Wednesday, October 18, 2017

Decoding Pigment Codes

limited palette painting demo in chromium oxide green and pyrrole red

     In the process of working on materials for my limited palette workshop at Evanston Art Center, I came across a number of interesting things in the world of colors and pigments and oil paints.

Now, I always assumed that a paint's pigment code (such as PB29 for Ultramarine Blue) was absolute. Not so much, really. It is pretty specific yes, but 2 paints from 2 different manufacturers can vary quite a bit even if they have the same pigment code. Also, 2 different-looking paint colors can be made from the same pigment, by varying the grind and including additives.

This whole labyrinth came to my attention when I realized that Cadmium Lemon and Cadmium Yellow Deep oil paints (both from Williamsburg) are each composed solely of PY35 pigment. Now, Cad Lemon is a lighter, cooler yellow and Cad Yellow Deep is more like an orangey yellow and darker in value. Pretty different while both obviously still being "yellow." How can they both be the same pigment? I emailed both Williamsburg and Gamblin these sort of questions and got helpful responses from both.

From Scott Bennett at Williamsburg/Golden:

"There are many examples of paint colors that have the same pigment ID number but exhibit different colors. Iron Oxides in general have a wide variety of colors due to little tweaks in trace elements and pigment size. Phthalo Blues and Quinacridone Reds can have the same number but with a color and a sub number indicating a variation. How the pigment is ground during paint making will change the color. PR 108,..the various Cadmium Reds, is similar but the differences have to do with the addition of selenium.

Differing hues of Cadmium reds, orange and yellow are made by increasing the percentage of selenium, zinc or sulfur during one stage of production. The range of cadmium pigments, yellow, orange, red are basically cadmium yellow (cadmium sulfide) with some selenium added in place of sulfur (cadmium selenide). Therefore cadmium sulfide can be made in various shades ranging from yellow, orange to red. Mineral pigment produced from cadmium sulphide when heated with selenium becomes red.

So it is not just one variable but a range of variables depending on the pigment. And the same pigment in different binders can look very different."

Another thing I found is that in researching colors, you really have to go to the manufacturer's site's to get accurate information on pigment content. It can seem handy to just look on the Blick site (or some other retailer), but they tend to just assume pigment content based on the name and I have found numerous errors.

Below, see Blick's pigment info on the left, and Michael Harding's on the R:

If you want to read info on specific pigments read up on:
The Color of Art Pigment Database

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