What Makes Organic Pigments Translucent ?April 14th, 2009 by Richard
In a previous blog, I noted that most inorganic pigments are opaque, except when a molecule of water is bonded to the pigment crystal. That “water of crystallization” makes an inorganic pigment, like viridian, translucent.
On the other hand, most organic pigments are translucent. Diarylide yellow (the pigment we use to make Indian yellow) is a good example of this. What makes organic pigments translucent is the relationship of a pigment’s density to the density of the medium it is in.
What’s the difference between organic and inorganic pigments?
Inorganic pigments are based on metals, such as cobalt, cadmium, titanium, chromium, etc. These are generally much denser than the organic pigments.
Modern organic pigments come with the unpronounceable names, such as phthalocyanine, quinacridone, dioxazine, diarylide, anthraquinone, etc. These pigments are hydrocarbon-based, which means that they are derived from coal tar or petroleum distillation. Prior to the 19th century, translucent colors were plant- or insect-derived lake pigments.
Organic pigments are less dense than the metal-based pigments. Compare 10 grams of chromium oxide pigment to 10 grams of diarylide pigment. The chromium oxide pigment has far less volume because it is denser.
This difference in density is what makes the inorganic pigments opaque and organic pigments translucent.
To understand this, let’s compare the densities of different pigments and mediums. Note that the organic pigments are grouped with the mediums, while the inorganic pigments are all lower down, or denser.
The density is expressed as a “refractive index.”
What is a refractive index?
The refractive index refers to the refraction, or bending, of light when it travels from a lighter matter, such as linseed oil to a heavier matter, such as chromium oxide.
Think of a car traveling down the road, suddenly driving into a thick layer of mud. What happens?
Instead of keeping a straight course, the car swerves. It has just traveled from a less dense medium (air) into a dense medium (mud). The denser the mud, the more it will swerve. In the same way, the denser the pigment is, the more it will bend the light that travels from the air through the linseed oil through the pigment.
What does this bending of light (refraction) have to do
The classic illustration of refraction is the pencil in the glass of water.
The degree to which the light is bent determines the angle at which it will get reflected. The greater the bending, the greater the shift of the reflection (which is why the pencil looks broken). It is also why the object is more opaque and less translucent.
Here is a graph showing light being bent by chromium oxide, which is opaque. (You may wonder why light can penetrate the pigment if it is opaque. The reason is that all pigments are translucent at particle level. What makes them opaque is not how they stop light but how they bend it.)
The light hitting the chromium oxide pigment is noticeably bent (refracted) because the pigment is much denser than the linseed oil. As a result the light reflected off the pigment is at a very different angle from the light reflected off the linseed oil and intersects it. That change in angle (like the pencil shifting) means that the pigment is very reflective, which is why it has a heavy reflection line. Being reflective makes the pigment opaque. This may not be very obvious at first, but it becomes more understandable when you consider the converse.
If the pigment had the same refractive index as the linseed oil, the light traveling through it would not be bent. It would travel a straight line from the linseed oil through the pigment. On a graph the reflection off the linseed oil and the reflection off the pigment would be parallel because the two reflections would be at the same angle. What this means visually is that the pigment would be transparent and not visible in linseed oil.
Diarylide yellow, on the other hand, is an organic pigment with a low density that approaches transparency in the linseed oil. Here’s a graph showing light traveling through linseed oil and being bent by the diarylide pigment.
The pigment barely bends the light. This makes the color deeper than if it were highly reflective, which is why in the graph it has a heavy refraction line. The reflection, which is nearly parallel to the reflection off the linseed oil, will be weaker and less reflective, which makes the pigment more translucent.
Going into the mechanics of light refraction like this may seem remote from artistic expression. But when you realize that much of the character of Western oil painting, since its development in the 14th century, has centered around the interplay of these opaque inorganic and translucent organic pigments — top tones against undertones, thick body paint against delicate glazes — then an understanding of the underlying physics can only enrich your appreciation of the effects that those mechanics produce.
Future blogs will explore how refraction and reflection affect pigment color and surface characteristics of paintings.