ETYM AS. cealc lime, from Latin calx limestone. Related to Calz, and Cawk.
(Homonym: chock).
1. A piece of chalk (or similar substance) used for writing on blackboards or other surfaces.
2. A pure flat white with little reflectance.
3. A soft whitish calcite.
Soft, fine-grained, whitish sedimentary rock composed of calcium carbonate, CaCO3, extensively quarried for use in cement, lime, and mortar, and in the manufacture of cosmetics and toothpaste. Blackboard chalk in fact consists of gypsum (calcium sulfate, CaSO4.2H2).
Chalk was once thought to derive from the remains of microscopic animals or foraminifera. In 1953, however, it was seen under the electron microscope to be composed chiefly of coccolithophores, unicellular lime-secreting algae, and hence primarily of plant origin. It is formed from deposits of deep-sea sediments called oozes.
Soft limestone (made from gypsum) used in stick form as a drawing medium; mixed with pigment and a binding agent it is an ingredient used to make crayons. Chalk drawings exist from prehistoric times and chalk was popular among Renaissance artists, for example, Leonardo da Vinci. It is still used in art today, for example, by pavement artists as it can be erased easily. + prikaži više