CERAMIC TILE

Historical Background

The history of tiles dates back as far as the fourth millennium BC where in Egypt tiles were used to decorate various houses. In those days, clay bricks were dried beneath the sun or baked, and the first glazes were blue in color and were made from copper.

As early as 4,000 BC, ceramics were also found in Mesopotamia. These ceramics bore decorations which were white and blue striped and later possessed more varied patterns and colors.

In China, the great center of ceramic art, a fine, white stoneware with the earliest Chinese glaze was produced during the Shang-Yin Dynasty (1523-1028 BC).

Through the centuries, tile decoration was improved upon as were methods of tile manufacture. For example, during the Islamic period, all methods of tile decoration were brought to perfection in Persia. Throughout the known world, in various countries and cities, ceramic tile production and decoration reached great heights. The tile mosaics of Spain and Portugal, the floor tiles of Renaissance Italy, the faiences of Antwerp, the development of tile iconography in the Netherlands, and the ceramic tiles of Germany are all prominent landmarks in the history of ceramic tile.

In the early days, the tiles were hand-made - that is to say - each tile was hand-formed and hand-painted, thus each was a work of art in its own right. Ceramic tile was used almost everywhere - on walls, floors, ceilings, fireplaces, in murals, and as an exterior cladding on buildings.

Today ceramic tile in the United States, as it is throughout the world, is not "hand-made" or "hand-painted" for the most part. Automated manufacturing techniques are used and the human hand does not enter into the picture until it is time to install the tile.

As in the early days, ceramic tile is still used indoors as well as outdoors. It can be found on floors, walls, counters, fireplaces, fountains, exterior building walls, etc.


Raw Materials

The majority of raw materials used by the ceramic industry are the oxides of metals. The three metals which have been the mainstays of the industry for many years are clay, flint, and feldspar. These are the major materials contained in what is sometimes referred to in the industry as "classical ceramic bodies."

Clay

Clays are hydrated aluminosilicates and are the end-product of the weathering of feldspathic rock. The most important clay mineral is Kaolinite which has the composition AL2O3 - 2SiO2 - 2H2O. Clay is the material that gives a ceramic composition the plasticity which facilitates the fabrication of the material into the desired form prior to heat-treating.

Flint

Flint is a form of silicon dioxide (SiO2) usually produced from quartzite, sand or rock. It is used in a finely pulverized form as a filler to give the clay and final product the desired properties.

Feldspar

Feldspar is a broad, generic name applied to a group of alkali-aluminosilicates. For example, feldspars in which the alkali is potassium (K2O - Al2O3 - 6SiO2) are called "potash feldspars," and those containing sodium (Na2O - Al2O3 - 6SiO2) are called "soda feldspars." Most feldspars, however, are combinations of these two types. Feldspar is used and known as a "flux" in the ceramic industry. The flux is the material which starts to melt at the lowest temperature during the heat-treating process, thereby acting as the cementing element which gives the ceramic body its strength.

A great many other naturally-occurring minerals and some synthetically produced chemicals are used as raw materials by the ceramic industry. A few of the more important ones, along with their chemical formulas, are now presented:


Raw Materials Listing


Naturally Occurring (Chemical Composition)

Clay (Al2O3 - 2SiO2 - 2H2O)

Feldspar (K2O or Na2O - Al2O3 - 6SiO2)

Flint (SiO2)

Whiting (CaCO3)

Magnesite (MgCO3)

Talc (3MgO - 4Si2 - H2O)

Nepheline Syenite (K2O - 3Na2O - 4Al2O3 - 9SiO2 + Feldspar)

Zircon (ZrO2 - SiO2)

Borax (Na2O - 2B2O3 - 10H2O)

Pyrophyllite (Al2O3 - 4SiO2 - H2O)

Spodumene (Li2O3 - Al2O3 - 6SiO2)

Beryl (BeO - Al2O3 - 6SiO2)


Artifically Produced (Chemical Composition)

Alumina (Al2O3)

Litharge (PbO)

Zinc Oxide (ZnO)

Tin Oxide (SnO2)

Barium Titanate (BaO TiO2)

Lead Zirconate (PbO ZrO2)

Iron Oxide (Fe2O3)

Silicone Carbide (SiC)

Titanium Carbide (TiC)

Boron Nitride (BN)

Cobalt Oxide (CoO)


Ceramics - A Definition

Ceramics are defined as products made from inorganic materials having non-metallic properties, usually processed at a high temperature at some time during their manufacture.

The word "ceramics" comes from the Greek word "Keramos" meaning "Pottery", "Potter's Clay", or "a Potter". This Greek word is related to an old Sanskrit root meaning "to burn" but was primarily used to mean "burnt stuff".

The technical definition of ceramics encompasses a much greater variety of products than is normally realized. To most people, the word ceramics means dinnerware, figurines, vases, and other objects of ceramic art.

