[Pigments and Binders]

Calcium Carbonate

The pigment commonly known as white chalk is technically known as calcium carbonate. Calcium carbonate is mined for both industrial and artistic reasons. Items such as plaster and common classroom chalk are made from the mineral. The formula for calcium carbonate is CaCO₃ (w/ small amounts of MgCO₃, SiO₃, and [FeAl]₂O₃ present). Calcium carbonate is very important in the field of art because it serves as a universally mixable and matchable color. Calcium carbonate, once made into paint, can be mixed with any other paint in order to make what might be a very brilliant hue into something paler or lighter in color.

To discern the chemical properties of calcium carbonate a few things were done in lab. By mixing equal parts of sulphuric acid and calcium sulphate, a base, one will observe the "absorbtion [of calcium sulphate] in a solution of this reagent(H₂O + H₂SO₄) gives rise to the formation of acicular crystals, singly and in irregular aggregates."(Chimot and Mason 323) These acicular aggregates show up in the form of starlike structures. According to Chimot and Mason, these results are similar and indicative of the presence of calcium carbonate.

The chemical formula for the above reaction goes like this: CaCO₃ + 2H₃O(minus one electron) which produces H₂CO₃ + 2H₂O + Ca(minus 2 electrons) The same experiment was conducted again only calcium carbonate was substituted for the calcium sulphate to see if the reactions were similar, if they were calcium carbonate's presence was to be assured. The same crystalline stars appeared confirming the hypothesis. In addition to the stars appearing in the solution, there was a certain amount of effervescence or bubbling when the pigment and the acid were mixed. This is characteristic of an acid-base reaction.

When not in solution, calcium carbonate particles appear rather dull. Under a microscope with objective set on X40 and using transmitted light, the particles appear to be small round circles with seemingly transparent bodies and distinct outlines. Using the same objective the particles appear roughly 1/10 of an ocular unit in diameter.

Section II

This is calcium carbonate as a pigment in an egg tempera binder.













This is calcium carbonate as a pigment in an oil paint.

David Price, 1997.