A bright white mineral (chemical formula CaCO3) added to paper pulp as a filler in alkaline papers or applied as a coating pigment. Like clay and titanium dioxide, it is added to the papermaking furnish to increase brightness and opacity. Calcium carbonate is a better optical brightener than clay, but it is not as good as titanium dioxide. It is, however, as inexpensive as clay. Calcium carbonate cannot be used in the manufacture of acid paper, as it is an alkaline material which reacts strongly with acidic papermaking conditions. It is used in alkaline papermaking, which is receiving increased interest due to the longer longevity of alkaline papers. The presence of calcium carbonate in papers creates difficulty in offset lithography, as the alkalinity of the paper filler or coating tends to react with the acidic dampening solution used on the press to keep non-image plate regions receptive to water. Neutralization of the dampening system impedes its effectiveness. (See pH.) Calcium has also been known to react with materials in the ink and the plate, causing serious printing defects as well as damage to the plate and/or the blanket.
Calcium carbonate is the most abundant mineral on earth that is not silicon-based. It is also called limestone or marble. It is found in chalk, coral, the shells of mollusks and other marine creatures, and eggshells.
Calcium carbonate is also used as a white pigment in many printing inks. It is a dull white, but can be coated to impart greater gloss to the ink surface. It is low in cost, and is commonly used as an extender in place of the more expensive titanium dioxide. It is also highly reactive when exposed to acids, and is not recommended for use in lithographic inks. Calcium carbonate may also contain trace amounts of magnesium carbonate, silicon dioxide, aluminum oxide, iron oxide, and water. (See also White Pigments.)
('CI Pigment White 18 No. 77220'.)