The majority of ceramic products not generally recognized as such are much more recent in development and are, in general, utilitarian rather than aesthetic. Examples are bathtubs, washbowls, sinks, electrical insulating devices, water and sewerage pipes, bricks, hollow tile, glazed building tile, floor and wall tile, earthenware, porcelain enamel and glass.

Ceramic products have a number of outstanding properties which determine their usefulness. One of the most unusual of these is their great durability. This durability can be divided into three types: chemical, mechanical and thermal.

Chemical Durability

The high chemical durability of the great majority of ceramic products makes them resistant to almost all acids, alkalies, and organic solvents. Of further importance is the fact that ceramic materials are not affected by oxygen. The materials generally contained in the ceramic products have already combined with all of the oxygen for which they have an affinity, and therefore, are not affected further by the presence of oxygen in their environment.

Mechanical Durability

The mechanical durability of ceramics is evidenced by their strength and hardness. The compressive strengths of ceramic materials are extremely high, normally 50,000 to 100,000 lbs/sq. in. This hardness makes ceramic materials very resistant to abrasion. It is this property which makes them useful for floors, and for the grinding of metals and other materials.

Thermal Durability

Most ceramics have the ability to withstand high temperatures. This is why they are useful in the production of all types of heat-containing equipment such as kilns for the ceramic industry, and such products as the inner linings of fireplaces and home heating furnaces.


Ceramics - Methods of Manufacture

Ceramic products can be divided into three broad groups according to the method of manufacture. They are as follows:


Group 1 Ceramics

Group one comprises pulverized products in which the raw materials have been heated to impart latent cementitious properties which become active upon the addition of water. This group includes cements, limes, and plasters.


Group 2: Ceramics

Group two consists of products which are heated until fluid and shaped while in this state prior to solidification by cooling. The most important material in this category is glass.


Group 3: Ceramics

The third group to which ceramic tile belongs consists of products which are shaped by various methods of compacting finely divided powders, followed by a heat-treatment which promotes partial fusion or sintering of these particles thus imparting strength to the formed object. The great majority of ceramic products are manufactured by one or more of a number of forming methods such as dry pressing, plastic molding, extrusion, and casting.


TILE FORMING PROCESSES

The processes specifically used for ceramic tile manufacture include 1) dry (dust) pressing, 2) auger extrusion, and in some cases, 3) plastic molding.


Dry (Dust) Pressing
The Dry Pressing method is used for forming many types of ceramic products, including tiles. However, the term "dry pressing" is not strictly accurate because in many cases, the materials being pressed contain from 3% to 15% water. Pressures used in pressing vary from a few hundred lbs/sq. in. to 100,000 lbs/sq. in. The pressure needed depends upon the material being compacted, the shape of the piece, and the quality necessary for the ultimate use for which the piece is intended. Pressing is usually accomplished in steel dies using either hydraulic or mechanical means of producing the desired pressures.


Plastic Molding
Plastic Molding is one of the oldest methods of ceramic forming, since it can be readily performed with little or no machinery. Tiles produced by ancient civilizations were made by plastic molding. Today, this method is not used much in the ceramic tile industry, except possibly in the production of ceramic-mosaic tiles.


Auger Extrusion

In Auger Extrusion, the rotation of an auger-type screw is used to force a plastic body through a confining chamber with a die located at its end. Quarry tile is produced by this method. It is a continuous process with the material being fed into the chamber located at one end of the screw and forced out the die end. The continuous ribbon of extruded material is then cut to the desired lengths by a wire-type cutter.


HEAT TREATING PROCESS
As was mentioned earlier, ceramic tiles are processed at a high temperature at some time during their manufacture. In the ceramic industry, this heat treatment is usually referred to as "firing" and furnaces in which the heat treatment is performed are called "kilns." The temperatures used in firing ceramic products range from approximately 1100 degrees F. to 3200 degrees F. Temperatures as high as 4200 degrees F. can be obtained in some special type kilns.


Types of Kilns
A kiln, in the broadest sense, is merely an enclosure for containing heat and the material to be heated. The sources of heat are usually electric elements, gas, oil, or coal. The enclosure itself is built of ceramic refractories (materials which resist the effects of heat), while the inner lining of the enclosure is composed of high melting-point refractories. This inner lining is backed by ceramic materials which are good heat insulators. These serve the purpose of keeping as much heat as possible inside the enclosure.

The kilns used for the production of the great majority of ceramics can be divided into two major types, periodic and continuous. All of the earliest types of kilns were periodic. In this type, the tile to be heated is placed in the cool kiln. The kiln is then heated to the required temperature, allowed to cool, and the tile is then removed.

Continuous-type kilns are called "tunnel kilns" since they are essentially long tunnels through which the tiles pass on either cars, belts, or sliding slabs. Tunnel kilns are from 80 or 90 feet to over 300 feet in length. The entrance and the exits are only a few hundred degrees above room temperature, but the temperature at each succeeding point in the kiln increases as the midpoint is approached, and reaches a maximum at a point near the center. The temperature at any given point within the kiln remains constant and is monitored by sensing equipment in the form of thermocouples which feed data into a computer.

