Mention a well-known pigment like Ultramarine or Cobalt Blue, and we instantly picture a very particular and unwavering color. And why not – it is easy to think of pigments as having characteristics that remain constant as one moves between different mediums such as acrylics, oils or watercolors. Even if we accept that the handling properties or the pigment load changes, certainly the color is constant, no? In this article we explore the surprising answer to that question and examine some of the ways a pigment’s color changes when used in different systems.
But first, a little background on how this theme came about. Every year Golden Artist Colors, along with other manufacturers, helps to organize and participate in a Materials Panel at the College Art Association (CAA). This last year the theme, “Pigments in a Bind(er)”, looked at the impact of different binders on the appearance of pigments. Working in collaboration with R&F Encaustics, Gamblin Artist Colors, and Natural™ Pigments, samples were generated using identical pigments of Ultramarine and Cobalt Blue prepared in egg tempera, watercolor, casein, encaustic, acrylic, and oil. No fillers were used, allowing them to represent as much as possible the interaction of pigment and binder alone. These were cast on black and white drawdown cards at similar film thicknesses then cut into 2”x 3” swatches and carefully assembled on a display board (Image 1). What follows is adapted from our presentation, which focused on the role of pigment load in the appearance and film qualities of these very different paints.
Certain aspects of a pigment are considered constant, such as the molecular weight, refractive index, density, and chemical composition. Very little that we ever do as paint makers or artists will change any of those things. Absent from that list, however, is the one thing we almost always think of when referring to a particular pigment – its color. So the question is a simple one: why? Why do some of the swatches, all using the same pigment, look so different from each other? As we can see in Image 1, our panel of two pigments in six binders roughly breaks into two groupings with very distinct appearances. In one group you have watercolor, casein, and egg tempera, where the colors are brighter, higher chroma, and more opaque, especially with Ultramarine Blue. In the others (acrylic, encaustic, and oil), the colors grow deeper, redder, and with Ultramarine Blue in particular, considerably more translucent.
One would typically think that the Refractive Index (RI) of the binders would be the culprit, but while this can sometimes be a crucial factor, in this case the data simply does not support that common theory. There clearly is not a broad enough range to account for the sharp differences (Table 1i).
Pigment Volume Concentration (PVC)
Looking past refractive index as a main cause, a more promising possibility is that the changes are driven by differences in the Pigment Volume Concentration (PVC), defined as the ratio of the volume of the pigment divided by the volume of both pigment and binder together:
PVC = pigment volume / (pigment volume + binder volume)
This represents the percentage of pigment in the paint layer after everything has fully dried and is what most people think of when talking about ‘pigment load’. And sure enough, if we look at the paints that were made, the division we noticed between the visual appearance of casein, watercolor, and egg tempera vs. oil, acrylic, and encaustic is clearly echoed by the sharp increase in the percentage of pigment in each of the dried films (Table 2).
Critical Pigment Volume Concentration (CPVC)
As the ratio of binder to pigment changes, one reaches a sweet spot where the pigment is at its maximum loading while still having all the air between the particles completely filled with binder. This optimal point is known as the Critical Pigment Volume Concentration, or CPVC. While every paint system will be different, the CPVC generally falls somewhere in the 30-60% range. As one moves along this continuum (Image 2), and past the CPVC, one moves towards a paint film that has an increasingly large number of voids, which in turn leads to a layer that is more matte, more permeable, and increasingly fragile.
Looking at these stages more diagrammatically, if we were to peer inside a paint film at different stages, we would see something similar to the following contrast between a starting point of dry pigment alone, a paint film at CPVC, and one that is well above that level (Image 3).
The actual surface of a paint film above CPVC, with a 60% pigment volume ratio, is dramatically captured in the following electron micrograph (Image 4).
As the surface texture changes, the appearance of the paint can change dramatically as well, as we saw in the contrasting samples shown earlier.
When paints are at or below CPVC, their smoother surfaces scatter less light, allowing more of it to penetrate and be absorbed by the pigment. As a result, the color will feel more saturated and deeper in value, as well as typically appearing more transparent since the difference in the refractive index between the pigment and binder is far less than the pigment and air. A smoother, glossy surface also reflects light away from the viewer in a more orderly, controlled manner. While one might occasionally get a sense of a highlight or patch of glare, one almost never sees the type of diffuse scattering associated with matte surfaces (Image 5).
As a paint film climbs above CPVC it becomes increasingly matte and textured until, if pushed far enough, the pigments become underbound and only partly held in or coated by the binder. At this point the pigment scatters light to a far greater degree, since there is a much wider difference between the pigment’s refractive index and that of the surrounding air. In addition, the rougher surface scatters light in a far more random pattern, and this haze of white, diffused light appears to blend with the color of the pigment, causing it to seem lighter and often chalky or washed out by comparison (Image 6).
Lastly, the overall scattering is affected by the number of voids or air pockets found within the paint as these will scatter additional light that happens to penetrate below the surface, causing the color to appear quite opaque as a result; the internal haze of light acting like a form of interior fog that blocks any ability to see the surface below.
Robert Feller, a major figure in conservation science, happened to illustrate many of these changes in appearance in 1981 by measuring the surface reflectance of Ultramarine Blue paint formulated at an ever increasing PVC. Readings were taken at 440nm, which is the wavelength of maximum reflectance when this pigment is fully encased in a binder (Image 7).
