Ceramic Tile

Tile manufacture is the largest sector of ceramic industry. Engineers overcome the very difficult technical challenges of drying and firing defect-free, flat and durable tile. Potters can do it too.

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The tile industry is the largest sector of the ceramic industry. Few people realize the enormous scale of the industry - for example in Morbi, India alone there are 1200 factories! Tile production is done at a wide range of temperatures using all manner of body and glaze types and process methods. While some countries use little ceramic tile, others cover every surface of their buildings with it. Even sidewalks. The quality of tile a company produces is a real testament to the expertise of their technical staff, this is because the engineering challenges in tile production are daunting indeed!

Wall and floor tile make up the largest portion of production. Floor tile must be durable, non slip, dense, strong, easily cleaned, very flat. These are less important for wall tile.

Tile can be formed using many techniques. Hand rolling of plastic clay is surprisingly common in traditional techniques. Tiles as thin at 3/16" x 12" or more can be dried very flat between flats of plaster wallboard panels (although it takes days). Tile can also be extruded and wet pressed (e.g. RAM pressing). Engobe (having the same firing shrinkage), in plastic form, can be laminated in a thin layer on a dark-burning body to produce tiles that glaze as if they were porcelain. Pretty well any hand-fabrication method suffers from a critical issue: Floor and rack space. Drying is slow so the higher the production the greater the floor space needed. And dehumidifying capacity. Greater capacity can be added by using a tunnel drier (where draft, heat and humidity can be controlled).

By far the most common fabrication method is dust pressing. Giant hydraulic machines are used to press moist powder (at 500kg/cm2 or more) into flats that can be larger than a meter square! Since shrinkage is almost zero, little warpage occurs during drying. Glaze and engobes can be applied to the dry or newly pressed tiles.

In industry, firing is done in continuous roller kilns that heat so evenly top and bottom that little or no warping occurs (despite significant firing shrinkage). Not only this, tiles move thru the kiln quickly. This ability to quickly fire such large flat shapes planar is perhaps the greatest achievement in kiln firing technology. Although small-scale operations can employ electric kilns that heat tiles top-and-bottom (to keep them flat), either continuously or periodically generally they are technically incapable of producing the type of tile that industry can make.

Huge quantities of unglazed porcelain tile are produced. Planarity is a big problem (because porcelain shrinks a lot on firing), so much so that tiles have to be ground flat after firing (that alone is an entire industry also). To achieve the lowest possible firing temperature tile bodies can contain up to 60% feldspar with no silica.

Much tile is covered with a white engobe before glazing. This makes it possible to employ a local clay that fires buff, brown or even red yet have a porcelain-like surface on which to apply glaze. Technicians expend much effort to match the engobe fired shrinkage/COE and glaze COE to the body. These can be applied as powder layers during the dust-pressing stage of the tile (or as slurries after pressing).

Another big issue is dry strength of the pressed tile (since the clay is low in plasticity and therefore has low dry strength). Companies add binders and glues as hardeners and even incorporate fiber (e.g. fiberglass) to make the pressed tiles stronger and less likely to be broken during handling.

As noted, some types of tile do not need to be vitreous to be strong and resistant to liquid penetration and crazing. Well-fitted engobes and glazes can make it possible to use non-vitrifying bodies of surprisingly high porosity to produce wall tile of very good quality. Such bodies are easier to fire flat because they shrink less. Regarding flatness (planarity): When tiles exit the production line and they are not flat this is a big problem for technicians. It is very difficult for them to replicate production conditions in the lab. Often planarity issues trace to misfit of thermal expansions of engobe, glaze and body (if the former are not under compression to the correct degree they can affect flatness). Lab personnel cannot test things on production lines (it’s too expensive) and in the lab they cannot easily replicate what is done there. So they have to make sense of everything in theory before trying it in production and hope for the best. The longer their experience the more likely it will be a success.

The tile industry is at the leading edge of decoration technology. Of course there are many traditional methods of decoration, but today there are just three important words: "Ink jet printing". Manufacturers can make a tile look like marble by simply printing a picture of marble on it! Photo realism is possible. Print heads are as wide as the fast-moving stream of tiles passing under them and there are separate heads for each color. Entire industries have formed around every aspect of printing (design, ink chemistry, ink rheology, nano particle pigments, machine design). Printers can cost $500,000 and ink thousands of dollars a kilogram.

Technicians fight a constant battle against pinholes. Just one pinhole can ruin a tile. Companies must go to incredible lengths in material processing, recipe development and production parameter control to create a pinhole-free tile. As already noted, they struggle to keep the tiles flat through drying and firing. Matching glaze and engobe thermal expansion to the body is very important in keeping tiles flat.

Huge supply industries have grown to provide tile manufacturers with everything they need. Cutting-edge technology characterizes the equipment industry especially. And, as noted, decorating. Suppliers are constantly developing new lines of glazes and novel surface treatments. There are even companies that do nothing but build tile factories, complete and ready to move in and switch on. And, of course, material suppliers (e.g. frits, feldspar, silica, talc, clays, pigments). Suppliers provide enough tech support and expertise that tile manufacturers can even outsource much of their engineering needs. The reason many countries do not have tile manufacturing industries, even though they do have clay and energy, is because they do not have these suppliers.