Casein Blues: Staining

Ask the Experts: Technical Questions Answered for Artists

By Bradley Lance Moore

Casein Blues

Q. I’ve read a lot about casein paint, and I thought I fully understood its history, its decline in favor of acrylics and its recent resurgence. Nowhere have I seen any mention that the blue color stains the brushes and contaminates the water, water container, clean-up sink and all other colors. How could this medium have been practical, especially among pre-1960s illustrators, with whom it was so popular? How did they avoid the color staining and contamination?
Sal Amendola, Brooklyn, New York

A. Casein is a binder used in paint; it’s made by treating the protein found in skim milk with an alkali. This process makes a versatile paint that dries to a matte, inflexible film and, therefore, should be applied only to a rigid support. The pH of casein can be as high as 9.8 so, as with fresco painting, only pigments considered stable in high pH should be used. Because of casein’s alkalinity (high pH), an artist should avoid intensive skin or eye contact with this paint. And never allow casein paints to freeze, as this can damage the protein binder.

Your staining problem suggests phthalocyanine blue (color index PB 15, 16, 17), which is the usual suspect in these cases. This color is also called phthalo blue, monastral blue, monestial blue, heliogen blue or blue lake. Phthalo blue is usually prepared as a lake pigment—a pigment formed when an organic dye colorant is used to stain an otherwise colorless powder base, such as aluminum hydrate. By 1937, phthalo blue had become available as an artists’ material, about the time of the revival of casein tempera painting. As you’ve discovered, this high-tinting culprit can also be highly staining. In fact, it’s tinting strength can be 40 times greater than ultramarine! Watercolorists know that phthalo blue often penetrates into paper fibers, which then absorb the dye. Thus, the dye stains the paper and becomes difficult to lift.

Three characteristics of casein can encourage these staining tendencies. Firstly, casein is a matte, water-based medium that doesn’t envelop the pigment particle to seal in the color to the degree that oil or acrylic does. Secondly, casein’s high pH seems to aid the redissolving of some of the dye into the water. And lastly, sometimes manufacturers leave an excess of soluble dye on the pigment during the laking process. The literature mentions very little about this, so you’re right to assume that this blue is considered water-fast. For this reason, phthalo blue’s high-tinting strength makes it more pernicious when one or more of the three factors I mentioned come into play. To avoid this staining and contamination problem, opt for cobalt or ultramarine blue.

Find more “Ask the Experts” Q&As here! Bradley Lance Moore has both a master of science degree in painting conservation and a master of fine arts degree in painting. He teaches art history and studio art at the Maryland Institute College of Art in Baltimore and exhibits his artwork internationally. Visit his website at


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