This elusive pigment may secretly be lurking quietly in any given tube of paint. It is used extensively in oils as an extender, but unfortunately its presence is not often disclosed on paint tubes. Since it is a pigment-- a white pigment that is transparent-- it may not be considered an additive, but it could be said to be a filler. It is actually quite useful when a painter is able to control its addition to the paint. Chalk in oil paint is almost transparent, but oddly enough it gives opacity to watercolor and is used in gouache.

The term chalk can also include marble dust, which comes in several very different particle sizes. The precise size of the chalk particles matters quite a bit. Sometimes the term chalk includes limestone or marble, but generally it refers to calcium carbonate. Artiscreation mentions that magnesium carbonate may be included as an impurity. In painting literature it is also sometimes called whiting.

Sometimes chalk is used as a base, and it is commonly used as an extender in oil paint. Unfortunately its use in titanium white can accelerate some forms of degradation, though this seems to happen to a lesser extent with chalk than it does with barium sulfate, another common extender. A fuller discussion of that study can be found in this article which was shared by Natural Pigments. We wish more paint companies would disclose its use in oil paint. While this color has an excellent blue wool scale rating, its role in lightfastness when mixed into other pigments is still being explored.

Monona Rossol mentions in her work to use the standards for nuisance dusts if working with chalk, and it can also contain impurities, so always handle pigments with care. It is available in several grades of coarseness or fineness. Sometimes PW18:1 Dolomite (with some magnesium carbonate) can be an impurity in chalk.