Chelsea Petersen, of Kaikoura Suburban School, asks :-

How is chalk made?

David Hawke, a chemist at the Christchurch Polytechnic Institute of Technology, responded.

Chalk is one of those unusual substances which occurs naturally but is also made industrially. Natural chalk is a very pure form of limestone which was formed in ocean sediments far from land. Many of the well-known chalk deposits, such as the white cliffs of Dover in Britain, formed during the Cretaceous Period when dinosaurs wandered the earth and the oceans were much bigger than they are now. Natural chalk is made of the skeletons of tiny marine organisms, mostly coccoliths and foraminifera. Because these organisms are so small (less than a hundredth of a millimetre), the grains that make up chalk are truly tiny.

Synthetic chalk has even smaller particles, around 150 times smaller than natural chalk. Synthetic chalk is made during soda ash production. Soda ash is an important industrial chemical (around 30 million tonnes each year), much of which is used to make glass. The part of the soda ash process involved in chalk manufacture uses limestone and coke to make a slurry of lime, which is then reacted with carbon dioxide. Another co-product of soda ash manufacture is high purity sodium bicarbonate, used in the food industry and in our homes as baking soda.

While blackboard chalk is the most obvious use of chalk, much larger amounts are used in making cement, cosmetics, fertiliser, paint, plastics, and putty. Natural chalk deposits in the North Sea and the Gulf of Mexico are also important sources of oil and natural gas.

Ask A Scientist, PO Box 31-035, Christchurch, New Zealand. email -:- questions@ask-a-scientist.net