In the beginning, ceramic tiles were made by hand. Wet clay was shaped, sometimes with a wooden mold, and then left to dry in the sun or fired in a small brick kiln. While a handful of artisans still craft ceramic tiles by hand, the majority of ceramic tiles now go through a process called “dry pressing” or “dust pressing.” This process requires far less labor and time, which is why ceramic tile is not just for Middle Eastern kings anymore!
Ceramic tile begins life as a clump of earth — everything in the final product is a natural product. Each manufacturer has its own time-tested recipe for ceramic tile, but clay is typically the main ingredient, along with such other items as sand, feldspar, quartz and water. The ingredients are mixed and ground up into a ball mill to create what's known as the “body slip.” Body slip is used to differentiate the body of the tile from its glazed topping; it's the bagel to the cream cheese.
At this point, the body slip contains about 30% water. That moisture helps adhere the ingredients to each other, but as soon as its job is done, it's gone. To accomplish this, the body slip is put into a dryer and heated; the moisture content is then reduced to about 6%.
After drying out, the body slip becomes essentially powder or dust. The dust is placed into a large press powered by electricity or hydraulics. The press pushes the dust into a set size and shape with a force ranging from a few hundred pounds per square inch to 100,000 pounds per square inch. We’re talking serious pressure here.
The pressure provides the finished project with its tensile strength. While square or rectangular ceramic tiles are most common, presses may have shaped imprints to create ovals, diamonds and other unique forms. The shaped body is called the “bisque.” After the body is formed, it's dried out to remove all of the final traces of moisture.
Glaze is the shiny substance typically applied to one side of the tile. The word comes from the Old English word for glass. Glaze can be sprayed or silkscreened onto the tile, finished in matte and high-gloss. To give the tile its color, pigments are mixed into the glaze. Though glazing is a standard step for ceramic tile, it's not essential. Not every tile has to be glazed to be considered ceramic.
There is one qualification, however, that ceramic tiles do have to meet. They all have to be baked. Before a tile goes in the kiln, it goes by another name: “green tile.”
After the glaze is on, it's time to fire the tiles in the kiln. Traditionally, ceramic tile baked for several hours in what's known as a periodic kiln, such as a beehive kiln. Over the last century, however, it’s the continuous kiln that has made the production process of ceramic tile more efficient. Continuous kilns include tunnel kilns and roller-hearth kilns.
These new types of kilns are like the conveyor belt pizza ovens you’ve seen at less-than-authentic Italian eateries. Rather than sitting in the heat for hours, the tiles roll through the contraption. The heat inside the kiln is precisely monitored and controlled by computer.
In the first half of the tile's journey, things start to get warm. At the center point, maximum temperature can get as high as 2,500 degrees Fahrenheit (1,371.1 degrees Celsius). The higher the temperature, the stronger the tile.
As the tile makes its way to the opposite side of the tube, it gradually cools down. The cooling period isn't as passive as it might seem — tiles are still changing color. With these continuous kilns, the baking process has gone from hours to less than one hour. This allows the manufacturer to make a lot more tile at a reasonable price.
This process was expedited by the resurgence of the “monocottura method.” Monocottura, an Italian term meaning "fired once," gives ceramic tile much greater strength. This additional strength is what allows tile to go from a product best suited for walls to one that is strong enough for floors. After just one trip through a hot kiln, tile made with the monocottura method is ready to be sorted and distributed.
Colors & Patterns
If firing a tile just once makes it so much stronger, why fire it again? Well, if the goal is a tile with many colors or elaborate patterns, then that tile will be baked using the “bicottura” method. Though the prefix of the word indicates that the tile is fired twice, it can actually be fired as many times as desired. Before each firing, a different colored glaze is applied to the tile and the process is repeated until the chosen design is complete.
Slippery When Wet
Depending on its finish, a wet tile can be slippery. Most manufacturers have a rating system that is based on or supported by the American Society for Testing & Materials (ASTM). Many times you can find these ratings on the tile sample or in the product catalog.
The most common system rates ceramic tile abrasion, resistance and the overall durability of the tile. There are 5 classes to be aware of:
Class 1: No Foot Traffic
These tiles are suggested only for interior wall applications — not for flooring. Serious slippage.
Class 2: Light Traffic
These tiles are suggested for interior wall applications and residential bathroom floors only. Minor slippage.
Class 3: Light to Moderate Traffic
These tiles can be used for residential floors and wall applications, including bathrooms, kitchens, foyers, dining rooms and family rooms. Slight chance of slippage.
Class 4: Moderate to Heavy Traffic
These tiles are recommended for residential, medium commercial and light industrial floor and wall applications, including shopping malls, offices, restaurant dining rooms, showrooms and hallways. Rare chance of slippage.
Class 5: Heavy/Extra Heavy Traffic
These tiles can be installed anyplace. They will work for both floor and wall applications in airports, supermarkets and subways. Zero chance of slippage.
The ceramic tile you choose may also carry a rating for Slip Resistance, which is measured by its Coefficient of Friction (COF). The higher the COF, the more slip resistant the tile is. Consider selecting a high COF tile for areas that get wet, such as your shower or bathroom floor.
Other ratings listed by manufacturers may include: scratch resistance, moisture absorption, chemical resistance and breaking strength.