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Transcript of Chalkboard
Sticks of processed "chalk" are produced especially for use with blackboards in white and also in various colours. These are often made not from chalk rock but from calcium sulfate in its dihydrate form, gypsum. Chalk sticks containing calcium carbonate typically contain 40-60% of CaCO3
(UK English) or
(US English; also blackboard) is a reusable writing surface on which text or drawings are made with sticks of calcium sulphate 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 Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.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.
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.
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 odour.
Chalk writing often provides better contrast than whiteboard markers.
Chalk can be easily erased; writing on a whiteboard left for a prolonged period may require a solvent to remove.
Chalk can be easily removed from most clothing; whiteboard markers often permanently stain fabric.
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 (
), 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.