Project Description

pigment science

Our  female ancestors, the Neanderthals, 40.800 years ago already knew where to find and prepare PR4(*) but, do you?

cave

40,800 years old crimson hand stencils on Spanish cave walls El Castillo cave

Encaustic paint is so new (and so old) that even without knowing it, you are already experimenting with pigments, binders and additives every time you heat your wax.

Basically, an artists’ pigment can be any substance with the following characteristics:

  • Allow being finely ground, without  becoming too abrasive
  • Have deep color
  • Don’t lose their color when mixed
  • Relatively weather-, light- and heat-resistant
  • Relatively insoluble in the binder and  chemically stable

There are two reasons why I hereby declare a crusade for pigments: (i) Artists cannot fear the materials they work with, and (ii) and more important, trying to understand pigments can be our modest artists contribution to bridge again humanities & sciences, a communication bridge that I firmly believe will help overcoming the global problems we are immersed in (wow!).

This is the first of the 3 posts that will be dedicated to mixing your pigments. In this first post will try to explain in an easy way the necessary concepts derived from pigment science that will allow you to understand how pigments work, in the second post I will focus on how to select your pigments and how to make encaustic paint, and in the third one how to deal with safety issues while manipulating pigments .

If you need to remember the principles of color vision, you can read this on light absorption, and this on light dispersion.

Pigment and dyes

Most pigments are dry colorants usually ground into a fine powder. This powder is added to a relatively neutral or colorless binder that suspends the pigment  to form a coating (what technically is called colloidal suspension)

Dyes are also colorants, but are soluble in the medium and color the materials by means of a chemical reaction and not by forming a coating. Dyes usually have specific chemistry for use with very specific materials (mostly textile, hair fibers and plastics). The chemistry of the dye  is a science all of its own.

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Pigments and dyes are often derived from the same basic substances. The fundamental difference between them lies, as said, in the fact that dyes are soluble and pigments are not. Some dyes can be precipitated with  salt to produce lake pigments.

Crystal structure

All pigments, with rare exceptions, have fixed crystalline structures that dictate not only their size and shape but also their color. There are pigments with identical chemical composition but with different hue, due to different crystal formation (they are called polymorphic).

These crystals’ smallest units are called primary particles, these particles are so small that they have great surface attraction for each other. This causes them to stick together and create agglomerates, these agglomerates is what we get when we buy pigments, as it would not be practical to supply pigments in the form of primary particles as they would be more like smoke than a powder.

Mixing with the binder and separating these agglomerates into the original primary particles is called the pigment’s  dispersion  (into the colloidal suspension) and, as said,I will specifically dedicate the next post to this procedure.

Particle Shape

The shape of a particle is determined by its chemical composition and its crystalline structure. The primary particles  are not usually spherical but present different shapes: nodular, spherical, prismatic, acicular or lamellae. These shapes can have different dimensions depending on whether one measures the length, width or height. Normally we talk about average diameters of primary particles, no matter which shape they have.

Particle Size

Primary particle size is so small, that needs being measured in the same length units (µm micrometer = 1/1000 mm) as bacteria or even wavelength of infrared radiation. (Have you ever heard about wave-particle duality? here it is).

Typical ranges for average diameter of primary particles are:

  • carbon black – 0.01 to 0.08 µm;
  • titanium dioxide – 0.22 to 0.24 µm.
  • organic – 0.01 to 1.00 µm;
  • inorganic – 0.10 to 5.00 µm;

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The pigment’s particle size affect its color, brightness, transparency and particle distribution and the stability of the dispersion.

Titanium white (rutile) for instance, as a visible crystal is transparent, but into micron particles is the opaque white we all know. The same effect as snow flakes crystals and solid ice-cubes show.

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The smaller these particles, the greater their surface energy and therefore the more likely it is that they will clump together. As said, the pigments most at risk to flocculate are the one with smaller primary particles like carbon black and organic pigments.

Surface area 

The surface area is the total area of the solid surface of all the primary particles contained in 1 gram of pigment, it is so big that is measured in squared meters (m2). Typical values for organic pigments are between 10 and 130m2 !

