![]() ![]() The mineral is mined in more than 90 countries, including Canada, Mexico, Spain and the United States. The most likely reason is that gypsum is abundant, easily mined and processed in enormous quantities. While historically chalk has been remarkably dusty, modern manufacturing methods, including baking the chalk and coating it with products like shellac, have reduced the problem for both materials. It’s not clear why gypsum has replaced chalk for writing on blackboards (which today are mostly green, but that’s another story). For the former, the chalk is baked, while with the latter, it is air-dried. With gypsum, it must also be dehydrated in a process that involves high temperatures to reduce its water content from nearly 21% to about 5-6% to make classroom chalk, the material is mixed, again, with water (and colored pigments, if desired), and to produce more exotic pastels, such as used for art drawings, pigments as well as clays or oils are also added. Nonetheless, both are susceptible to a process that produces sticks of themselves that, when pressed against certain services, leave washable marks.Īfter quarrying, each is crushed, ground, washed and sifted. Gypsum on the other hand, had been used primarily in construction, such as for the aforementioned mortar, as well as in the manufacture of windows. Traditionally chalk has been used for drawing and writing, and by the end of the 18th century, with advances in slate quarrying (slate was originally used for writing tablets and blackboards), writing slates covered with chalk letters, symbols, numbers and figures were commonplace. Gypsum’s origins are similar, but in addition to being comprised of the calcium produced by the deaths of millions of plankton, gypsum also contains some of the salt that was left behind as the ocean evaporated. Chalk is a limestone deposit created as plankton (tiny marine organisms) concentrate calcium in their bodies while living, then leach the calcium out after they die and settle onto ocean floors over millennia, large deposits are formed, and as the seas recede, chalks deposits remain. ![]() Similar and yet distinct, chalk is a base (an alkali that neutralizes acids) that is composed of calcium and oxygen combined with carbon (CaCO3), while gypsum is a salt (the product of a base and acid reacting and both becoming neutralized), made up of calcium and oxygen combined with sulfur.īoth are believed to be formed in similar fashion. Chalk (calcium carbonate) has been found in cave paintings that date back to 40,000 BC, while gypsum (calcium sulfate) has been used as a mortar for construction since the dawn of civilization, and is even found in the Egyptian pyramids. Notably, however, most chalk today isn’t technically chalk at all, but gypsum.Ĭhalk and gypsum have both been mined since ancient times. White, powdery and prone to sticking to those surfaces where it is put (and just as easy to wipe away), chalk and its accompanying board are excellent instructional aids. Ubiquitous in many classrooms since the 19th century, chalk and chalkboards are familiar to most of us.
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