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A blackboard (UK English) or chalkboard (US English) is a reuseable 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 Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.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 color 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). 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.
Chalk sticks 
Sticks of processed "chalk" are produced especially for use with blackboards in white and also in various colours. These are not actually made from chalk rock but from calcium sulfate in its dihydrate form, gypsum.
Advantages and disadvantages 
As compared to whiteboards, blackboards have a variety of advantages:
- Chalk requires no special care; whiteboard markers must be capped or else they dry out.
- Chalk is an order of magnitude cheaper than whiteboard markers for a comparable amount of writing.
- It is easier to draw lines of different weights and thicknesses with chalk than with whiteboard markers.
- Chalk has a mild smell, whereas whiteboard markers often have a pungent odor.
- Chalk writing often provides better contrast than whiteboard markers.
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 to the American Academy of Allergy, Asthma and Immunology (AAAAI), there are links between chalk dust and allergy and asthma problems. The dust also precludes the use of chalk in areas shared with dust-sensitive equipment such as computers.
The scratching of fingernails on a blackboard, as well as other pointed, especially metal objects against blackboards, produces a sound that is well known for being extremely irritating to most people. Many are averse also to merely the sight or thought of this sort of contact.
Etymology and history 
They use black tablets for the children in the schools, and write upon them along the long side, not the broadside, writing with a white material from the left to the right.
The first classroom uses of large blackboards are difficult to date, but they were used for music education and composition in Europe as far back as the sixteenth century.
The term "blackboard" is attested in English from the mid-eighteenth century; the Oxford English Dictionary provides a citation from 1739, to write "with Chalk on a black-Board". The term "chalkboard" was used interchangeably with "blackboard" in the United Kingdom in the early nineteenth century, but by the twentieth century had become primarily restricted to North American English.
The blackboard was introduced into the US education system from Europe in 1801. This occurred at West Point, where George Baron, an English mathematician, used chalk and blackboard in a lecture on September 21. James Pillans has been credited with the invention of coloured chalk (1814): he had a recipe with ground chalk, dyes and porridge.
See also 
|Look up blackboard or chalkboard in Wiktionary, the free dictionary.|
|Wikimedia Commons has media related to: Blackboards|
- Chalkboard gag from The Simpsons
- Interactive whiteboard
- Sidewalk chalk
- Sound of fingernails scraping chalkboard
- WebMD. "Reading, Writing, and Wheezing? Not Necessarily". Asthma Health Center. WebMD. Retrieved Sept. 19, 2000.
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- Owens, Jessie Ann. Composers at Work: The Craft of Musical Composition, 1450-1600. Oxford University Press, 1998.
- Entry for blackboard, n, in the Oxford English Dictionary (Third ed., 2011)
- Entry for chalkboard, n, in the Oxford English Dictionary (Third ed., 2011)
- Stephen E. Ambrose (1 December 1999). Duty, Honor, Country: A History of West Point. JHU Press. p. 19. ISBN 978-0-8018-6293-9. Retrieved 14 February 2013.
- Jo Swinnerton (30 September 2005). The History of Britain Companion. Anova Books. p. 128. ISBN 978-1-86105-914-7. Retrieved 14 February 2013.