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Pigments – versatile play with light

Pigments – Versatile Play with Light

The industry uses three key pigment classes: absorption pigments, metal effect pigments and pearlescent pigments. We are familiar with absorption pigments from watercolor paints, for example. They represent "true" colors as they absorb part of the light which hits them and scatter the rest. This gives them their own body color. In contrast, silver metal effect pigments throw almost all the light back like lots of tiny mirrors. This gives them their great surface luster, which is sometimes considered dazzling.

Pearlescent pigments are truly multi-talented pigments. They combine the properties of the other two groups: thanks to their particular layered structure, the light is reflected at different levels in the pigment. Under the right circumstances, the reflected waves can interfere with each other, causing amplification or cancellation. Depending on the structure of the layers in each case, this creates the brilliant interference color which gives the pigments their unique character.

 
The pearlescent effect: derived from nature

The Pearlescent Effect: Derived from Nature

Pearlescent pigments work based on the following phenomenon: behind their attractive shimmer is the simple process of refraction. At boundary layers, such as between air and glass, white light is broken up and divided into its components – the colors of the rainbow. The greater the difference between the refractive indexes of the adjacent materials, the stronger the refraction. Depending on the angle at which the light hits the boundary layer, it may be completely reflected or passed through unchanged.

A key feature of the classic pearlescent pigments is the difference in color   between the share of the incoming light which is reflected by the pigment and the share of light completely passing through. The color of the transmitted light is complementary to the color of the reflected light. For example, a red reflection color goes with a green transmission color, while yellow and blue form another pair of reflected and transmitted light.

A piece of nature at its core: mineral mica
Mineral mica is a natural inorganic pigment. The mineral is the starting material for some of Merck's effect pigments. After extraction, it is cleaned, ground and then coated with one or more metal oxides in a precipitation process. The result: a layer-substrate pigment which interacts in a specific way with light called interference.  The result is a specific interference color, depending on the thickness of the metal oxide. A wide variety of effects can be achieved, from matte shimmer similar to that of pearl or mother of pearl to interference looks with significant shimmer in all colors of the rainbow.

Innovative substrates for novel effects
The combination of the substrate and coating is key to the look of every effect pigment. A transparent substrate is an important pre-condition for the perfect interaction between the light and the pigment. Natural mica was just the start – today it is possible to produce transparent substrates with similar properties artificially. By coating them with metal oxides, completely new effects can be achieved, far beyond the options offered by natural mica. The innovative substrates include synthetically produced aluminum oxide (Al2O3) platelets, silicon dioxide (SiO2) platelets and glass platelets made from calcium aluminum borosilicate.