A blackboard (also known as a chalkboard) 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 Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone. Modern versions are often green because the colour is considered easier on the eyes.

A blackboard can simply be a piece of board painted with matte dark paint (usually black or dark green). Black plastic sign material, using the trade name sintra 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 (black, green, blue or sometimes other colours). Porcelain is very hard wearing and blackboards made of porcelain usually last 10–20 years in intensive use.

Lecture theatres may contain a number of blackboards in a grid arrangement. The lecturer then moves boards into reach for writing and then move 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 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.

Source: Blackboard on WIkipedia

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