About Tile

Ceramic tile is a mixture of clays which have been shaped and fired at high temperatures to produce a hard body. This hard body may then be left untreated or it may receive a glazed wear layer.

The clay body, also known as bisque, is made of various types of clay and minerals which give the bisque strength and stability. The bisque’s strength is also a matter of its density. The strongest bisques, those typically used in commercial applications, have the smallest and fewest numbers of air pockets. This density also determines if a tile can be used in outdoor applications. Tiles which absorb more than 3% moisture are considered too porous and will freeze and crack in cold climates.

The glaze on tile is a liquid glass that has been sprayed or poured over the tile. The glaze is then fused and hardened through tremendous heat. Colors found in glaze are made from a mixture of minerals such as gold, silver, cobalt and many others. Like bisque, the glaze’s strength and wear resistance is determined by hardness. Hardness of a glaze is a matter of three factors:

• Temperature: higher kiln or oven temperatures produce harder glaze
• Color: dark-colored glazes are typically softer than light-colored ones
• Gloss level: shiny glazes are typically softer than matte-finished or satin-finished ones.

Porcelain Enamel Institute Wear Rating System

Most manufacturers use the following P.E.I. guide to rate the durability of each tile. The ratings go from Group 1 to Group 5, with Group 5 being the most durable. It is important to note that wear rating of the tile has nothing to do with quality or price, only durability. Some of the most expensive and luxurious tiles in the world rate only in group 1 or 2.

Tile Density Terms

Tile density is measured by the amount of water it absorbs.

Non-vitreous Tiles: are ones that absorb 7% or more of their weight in water.
suited for indoor use only.

Semi-vitreous Tiles: absorb 3% to 7% water.
suited for indoor use only.

Vitreous Tiles: absorb 0.5% to 3% water.
suited for both interior and exterior use, these are frost-resistant tiles.

Impervious Tiles: are the strongest. They absorb between 0 and 0.5% of their weight in water.
Suited for both interior and exterior use, these tiles are frost-resistant.

Production Methods

There are several methods for making tile, but two methods are most common for producing tile.

Monocottura

Italian for "single-fired," this is the newest method of producing tiles. The tile is shaped, glazed and fired in one step which speeds up the production time of tile resulting in lower priced products. These tiles also have a higher density than previous methods of construction and result in a flat backed product which is easier to install.

Bicottura

Italian for "double-fired," the clay body is baked on the first pass, then glaze is applied and a second baking occurs. Bicottura tiles can go through the kiln as many as four times resulting in a softer body and weaker glaze. Bicottura has been largely replaced by monocottura, but is still considered the best method for making decorative wall tiles which require multiple colors.

Types of Tile

Glazed Tiles

Glazed tiles are made the same way that unglazed are, except that a glass wear layer, or glaze, is applied. Most glazed tiles are not suited for heavy commercial installations, but are suitable for light to medium residential or commercial use. Advancements in technology have increased the durability of glazed tile so that some may be used in heavy commercial settings. Glazed tiles allow manufacturers to produce an unlimited array of colors and designs and the glazed surface is non-porous making it practically stain proof. While most quarry, porcelain, terra-cotta and mosaic tiles are unglazed; all of them can be produced with a glaze.

Unglazed Tiles

True inlaid tiles which are baked pieces of clay whose colors run throughout the entire body, unglazed tiles are generally thicker and denser than glazed tiles. Unglazed tiles include quarry and porcelain tiles, and are ideally suited, though not limited to, heavy commercial installations. Unglazed tiles are limited to the natural colors of the clay, but some manufacturers achieve an array of colors by adding pigments in with the bisque. Unglazed tile also provides a measure of slip-resistance because of the texture of its surface, making them ideal for use in wet areas.

Quarry Tiles

Made form natural clays and shale, these tiles are unglazed with a thick and dense body making them popular for heavy commercial and residential use. Colors are generally limited to earth tones ranging from red to a light tan, though some come in a color array featuring added pigments.

Porcelain Tiles

Porcelain tiles are made up of special clays and minerals similar to those found in chinaware. These special clays can be fired at extreme temperatures (2500 degrees Fahrenheit) resulting in a denser and harder body than most other tiles. Porcelain tiles come in plain, rough and polished finish and are highly stain resistant and strong.

Terra-Cotta Tiles

Machine-made terra-cottas are much denser than handmade tiles and usually can be installed outdoors. Handmade tiles are very porous and must be sealed and waxed to prevent staining and wear and are not suitable for outdoor use. The most popular handmade tile is the Saltillo tile from Mexico, which are crudely made varying in size, shape and texture from one tile to another and are occasionally marked with animal tracks.

Mosaic Tiles

Mosaic tiles are small, typically less than six square inches in size and come mounted on perforated paper sheets or latex mesh for easier installation. Mosaics are typically very dense and stain-resistant and are ideal for shower floors or counter tops because of their toughness and flexibility to follow the contours or areas and slopes due to small size.