The tiles enter at one end, are gradually heated (Pre-heat zone) as they progress slowly towards the center (Firing zone) of the kiln, and then are cooled slowly as they approach the (Cooling zone) opposite end of the tunnel. This process is widely used in the ceramic floor and wall tile industry. The most efficient and economical use of these kilns requires constant flow of tiles through them twenty-four hours a day. Therefore, most large ceramic tile manufacturers have firing crews working around the clock.


CERAMIC TILE PRODUCTION PROCESS

Dry-Press, Single-Fire Methodfor White-Body, Glazed Ceramic Tile Several of the tile-making processes as well as some of the specialized pieces of equipment were identified above. We will now describe the total production process for the manufacture of white-body, glazed ceramic tile. This type of ceramic tile is used widely throughout the US for a myriad of applications.

The production of white-body, glazed ceramic tile begins with what is known as "dust," a combination of raw materials which will ultimately be formed into the tile "body." As was discussed earlier, "dust" is composed mostly of talcs and clays which are formulated to give a desired strength and shape to the finished tile. In this case, the "dust" is composed of 62% talc and 38% clay (Ball clay + China clay/Kaolinite).

Quantities of these materials as determined by specific formulation are put into a ribbon blender which blends the clays and talcs together. From the blender, the mixture is transported to a mix-muller where it is mulled or kneaded by two steel wheels weighing 1800 lbs each. During the mulling procedure, water is added to the dusty mixture to bring its moisture content to 8% by volume. The moisturized mulling of the dust causes the separate particles of talc and clay to adhere to each other thus completeing the combination of the materials into a solid, dusty state.

From the mix-muller, the "dust" is transported to a pulverizer which breaks down the dust globules created by the muller into a fine, dusty form. From the pulverizer, the "dust" is put into a storage bin where it is allowed to set for a given period of time while the moisture content of the full load of "dust" becomes uniform due to capillary action of the moisture which has been added.

From the storage bins, the "dust" is transported to the press room where the second stage of the production process takes place. Electrically and/or hydraulically-driven presses are employed to transform the "dust" into a solid body of specific size, shape and tensile strength. In the pressing process, the "dust" is molded into shape with pressures of about 2,400 lbs/sq. in. on four cavity (4-1/4"x4-1/4", etc.) presses. The "greenware" or "body" thus formed by the press has a tensile strength of about 8 lbs/sq. in. The body is then stacked on metal racks and is now ready for the next stage of the production process.

From the press room, the racks of tile are moved to designated areas to await space in the drying rooms. The tiles are allowed to set at room temperature for a period of from 12 to 24 hours during which time they will lose by evaporation approximately 2% of the 8% moisture content. After this waiting period, the tiles are put into a drying room for the first stage of the drying process. The drying process consists of three 12-hour cycles. The cycle temperatures are approximately as follows: First Cycle - 100 deg. F .- 110 deg. F., Second Cycle: 160 deg. F. - 170 deg. F., and the Third Cycle: 210 deg. F. - 230 deg. F. After the 36-hour drying period, the "body" is void of moisture and has a tensile strength of 16 lbs/sq. in.

The "body" is then placed on the spray booth chain where the fourth stage of the production process takes place. This moving chain carries the tile body through the spray booth or area where the ceramic tile glaze is uniformly applied to the face of the tile to a thickness of between 17 and 20/1000ths inches, depending upon the glaze.

The glazed tile is then removed from the spraying area and placed in fire-resistant setters which are likewise placed on small cars which move on railroad-type tracks through the tunnel kiln. The cars are moved through the kiln by means of a chain which is driven by a motor and reduction gears so that the car with its tile travels at a speed of 1.4 inches per minute. During the approximately four hours the cars are passing through the firing zone of the kiln, the tile is exposed to radiant heat of about 2000 deg. F. At this temperature, the glaze on the face of the tile becomes fluid and gasses are released. The glaze actually attacks the body of the tile absorbing some of the chemical properties of the body and creating a bond between the glaze and the body. It is during this period that the inherent shade variation in ceramic tile actually occurs.

After the tile has completed the firing cycle, it is stacked on pallets and taken to the sorting department where it is separated by shade and grade. To insure uniformity in shading, one person does the shade separation for each complete run of a single color. The tile is shaded and graded from a continuously moving belt. The grades into which tile is separated are established in accordance with specifications written by the U.S. Bureau of Standards, and the U.S. Department of Commerce. The grading of the tile is the last step of the production process before the tile is packaged. The graders fill the cartons and seal them.

From the grading and packaging area, the cartons are moved by conveyor belt to the storage area where they are set aside by order to await transportation to the distributor.