Most ceramic tiles are made from a combination of refined clay and ground shale or gypsum, plus talc, vermiculite, and sand; the latter elements control shrinkage. The combination of all these elements and water forms a mixture called a bisque. The bisque is formed into a tile by one of several different methods, all of which have the same goals; To cast the tile shape and remove the water from the bisque prior to firing. Most commercial tile manufacturers use a method known as extrusion. In this method, green (unfired) bisque is squeezed through a press into a die in the shape of a tile. Smaller manufacturers often use the ramming to form tiles-pushing green bisque into a die and stamping it down.

Another method of tile formation is to roll the bisque flat and cut the tiles with a form much like a cookie cutter. Modern craftspeople use a method not unlike that of their ancient predecessors, which is to form the tiles by hand using a wood or metal frame. Whatever the method of tile formation, the bisque must loose its plasticity before being fired in a kiln. All ceramic tile, whether glazed or unglazed, is fired; some varieties just once, some many times. The purity of clay, the number of firings, and the temperature of the kiln determine the priced of the tile. Because of the cost of energy, tiles that are fired longer and at higher temperatures are more expensive.

Kiln temperatures vary from about 900° to 2500° F. Lower firing temperatures produce more porous tile and soft glazes; higher temperatures produce dense, non-porous tile and hard glazes. Kiln time varies from several hours to several days. The less time spent in the kiln, the more porous the tile.

Glazes

The transparent or colored coating on the top side of a ceramic tile is called a glaze. Glazes made up of lead silicates and pigment and are brushed or sprayed onto the surface of the bisque and fired. Glaze can be applied to green bisque and fired with the bisque or applied to fired bisque and fired again. Unglazed tiles derive their color from the clay. Glazes add color and protect the surface of the tile. Additives are sometimes mixed with the glaze to produce textured tiles. Sawdust is one popular additive; in the kiln the sawdust burns, leaving the tile surface slightly roughened more slip resistant. Silicon carbide sprinkled on top of the glaze also roughens the surface.

Some manufacturers add bits of carbide and stone chips to the glaze to achieve interesting effects. The hardness of the glaze depends on the temperature of the kiln and the length of time the tile is in the kiln. It is important to know the quality of the glaze when purchasing tile. Soft glazes, for example are inappropriate for floors and counter top installations, where everyday use would chip the glaze. These same tiles are perfectly adequate for wall installations, however, where they are less likely to get knocked or bear weight.