Chalk and gypsum have both been mined since ancient times

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. After quarrying, each is crushed, ground, washed and sifted. 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. For the former, the chalk is baked, while with the latter, it is air-dried.


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