Why do some Oil Pigments make Opaque Paints while others make Transparent Paints? A Book Review, kind of...
- Katherine Griffin
- 3 days ago
- 5 min read
Oil paints come with three levels of transparency: opaque, semi-transparent, and transparent. All three have their uses and can aid an artist in their art making. You can read more about how here.
Before I get too far into why some oil pigments lend themselves to opaque paints while others are transparent, I must first take you back to last Christmas.
Sometime last year I informed my husband I that I wanted the book The Artist's Palette, by Alexandra Loske.
True or false, does "the palettes behind the paintings of 50 great artist" in full color, with large image spreads, not sound absolutely divine? The resounding answer is TRUE! I imagined myself reading through lists of paints used by each artist with historical anecdotes about pigment history as one color came into vogue and then out of the collective artistic conscience, all in its full color coffee-table sized glory!!!
I will be giving a full review of the book shortly, but spoiler alert, I was disappointed. It all began on page 16 of the introduction with this statement:
"Most palettes used for oil- and water-based paint were made of thin pieces of polished hardwood, commonly cherry, walnut, or rosewood, although many other materials are also known, such as ceramic or enameled metal. The choice of material may depend on which medium is used or which colours are preferred. Oil paint is opaque, unless diluted heavily and used as a glaze, so a dark hardwood is suitable for oils..."
In this statement Alexandra errs in two places. One, while the choice of palette material is based on the type of paint used, oil and water colors have always used two very different types of palettes. In oil paint the palette material is very specific to the artist's choice of imprimatura (a future post on imprimatura is forthcoming). Which means a dark hardwood is not always suitable. Two, oil paints are not always opaque by nature. It is this second statement that we will strive to correct today.
Why do some Oil Pigments make Opaque Paints while others make Transparent Paints? Let's get into it.
Pigment Characteristics
There are a few characteristics of a pigment that must be taken into consideration in order to create the ideal paint out of it:
1. Refracting Index (RI): Refers to a pigments ability to slow down light and bend it.
The higher RI a pigment has the more opaque it is.
The lower RI a pigment has the more transparent it is.
The RI does not indicate how much oil is necessary to make a paint with the pigment. Some lower RI pigments require very little oil, while some higher RI require a larger amount of oil to make an opaque paint.
Particle Size: Refers to the ideal particle size of a pigment in order to create the ideal consistency of a paint.
The larger the particle size of a pigment, the higher RI it will have, and the more opaque the paint will be.
The smaller the particle size of a pigment, the lower RI it will have, and the more transparent it will be.
Particle size is important to note because some pigments are naturally more fine than others, while being ground at the same rate and with the same force. An example would be the difference between grinding a diamond verses a piece of charcoal at the same rate and force. You will have two different outcomes.
Particle size does not indicate how much oil it will take to make a paint.
Pigment Volume Concentration (PVC) or Oil Absorption Rate: Refers to the ratio of the amount of oil by weight that is needed to make an ideal paint for a specific pigment. This is usually expressed as the number of grams of oil needed for 100 grams of pigment.
It is interesting to note that the oil absorption rate does not indicate how fast an oil paint will dry. That is completely based on the pigment used and not dependent on how much oil is used to make the paint.
Critical Pigment Volume Concentration (CPVC): Refers to the minimum amount of oil that is necessary to add to a pigment in order to make the ideal paint.
This means that every pigment particle is fully coated and all of the gaps between the pigment particles are filled.
Too much oil to pigment ratio will cause the paint to bead on your canvas.
Too little oil to pigment ratio will cause the paint to be grainy.
To calculate the CPVC you must know the other three characteristic of the pigment. The formula, when using linseed oil as your binder, is:
CPVC = 1/(1+[(OA)(p)/93.5])
OA = oil absorption value or PVC
p = Density (g/ml)
93.5 is the density of linseed oil X100.
So lets put all of this together to really drive home my point:
To the left is a visual representation of the CPVC of Titanium Zinc White and Prussian Blue.
Titanium White is opaque and slow drying. Prussian Blue is transparent and fast drying. But they are both made with the same amount of oil!
Titanium Oxide is heavier at 3.84 to 4.26 g/ml. It has a higher RI and a larger particle size. It also has a low absorption rate of 20 wt/100wt.
Prussian Blue is lighter at 1.8 to 1.9 g/mL. It has a low RI and a very fine particle size. It has a high absorption rate of 40 wt/100wt.
Add it all together and Titanium White has a CPVC of ~55% and Prussian Blue has a CPVC of ~56%. MEANING, again, it takes the same amount of oil to make the opaque Titanium White as it does the transparent Prussian Blue.
WHAAAAATTTT!!! YES!
But we aren't going to stop there!
Drying Times
Although we have just disproven it, it can be easy to assume that an opaque paint is made with less oil and and therefore it must also have a faster drying time, but NO! The two paints' different drying times are based on the pigment's ability to chemically react with their oil binder, and not the amount of oil that is in the paint. Even though both of our paints are made with the same amount of oil, our opaque paint, Titanium Zinc White, has a low siccative rate and the drying process of oxidation is slowed down. Our transparent paint, Prussian blue, on the other hand has a high siccative rate and the drying process of oxidation is sped up. You can read more about how oil paint dries here.
In summary, "oil paint is {not} opaque, {only by being} diluted heavily." Oil paints are opaque, semi-transparent, and transparent all with varying amounts of oil based on the pigment's needs.
You can read more about Critical Pigment Volume Concentration by following these links:
What do think? A little much, I know. Possibly extremely nit-picky about one sentence in a very large book? Perhaps, but a point needed to be made! For no reason, other than that is has been bothering me for almost a year now. But also, there were multiple times I came across such sentences and thought, "No, I don't believe that to right." This happened often throughout the book, and I'm not even through it all. Anyways...
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Hoping you are having a great day!
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