Particle size often determines practical properties of a pigment such as processability but also its color shade. For transparent paint or varnish layers, the primary particle should be as large as possible, because clumped particles make the color appear more opaque than large individual crystals. As a general rule, the finer the pigment, the more binder is required. For example: Blanc Fix needs a very small amount of oil while Terra di Siena and Furnace Black need a lot of oil.
In addition to the particle size, the particle size distribution also plays an important role. In the case of synthetic pigments, the distribution is usually quite homogeneous, i.e. the particles are all approximately the same size. In the case of earths and minerals, however, the particle size distribution can be very inhomogeneous, depending on the raw material. Basically, the more similar the individual particles and the more homogeneous the particle size distribution, the clearer the color. Finer particles form a fine dust which settles on the coarser particles and therefore changes their color.
Depending on the material, production and fracture behavior, pigment particles also have very different shapes. Particles can be round grains or sharp-edged small splinters, flat plates or thicker sheets and even rod-shaped, acicular or fibrous. The particular shapes of glass, quartz or lapis lazuli particles, for example, make them more difficult to process than pigments with a rounded particle shape. The sharp-edged small splinters tend to get stuck in the brush which makes it harder to spread the paint. Platy particles such as graphite or mica align themselves parallel to the paint layer. The surface of the particles reflects the light and gives a glossy or glittery effect. However, the platy shape can slow down drying, especially with oil paints. Porous particles such as chalk or carbonaceous blacks, on the other hand, tend to have a matte appearance. Due to their porosity, they act like small sponges and require a lot of binder. Acicular or fibrous particles can occur with malachite, magnetite, gypsum or other minerals but also with some synthetic pigments.
Pigment hue
The color shade of a pigment depends, among others, on the particle size. The 5 different grinding degrees of Kremer Pigmente colored glass pigments demonstrate this effect very beautifully. The choice of binder also changes the color hue. For example, most green earths turn out very dark when worked into oil.
In addition, the shade of a pigment is also influenced by lighting. The effect that a pigment looks different depending on the fall of the light is called metamerism. This is particularly evident in blue, green and violet pigments. Reproducing paints that contain strongly metameric pigments can be very difficult. Color systems according to RAL or NCS offer orientation, but do not provide any information about pigments or mixing ratios present.
