MITRA Forum Question Details

ApproveRejectUn-ApproveCalcite is a common constituent of sedimentary rocks, such as limestone. Calcite is also the primary mineral in metamorphic marble. Chalk is a form of limestone. Hence, marble and chalk are all forms of the mineral calcite. There are also human-made forms of calcium carbonate, known in the industry as precipitated calcium carbonate or PCC. Calcite, barite, kaolin, talc, silica and bentonite are all white pigments of low refractive index (less than 2). They are also known as extender or filler pigmentsin the paint industry, because they do not add to the opacity of paint, but can be used to increase the bulk of paint without lowering the pigment volume concentration (PVC). They also add important properties to paint, such as hardness, abrasion resistance, decreasing bulk weight, etc. Extender pigments are effective in bulking artists’ paint and adjusting the opacity, and a much more effective means of decreasing opacity by adding medium or oil to paste paint, which lowers the PVC. However, I must also caution about the excessive use of extender pigments in paint, because if used excessively they will result in more pronounced yellowing of the paint film. The best way to use extender pigments without increasing the PVC of the paint and the subsequent danger of creating an underbound paint film, is to grind the extender pigment in oil to form a stiff paste prior to adding to paint. —George O’Hanlon, Natural Pigments