Blackboard ‘chalk’ are produced especially for use with blackboards in white and also in various colours. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk rock or limestone, while colored or pastel chalks are made from calcium sulfate in its dihydrate form, CaSO·2HO, derived from gypsum. Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO (calcite). As compared to whiteboards, blackboards still have a variety of advantages: On the other hand, chalk produces dust, the amount depending on the quality of chalk used. Some people find this uncomfortable or may be allergic to it, and according
Einstein's Blackboard writing on the blackboard, although ephemeral in nature, is of historic interest because the equations displayed are taken from a model of the universe proposed by Einstein in May 1931 known as The Friedmann-Einstein universe. The last three lines on the blackboard are estimates of the density of matter in the universe "ρ", the radius of the universe "P" and the timespan "t" of the expansion of the universe respectively ("L.J." on the blackboard indicates "light years" in German). It has recently been shown that these estimates contain a systematic numerical error. The blackboard is considered a "mutant" object or