If you make pottery or clay sculpture, you need a kiln. Essentially an airtight oven, a kiln heats to high enough temperatures to cause clay to 'fire', which means that the glass particles in the clay melt and fuse, making it immune to dissolving in water.
Kilns are usually electric, sometimes gas fired, and of varying sizes. Temperatures in a kiln are strictly controlled and the oven is highly insulated. A kiln is an essential tool for any potter.