Chalk /ˈtʃɔːk/ is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores.
Pig bladders were used in the 19th century to store an artist’s paint. The bladder would be sealed with a string and then pricked to get the paint out. This option wasn’t the best because it would often break open. American painter John G. Rand was the innovator who made paint tubes from tin and screw cap in the 19th century.