Which one of the following is not a pigment?

In the context of paint chemistry, pigments are substances that provide color and opacity (hiding power). Zinc oxide (zinc white) and white lead (basic lead carbonate) are historically significant white pigments used for their high opacity and coverage [1]. Chalk, which is a form of calcium carbonate, is often used as a pigment in specific mediums like fresco or water-based paints, though it acts as a filler in oil-based paints due to its low refractive index [2]. Silica (silicon dioxide), however, is primarily classified as an extender or filler rather than a pigment because it lacks the necessary refractive index to provide significant opacity or color to a paint film. While it improves the durability and texture of the paint, it does not function as a primary coloring agent. Therefore, silica is the substance among the options that is not classified as a pigment.

Sources

1. [1] https://www.govinfo.gov/content/pkg/GOVPUB-C13-df487e45584ce466ccd4c8131523ddbf/pdf/GOVPUB-C13-df487e45584ce466ccd4c8131523ddbf.pdf
2. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 8: Limestone and Chalk Landforms > Limestone and Chalk > p. 76