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A blackboard or a chalkboard 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 Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.
[1] [5] Most of the photographs show the chalkboard in a moment drawn from the mathematician's work on it, [1] depicting how mathematicians think, work, and communicate with each other. [5] Some other photographs show chalk drawings that were deliberately created to be photographed for this book. The mathematicians themselves are not depicted. [1]
That's a regular kitchen sponge. If sponges are ever used for chalkboard erasers it is so rare as to be invisible in all popular media from the time period in which chalkboards were common. Most of my public school attendence was during chalkboard usage and I didn't see anyone use a sponge to erase a chalk board even once.
Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world , use of carbonate-based chalk produces larger particles and thus less dust, and it is ...
William Brownie Garden (December 2, 1869 – 1960) was a Scottish inventor and entrepreneur. He is probably best known for his invention of the "revolving blackboard". Up until his death at age 90, in 1960, the ‘Inventor’ still worked at his own bench in the factory, from which came innovations like an improved saw which cut ten times quicker than any other, and an eight-wheeled car for ...
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