| Carbon black agglomerates as sold | 100–1000 nm | Must be broken down by shear to develop jetness |
| Rubber-grade carbon blacks (N110 to N770) | 20–25 nm to 70–96 nm | Grades are classified partly by particle size |
| Pigmentary titanium dioxide | About 220 nm (often quoted) | Near-optimum scattering of visible light |
| Metal effect flakes | About 20 µm long, up to 1 µm thick | Mirror-like reflection, flop and sparkle |
Why: Brushing, rolling and spraying shear the paint differently; if it is prone to flocculation, the colour depends on application.
Fix: Stir thoroughly, apply touch-ups the same way as the original, and blend to a natural break such as a corner.
Why: Pigments in a mixture have separated or floated during drying.
Fix: Mix well and do not over-thin; if it persists, the paint's dispersion is at fault rather than the technique.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Carbon black primary particles are generally 10–300 nm in diameter, and the product as placed on the market consists of agglomerates of about 100–1000 nm that break down only when adequate shear force is applied.
Source: What are the Physical & Chemical Properties of Carbon Black?
FactStrong evidence
Particles much smaller than the wavelength of light scatter it weakly and more strongly at short wavelengths (Rayleigh scattering, varying roughly as the inverse fourth power of wavelength).
Source: HyperPhysics (optics, atmospheric optics and vision pages)
FactModerate evidence
Metal effect pigments are platelets roughly 20 micrometres long and up to about one micrometre thick, whose effect depends on aligning parallel to the coating surface.
Source: Effect pigment
Colourwise interpretationLimited evidence
Finer grades of an absorbing pigment tend to be stronger and more transparent but can be less lightfast and harder to keep dispersed, because more of the pigment's surface is exposed.
Based on: Colourwise synthesis of the size-regime physics above with the surface-exposure argument for why pigments outlast dyes; not a measured comparison of specific grades.
Caveat: Depends heavily on chemistry and surface treatment; some fine pigments are extremely lightfast.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.