As you can see, as the pigment volume ratio crosses the CPVC mark of 40%, there is a sudden and quite dramatic increase in the amount of light that is reflected from the surface, going from a mere 10% to nearly 60% by the time 80% PVC is reached. If we now look at the spectral reflectance curves from our six swatches of Ultramarine Blue, we see a similar uptick at the 440nm mark. Here the three paints with PVCs running from 14-46% (oil, acrylic, and encaustic) indeed top out a little above 10% reflectance, while the three systems with PVCs running from 76-81% (watercolor, egg tempera, and casein) reaches reflectance levels of 50% or more (Image 8).
While Cobalt Blue is clearly a different pigment, with a very different spectral reflectance and refractive index, we can still see a similar pattern, albeit not quite as dramatic (Image 9).
While there can be little doubt that Pigment Volume Concentration impacts the ultimate appearance of a color, there are other differences that need to be looked at and which can be easily overlooked. To do that, we looked at three paints in our study – casein, acrylic and oil – and unpack the implications of their different PVC ratios, especially given their very different formulations.
If we take the PVC ratios of dried paint films for Cobalt Blue in acrylic, oil and casein, we get the following diagram showing the relative amounts of pigment to binder for each system (Image 10).
In this view, acrylics and oil do not appear wildly different, even if oil does have a PVC that is 8% higher. However, when placed next to Casein, which has an incredibly high pigment load of 72%, that difference seems to pale by comparison. However, focusing on these numbers alone can give a somewhat misleading sense of the systems as a whole. To do that, we need to add back in the one large component that is missing – namely the water found in both acrylic and casein – while keeping in mind that oils, by their nature, are a 100% solids system with nothing that evaporates away. This leads to a very different picture (Image 11).
This is a very different type of comparison and one that points to important issues masked by the broader, simpler PVC ratios when taken in isolation. All of a sudden one can see that acrylic and casein have similar percentages of pigment when compared to their overall systems (10%), and because water is necessary in their formulations, there is no practical way they can ever match the 28% pigment load of Cobalt Blue in oils. Thus the very real and frequent sense that oils possess a density that is unique and unrivaled, and that goes directly to not only the nature of oil and pigment when milled together, and the fact that oil molecules are exceedingly small and very efficient at wetting out pigments, but the sheer fact that nothing evaporates or leaves the film. Acrylics, and other water borne media by contrast, must always contend with having to accommodate a large percentage of water in their formulations. So, even when the final pigment to binder ratios might be close, as with Cobalt Blue in acrylics and oils, the actual experience of the paint in its wet state is of something with far less pigment load and density.
In Image 12 the only change is the removal of water, thus showing an illustration of the pigment to volume ratios after they have dried. Continuing our unpacking of how simple ratios can sometimes mask other relationships, we can see how the original PVC percentages do continue to hold true. The initial 10% pigment in the acrylic does represent 20% of the final dried film, while with casein, because we rounded numbers to make the graph easier to read, the ratio of 10/13 comes to 77% rather than the desired 72% that was reported. But still roughly correct. Also note the comparatively small percentage of binder remaining in the casein, not to mention how much thinner the resulting film is due to the extremely high percentage of water at the outset. Overall, it speaks to a film that, while being exceptionally matte and opaque, is also extremely brittle and porous and suitable only for inflexible supports. Acrylics, on the other hand, are by far the most flexible of the three systems, and have a strong enough and high enough level of binder to allow them to be reduced with as much as 1:1 with water and still produce a durable film with good adhesion, while the clarity of the acrylic binder allows for the Cobalt Blue color to retain its saturation and clarity far into the future. Oils, on the other hand, need to constantly contend with having a binder that will eventually grow yellow over time, and the 72% of oil in this color suggests why so many manufacturers will grind Cobalt and other blues in safflower or poppy oil, even though those oils produce weaker and more fragile films – a trade off one needs to be careful about.
i A brief note is also in order on the issue of the refractive index of both casein and egg yolk. While The Science of Painting (Mayer,Taft, 2000) gives this as 1.338, which is similar to sources reporting the refractive index of milk or casein dissolved in water, most commercial references list the refractive index for dry or powdered casein as between 1.54-1.67, which is what we have decided to use here. Similar discrepancy can be found with egg yolk. In “Light: Its Interaction with Art and Antiquities” (Brill, 1980) a figure of 1.353 is given, which is a common figure given in many commercial reports on egg constituents in their liquid form, while Alan Phenix, Conservation Scientist at the Getty Museum, in his article “The Composition and Chemistry of Eggs and Egg Tempera”, reports a refractive index of dried egg yolk at 1.525, which is likewise what we have chosen to report. This divide comes about as both egg yolk and casein, in their natural states, are complex emulsions where a high percentage of water plays a large role and causes the refractive index to appear to be much lower then it eventually becomes once everything has evaporated and formed a solid film. As it is this dried state that we are studying, and which ultimately shapes our perception of the paint swatches, we feel that the refractive index values given for dried egg yolk and powdered casein are more accurate.
ii Feller, Robert L. Figure 3, Percent reflectance vs. pigment volume concentration, ultramarine UB-6917 in dammar, in “The Effect of Pigment Volume Concentration on the Lightness or Darkness of Porous Paints.” Preprints of Papers Presented at the Ninth Annual Meeting, Philadelphia, PA, 27–31 May 1981. Washington, DC: American Institute for Conservation of Historic and Artistic Works (AIC), 1981, pp. 66-74.