The surface area is closely linked to the pigment’s demand for binder for instance.  1 gram of larger particles have a smaller surface area in comparison, and therefore a lower demand for binder. As the size of particle of pigment decreases, the area of surface per gram become larger. As a result, the paint need large amount of binder to “wet” each of pigment particles during the dispersion process.

 

Pigments performance

Color, color strength and brightness

Particle size  plays a key role in color attribute, as smaller pigments will present a greater surface area to light, giving them a higher tinting strength and allowing even in a very thin film to possess rich color. 

Differences in the purity or uniformity of shape or size distribution of a particular pigment  are responsible for coloration nuisances. Natural mined earth pigments have a wide lot-to-lot color variation depending on the level of impurities they contain. Ultramarine Blue on the contrary, being one of the earliest synthetic pigments, is richer and more saturated than the genuine Lapis Lazuli it replaced, which as a mined rock always came with impurities that muted its tone.

Opacity or transparency

You are not totally wrong when you think that the thickness of the layer or its pigment load play a fundamental role in the transparency of a paint, of course they do, but just as a side effect of reducing scattered light through subsequent and repeated absorption.

What primarily makes a color to appear transparent is  the pigment’s ability to scatter light (see my previous post on optical properties). As a particle becomes smaller it scatters light more effectively until a certain optimal size is reached, after which the percentage of scattered light  drops off sharply. Below this threshold the pigment particle grows increasingly transparent while simultaneously reaching a maximum of tinting strength (see figure)

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Because the scattering of light is greatest at the boundary of two materials with very different refractive indices, the closer a pigment’s index comes to the medium that surrounds it, the more transparent it will appear. Pigment “transparency” or hiding power actually depends on the ratio between the refractive index of the pigment and the medium around it. Pigments appear transparent when this ratio is close to 1,0.

Inorganic pigments have a high refractive index and organic pigments have much lower values. Consequently, most inorganic pigments are opaque, whilst organic pigments are transparent (see table bellow).

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Just as an example: Titanium Dioxide (Titanium white) is opaque because its refractive index, 2.76, is much greater than the wax used as binder (1.44), so it tends to scatter a maximum amount of light in the surface where the binder is in contact with the pigment, and for this reason possess excellent hiding power. Phthalocyanine, however, is highly transparent because its index of 1.4 is much closer to the binder’s own.

The best colors for glazing, therefore, will nearly always be those that have a relatively low refractive index, like the organic pigments. Since the binder ‘s refractive index plays such a critical role in limiting the scattering of light, having the particles well bound in a paint film (surface area) becomes crucial for creating a luminous translucent glaze. For this reason a glaze needs to be made with plenty of medium to insure the pigment is fully suspended.

Since the refractive index of a compound cannot be altered in laboratories,  it is the adequate particle size and shape what  the today’s pigment technology is all about.

Unusual effects

There are pigments that create unusual absorbency and scattering effects. This is an area where much development work in the pigment industry is still in progress.

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Fluorescent materials absorb light at one wavelength and emit the light energy at another. Often the light energy is absorbed in the UV range, outside the visual range, and emitted in the visual range.  Optical brighteners used in paper and textiles are  examples.

Metallic effect coatings  usually contain aluminum or other metal flakes that act as tiny mirrors, they add only a gray nonselective color effect of their own, but based on their particle shape and size distribution, they can shift  the perception when looking at a sample from several angles.

Pearlescent effect coatings usually contain semitransparent mica flakes that allow some light to pass through their surface and absorb and scatter light as well. The net effect is to bend the light waves and create a multicolored effect, such as oil on water.

Now I think we are ready to understand why different pigment families show different properties, I will try to resume these in the following table.

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(If you are not familiar with the different pigment families, a good resource to update is here or here)

Now that we understand why different pigments behave differently, in the next post I will give some hints as how to  select the right pigments for the specific use and some practical tips for making our own encaustic paint.

 (*) Crimson red