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Blackboards, also known as chalkboards, were originally made from sheets of slate, a fine grained metamorphic rock derived from clay. Slate had the double advantage of being easily split into fine sheets, and having a surface just rough enough for chalk to adhere to. Quarrymen would split the sheets of slate and dress them into squares or rectangles to be mounted in classrooms.
Chalk originally was also a rock: a soft, white, porous limestone composed of calcium carbonate.
Blackboards or chalkboards were used in schoolrooms from at least the early 19th Century right down to the present, but more recently have been largely supplanted by whiteboards or dry erase boards which do not suffer from the dust problem generated by chalk. Chalkboards themselves, where still used, have not been made from slate for a long time, nor has chalk been made from limestone. Both were replaced by manufactured substitutes.
Despite the chalkboard being today pretty much obsolete, you will still hear the term "chalk talk" used when a speaker or teacher illustrates points by drawing or writing on a dry erase board or easel tablet.
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