Chalkboard
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A chalkboard or blackboard is a reusable writing surface on which text or drawings are made with chalk or other erasable markers. Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.
A blackboard can simply be a piece of board painted with matte dark paint (usually black or dark green). 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 chalkboards are made of a rougher version porcelain enameled steel (black, green, blue or sometimes other colours). Porcelain is very hard wearing and chalkboard made of porcelain usually last 10-20 years in intensive use.
Blackboards are commonly used in teaching. The chalk marks can be quickly and easily wiped off with a damp cloth or a special blackboard eraser consisting of a block of wood covered by a felt pad. Chalkmarks made on some types of wet blackboard can be difficult to remove. Blackboard manufacturers often advise that a new or newly resurfaced chalkboard 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 made especially for use with blackboards in white and also in various colours. These are not actually made from chalk rock but from gypsum.
Blackboards have disadvantages: they produce a fair amount of dust, depending on the quality of chalk used. Some people find this uncomfortable or may be allergic to it, and there has been speculation about links between chalk dust and respiratory problems. The dust also precludes the use of chalk in areas shared with dust-sensitive equipment such as computers. These disadvantages have led to the widespread adoption of the whiteboard, which uses ink markers that produce almost no dust. However, these alternative methods of displaying information have drawbacks of their own.
The scratching of fingernails on a blackboard is a sound that is well-known for being extremely irritating.
Blackboards are also used in many establishments (typically Public Houses ) as a form of advertising often for upcoming events and menus - as well as to keep the score in Darts Matches.
[edit] History
The chalkboard was introduced around 1801[1]. The term "blackboard" was invented about 1820, while the newer term "chalkboard" was first coined in the 1930s.[2].
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