Sally Berman, age 13, of Atlanta, Georgia, for her question:

What are bricks made of?

Brick making is older than recorded history    and we still use the same old dependable materials. Our remote ancestors no doubt noticed that the hardest dirt is sun baked mud. So they copied Nature's recipe and improved it to make bricks. They selected clay type soils made from fine particles and added water to make muddy paste. They patted this into usable shapes and let their bricks dry hard in the sun.

Clay type soils are plentiful, and a modern brick works digs its handy ingredients from the ground. Sometimes hard clay type shales are pulverized and added. The dry, fine particles are mixed with water and the paste is shaped into standard building bricks. The drying process is hastened and improved in a huge, very slow oven called a kiln. To make special firebricks for lining furnaces, alumina and silicon carbide are added to the dry ingredients. These are as durable as ordinary bricks and can withstand temperatures of 3000 degrees to 4000 degrees Fahrenheit.