What do you think the first storage containers were made of? They may have been made of animal skins or wood on even stone. But around 8,000 B.C., people learned to dig clay from the ground, mold it into a shape, and let the clay dry out until it was hard. The clay containers worked well for storing dry things. But they were not watertight.
The Chinese discovered how to make clay watertight. If clay is heated to a high enough temperature for a long time, it changes chemically. Clay is made of many different minerals. Dirt and mud have some of these minerals, but not enough for the chemical change needed. The chemical changes make the clay stronger and watertight. Modern pottery ovens, or kilns, use gas, oil, or electricity to get the heat needed. The temperature must be about 1200 °C for 14 to 30 hours. This process is called firing.
In modern factories, the clay is shaped by a machine or in a mold. When the clay has been fired, it is ready for glazing. Glazing means covering the clay with a thin layer of liquid glass. The object is then fired again, and the result is a clay object covered with shiny glass. It is watertight and ready to use.
Picture Credit : Google
