Wiki article on this topic: A blackboard is a reusable writing surface on which text or drawings are made with sticks of calcium sulfate or calcium carbonate, known, when used for this purpose, as chalk. Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.
A blackboard can simply be a board painted with matte dark paint. Matte black plastic sign material is also used to create custom chalkboard art. A more modern variation consists of a coiled sheet of plastic drawn across two parallel rollers, which can be scrolled to create additional writing space while saving what has been written. The highest grade blackboards are made of a rougher version porcelain enamelled steel. Porcelain is very hard wearing and blackboards made of porcelain usually last 10–20 years in intensive use.
Manufacturing of slate blackboards began by the 1840s. Green chalkboards, generally made of porcelain enamel on a steel base, first appeared in the 1960s.
Lecture theatres may contain a number of blackboards in a grid arrangement. The lecturer then moves boards into reach for writing and then moves them out of reach, allowing a large amount of material to be shown simultaneously.
The chalk marks can be easily wiped off with a damp cloth, a sponge or a special blackboard eraser usually consisting of a block of wood covered by a felt pad. However, chalk marks made on some types of wet blackboard can be difficult to remove. Blackboard manufacturers often advise that a new or newly resurfaced blackboard be completely covered using the side of a stick of chalk and then that chalk brushed off as normal to prepare it for use.
Sticks of processed ‘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, CaSO4·2H2O, derived from gypsum. Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO3.
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