Chalk is one of earth's minerals. We find it in thick layers, both on and below the ground. How it got there is one of the most fascinating stories of life on earth. The chief ingredient in chalk is calcium carbonates also known as lime. Our vast stores of chalk were gathered together and spread out in layers by some of the smallest creatures that share our world with us.
These little fellows are protozoa, one‑celled creatures rated among the simplest of living things. The chalk making protozoa are called foraminifera, a fancy word which, in plain language, means the window makers. These little fellows build a shell of lime to protect their soft bodies. But they are careful to leave at least one window in the walls through which they can keep in touch with the world.
They live in a watery world, for most of them dwell on the floor of the oceans or drift in salty sea water. We must admit that the window makers are a very successful group of animals. Their family tree goes back 500 million years. We have found their fossil shells dating back to the dawn of life on earth.
In their long history, the window makers have gone through changes. At one time in the dim past some of them made limy shells the size of quarters. Nowadays, the largest of them are about the size of pinheads and most varieties are barely large enough for our eyes to see.
The shapes of the limy houses vary. Some are flat, some coiled like snail shells. Some have many windows and some have but one. Through these the little fellows send out threads of their soft bodies. These threads form nets to trap food ‑ bacteria, diatoms and bits of floating debris.
These tiny creatures have lived and died in countless tons through the ages. Their vacant homes sink to the sea floor in layer upon layer. Seas and land sometimes change places and, when an old sea bed becomes dry land, we often find it covered with layers of these limy shells, mixed with mud. This is a bed of chalk, sometimes 1,000 feet thick. There are beds of chalk in Georgia and the Mississippi Valley and, of course, the White Cliffs of Dover are made of chalk.
Nature's chalk is refined to make our sticks of blackboard chalk. It is washed free of clay and mud and finely powdered. A little cementing material, about 5%, is added to the purified powder to hold the sticks together.
Chalk also has a host of chores in industry. In natural form it is sometimes added to cement. Purified and powdered it is called whiting which is used in paints, ink, paper, rubber and, of all things, toothpaste. Yes those busy little window makers help us put a polish on our teeth.