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Pigment Volume Concentration and its Role in Color

Image 1: CAA Materials Panel Presentation Board showing Cobalt and Ultramarine Blue in watercolor, casein, egg tempera, acrylic, encaustic, and oil.

Image 1: CAA Materials Panel Presentation Board showing Cobalt and Ultramarine Blue in watercolor, casein,
egg tempera, acrylic, encaustic, and oil.

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.

Constants
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).

Table 1: Binders and Refractive Indexes

Table 1: Binders and Refractive Indexes

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).

Table 2: Pigment Volume Concentration (PVC) Percentages for both Cobalt and Ultramarine Blue in the 6 different paint systems.

Table 2: Pigment Volume Concentration (PVC) Percentages for both Cobalt and Ultramarine Blue in the 6 different paint systems.

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.

Image 2: Pigment Volume Concentration Ladder, showing increasing PVC ratios.

Image 2: Pigment Volume Concentration Ladder, showing increasing PVC ratios.

 

 

 

 

 

 

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).

Image 3: Illustrations showing dry pigment, the pigment when at Critical Pigment Volume Concentration (CPVC).

Image 3: Illustrations showing dry pigment, the pigment when at Critical Pigment Volume Concentration (CPVC).

 

 

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).

Image 4: Scanning Electron Micrograph of paint film above CPVC, with pigment volume ratio of 60%. Gloss And Surface Structure Through A Paint PVC Ladder, N J Elton, A Legrix. © 2008, Suroptic Ltd.

Image 4: Scanning Electron Micrograph of paint film above CPVC, with pigment volume ratio of 60%. Gloss And Surface Structure Through A Paint PVC Ladder, N J Elton, A Legrix. © 2008, Suroptic Ltd.

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).

Image 5: Reflection from a glossy paint film at or below CPVC.

Image 5: Reflection from a glossy paint film at or below CPVC.

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).

Image 6: Reflection from a matte paint film above CPVC.

Image 6: Reflection from a matte paint film above CPVC.

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).

Image 7: Percent reflectance of Ultramarine Blue at various Pigment Volume Concentrations. ©American Institute for Conservation of Historic and Artistic Works (AIC). ii

Image 7: Percent reflectance of Ultramarine Blue at various Pigment Volume Concentrations. ©American Institute for Conservation of Historic and Artistic Works (AIC). ii

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).

Image 8: Ultramarine Blue Spectral Reflectance Curves showing increase in reflectance at 440nm.

Image 8: Ultramarine Blue Spectral Reflectance Curves showing increase in reflectance at 440nm.

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).

Image 9: Cobalt Blue Spectral Reflectance Curves showing increase in reflectance at 440nm.

Image 9: Cobalt Blue Spectral Reflectance Curves showing increase in reflectance at 440nm.

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.

Cobalt Blue
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).

Image 10: Comparison of the PVC for Cobalt Blue in Acrylic, Oil, and Casein.

Image 10: Comparison of the PVC for Cobalt Blue in Acrylic, Oil, and Casein.

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).

Image 11: PVC for Cobalt Blue in Acrylic, Oil, and Casein with the water component for Acrylic and Casein added back in.

Image 11: PVC for Cobalt Blue in Acrylic, Oil, and Casein with the water component for Acrylic and Casein added back in.

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.

Image 12: Showing the relative PVC for Cobalt Blue in acrylic, oil and casein after drying.

Image 12: Showing the relative PVC for Cobalt Blue in acrylic, oil and casein after drying.

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.

5 Responses to Pigment Volume Concentration and its Role in Color

  1. Koo Schadler January 19, 2017 at 11:27 am #

    Hi Sarah,

    This is an excellent article; such good and thoughtful research! It especially helps me to further understand and differentiate egg tempera from other mediums. I’ll be sure to share it with students and fellow tempera painters. Thanks to you and your collaborators.

    Koo Schadler

    • Sarah Sands January 19, 2017 at 11:52 am #

      Hi Koo – Thanks so much! Appreciate the positive feedback and so glad that you found the article useful. And definitely please share it with anyone that might benefit. Warm regards, Sarah

  2. Koo Schadler January 21, 2017 at 12:13 pm #

    Hi Sarah,

    It’s Koo again. In thinking about the article, I realize I have some questions. I’m confused what is the ultimately visual affect of PVC on chroma. On the one hand, high PVC means higher chroma. On the other hand high PVC means the irregular surface scatters light and creates a “haze of white, diffused light that appears to blend with the color of the pigment, causing it to seem lighter and often chalky or washed out.” So, does high PVC mean color is ultimately higher in chroma or washed out? Is it initially purer chroma that is subsequently diminished by scattered light? Does the local value of the color in question factor in; i.e. would the chroma of a very light value yellow be impacted differently by PVC than the more or less mid-value blues tested?

    Also, why do the colors you tested appear reddish in the acrylic, wax and oil binders? I know ultramarine has a fair amount of red in it, but I thought cobalt was more neutral..? In other words, what is producing the increased red, and would other colors in those binders also show hints of red or a more pronounced appearance of whatever is that color’s overtone?

    Well, as you know I tend to have questions. Thanks for being available to answer them.

    Koo

    • Sarah Sands January 23, 2017 at 5:44 pm #

      Hi Koo –

      Great questions and I will try to answer them as best I can. In terms of chroma and PVC, you are right that in general higher PVC should mean higher chroma up to the level of CPVC – or Critical Pigment Volume Concentration. It is when crossing that threshold that you begin to have the pigment interacting more and more with voids and the surrounding air, and thus introducing more of the haze from scattered light which will tend to lighten and desaturate the color. And yes, the value and likely even the hue of the colors will make a difference, with lighter colors (especially yellows) being far less impacted. but then, that is true when adding white to these colors as well, no? Not a dissimilar impact. And I can think of some colors that gain in chroma when white is added, or they have lower PVC, as in a glaze, such as Phthalo Blue – which in masstone, when richly pigmented, appears quite blackish but add a touch of white or use in a glaze, and the color is clearly much higher in chroma, becoming almost electric. Some of this is discussed in the article on the Subtleties of Color, which you can find here: http://www.justpaint.org/the-subtleties-of-color/

      The question of the apparent redness of the colors in oil, wax, and acrylic might be actually easier to see if thought of as the increased blueness of these colors in high PVC systems. That you can clearly see in the color spectra that are shared in the article, where clearly there is a very dramatic jump up in the reflectance of blue wavelengths. In particular Ultramarine Blue seems quite dramatic, jumping some 40% in that area. And while lesser on the Cobalt, the jump is still quite pronounced, and it is this contrast – of the very blue masstones of the high PVC paints, that make the ones where the pigment is fully encased in a binder appear much deeper and “redder”. And if you think of it, this is not unlike the cooling effect we experience when adding white to a color, especially if its Titanium Dioxide, with its strong scattering effect. Also, keep in mind hat we all settled on a specific shade of cobalt and ultramarine blue pigments and there are certainly many, many other shades that could have been chosen. So I do think overall the Cobalt Blue that was chosen is towards the warm side of that pigment, as too the Ultra blue. If you are used to working with a different shade it could definitely make sense that you might experience these shifts less dramatically.

      Hope that helps as always!

  3. Koo Schadler January 24, 2017 at 8:24 am #

    Yes, your answer is very helpful and interesting. I am always so impressed and fascinated by the complexity of color – there’s no end to learning about it. Thanks.

    Koo

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