At one time, all material for tile basically came from the banks of rivers and streams. The clay was pure. As Tile makers gathered the wet clay, crudely processed it to eliminate stones and debris, they formed individual tiles by hand and let the pieces bake in the sun. Early in tile’s history, which dates back at least 6,000 years, tile makers developed frames for shaping the body of the tile. And then, historians speculate, someone noticed that the fired clay inside the ovens used for baking bread was sturdier than the sun-dried clay and was also unaffected by water. This significant observation led tile makers to try baking their tiles in these ovens and, in turn, to create special ovens, or kilns, for firing tile. These kilns were probably reserved for special tile, while ordinary tile continued to be baked in the sun.
After these advances—or perhaps before them—another important development took place that changed the laborious task of gathering clay, a process that involved moving the tile maker’s household each time a source of clay was exhausted. At some point, a weary tile maker must have realized that it was possible to extend raw clay by mixing it with ground shards of pottery and broken tile. This practice probably took hold quickly and set a precedent for later extending clay even further by mixing it with ground shale and eventually ground gypsum.
Tile Making of Today
Nowadays, while some commercially produced tile is made of pure clay or pure gypsum, the vast majority is made from a combination of clay, ground shale or gypsum, and other ingredients such as talc, vermiculite and sand. These latter materials serve both to extend the clay and to control shrinkage in the tile. After the clay has been refined and mixed with water and these additives, it’s shaped into a bisque, or biscuit, which is the body of the tile.
Bisque
The green (unfired) bisque can be formed in several ways. It can be extruded through a die, like toothpaste squeezed from a tube, or it can be rammed into a die, in the same way that a piece of metal can be stamped. It can also be cut from a sheet, cookie-cutter style, or be formed by hand in a wooden or metal frame.
Manufacturers who produce tile in great quantity form the bisque in a die, which yields uniformly shaped and sized tiles. Most tile today is made by the dust-press method, in which the moisture content of the dry ingredients is so low that they hold together only when rammed into a die. Tile producers with smaller, less mechanized operations and makers who work by hand cut the bisque from sheets of clay or form the tiles in frames.
However the bisque is formed, it must leather-harden, or dry enough to lose its plasticity, before being fired in a kiln. Most tile, whether glazed or unglazed, is fired only once, though some tile is fired several times, and some highly decorated tile may even undergo five or more firings. The number of firings, as well as the purity of the clay, largely determines the eventual cost of the tile.
The temperature at which tile is fired ranges from as low as 900°F to as high as 2500°F, but, generally speaking, most tile is fired at temperatures between 1900°F and 2200°F. Low-temperature firing usually produces porous tile and soft glazes, whereas high-temperature firing yields dense, nonporous tile and hard glazes. The amount of time a tile remains in the kiln also contributes to the tile’s porosity, and while some tile may be fired for only a few minutes, other tile may stay in the kiln for as long as a week.
The demand for commercial ceramic tile, efforts to control air and water pollution, and the need to conserve energy have led to the development of significant advances in kiln and production technology. With the latest equipment, ceramic tiles that used to linger for days within the kiln can now be formed, glazed, fired and packaged in less than two hours.
Glazes
Although the bisque may be the backbone of the tile, it’s the glaze that gets all the attention. The earliest examples found of tile finished with colored glaze date to ancient Egypt, where copper particles formed the base of this glaze. Today, most types of glaze consist of a transparent or opaque coating of silicates and pigment, which is brushed or sprayed on the surface of the bisque and fired. The glaze can be applied to the tile’s green bisque and be fired along with the tile itself, or it can be added after the tile has undergone one or more firings. Either way, it serves both to decorate the tile and to protect its surface. While much of the tile produced today is glazed, some, like quarry tile and many Mexican pavers, is left unglazed and derives its color only from the clay from which it’s made.
In some cases, a glaze is intended not only to color and preserve the surface of the tile but also to give it texture. Sawdust is one of several substances added to the glaze of some tile for this purpose. As the temperature in the kiln rises, the sawdust burns, leaving behind a bumpy covering that’s not as slick as the traditional, smooth glaze. For another type of tile, particles of silicon carbide are sprinkled on the wet glaze before firing to produce a nonskid surface. (Some unglazed tile has carbide bits pressed into its surface before firing for the same purpose.)
When selecting tile for a particular installation, consider the use the tile will receive and how its glaze will hold up.
Depending on the temperature at which it’s fired and the amount of time spent in the kiln, a glaze may be soft, easily scratched and unable to withstand the wear a floor or kitchen countertop receives. Or the glaze may be hard, quite durable and serviceable for any installation.
