Pigment Chemistry and Structure

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Pigment Chemistry and Structure

From George O'Hanlon

Published before 2005

Posted by Brian van der Spuy:

I am endlessly fascinated with the pigments of pre-industrial times. They seem to have a different character, even if many are more subdued than the ones we have today.


It is true that many of the pigments on the Old Masters' palette are more subdued than today's modern pigments, but this appears to be a benefit for artists who are concerned about portraying natural colors.

The different character of pigments from pre-industrial times does not stop with color. I have found many more interesting differences than simply the color of the pigment; an entirely different range of paint body is possible with the coarser and heterogeneous pigment particles used by the Old Masters. The particles of the pigments used by the Old Masters vary from today's pigments in three significant ways: size, size distribution and shape.

Size Matters
Today, most modern pigment particles are less than 10 microns, and many are sub micron (less than one micron) in size. Some are so small that they approach the size of the wavelengths of visible light. This is one way to achieve transparency with some pigments that are normally opaque. The particle size of pigments found on the Old Masters' palette was often larger than one micron and many were as large as 70 microns (smalt is a good example). To put these measurements in perspective, very fine flour is typically milled to about five microns, whereas fine sand is about 100 microns. What does large pigment particles mean for artists' paint? Given the same pigment, a paint made with larger particles allows more light to pass through it and appears more transparent than a paint made with the same pigment, only with smaller particles. (This is one reason why modern paints are formulated with pigments consisting of the smallest possible particles—more opacity and better coverage or hiding power—means less pigment is required and less cost.) Perhaps this was one of the secrets of the Old Masters' glazing technique—pigments that are more transparent preclude the need to dilute paint with mediums in order to achieve greater transparency yet maintain paint body or consistency. Larger particles can also create both visual and physical texture in the paint layer; a feature often exploited by ancient painters.

Size Matters, Again
Another important factor is the distribution of particle sizes. When you examine paint layers from Old Master paintings, you see a wide range of pigment particle sizes. The paint layer may contain very fine particles; the median particle size may be 10 or 20 microns, but the distribution of sizes may be from one micron to 50 microns. This is very different from the paint layers of today's artists using modern pigmented paints. Modern pigments typically have a more compact distribution of particle sizes—one to ten microns is typical. And the industry is demanding even tighter ranges.

The reason for this is simple, the more homogeneous the particle size is within paint, the less likely the pigment will flocculate or settle in the can (or in the case of artists' paint—tube). However, this reason is not necessarily conducive for artists' paint. While homogeneous particle size pigments help avoid the problem of separation in the tube for ready-made paints (thereby also extending shelf life) and makes paint more opaque, it also tends to make paint exhibit the same body—short and buttery. Today's artists tend to believe this is how all paint behaves, not realizing however, the Old Masters did not work with paint of a singular body or consistency. They used paint with all sorts of different consistencies: "long", "ropey," "tacky," "flowing" and, yes of course, short and buttery. We have found that pigments with a wider distribution of size and shapes vary the consistency of the paint, creating these types of paint body.

Oh yes, let's not forget about the effect believed to be caused by homogeneous pigment particles that are well dispersed in a paint vehicle: the "suede" effect.

Different Shapes for Different Paints
Most pigments today are manufactured through chemical precipitation, creating particles that are amorphous or without a distinct shape. That is not the case with pigments used by the Old Masters. Few pigments then were the result of chemical precipitation (the lakes are about the only exception). Pigments were made by smashing, crushing, pulverizing and grinding stones or friable earth into powder. This created a large variation in the shape and size of the pigment. The wide variety of shapes, even within the same pigment sample, means that paint was less opaque and often had unique rheological properties different from modern paints. Paints exhibiting these characteristics are called thixotropic or rheopectic.

Thixotropy is the property of some paint to show a change in viscosity with time and strength of shear; in other words, the longer the paint is stirred or agitated (undergoing shear), the lower its viscosity and the more fluid it becomes. As soon as the shear is removed, it resumes its former viscosity. For an artist, thixotropic paint means that a fluid paint will not run off the brush, but will still spread easily and evenly, since the gel-like paint "liquefies" when brushed out. For paste paint, such as that squeezed from the tube, this means it will become softer, more workable when agitated with the brush or palette knife, but as soon as the brush or knife is lifted from the paint, it holds its shape.

Rheopecty or rheopexy is the opposite of thixotropy and is the property of paint to show a time-dependent change in viscosity; the longer the paint undergoes shear, the higher its viscosity. Rheopectic paints thicken when stirred or agitated.

Interestingly, a recent trend in the pigment industry exploits the particle shape of pigments to create new and interesting visual effects, such as interference pigments.

While pigments of the industrial epoch offer artists a wider range of highly-saturated colors than was available to former painters, let's not forget the unique properties of the pigments they used and the "new" effects it can give paints today.

George O'Hanlon
Natural Pigments LLC