|Monthly Tech-Tip |
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. 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. Wall tile does not have to be any of these, although of course, many types would benefit having one or more of these properties, density especially.
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. Small scale operations can employ electric kilns that heat tiles top-and-bottom (to keep them flat), either continuously or periodically. These are available from hobby kiln manufacturers.
Huge quantities of unglazed porcelain tile are produced. Planarity is a big problem (because porcelain shrinks a lot on firing), so much so that tile 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 a 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.
It would seem that tile needs to be vitreous to be strong and resistant to liquid penetration and crazing but this is not the case. Well fitted engobes and glazes can make it possible to use non-vitrifying bodies of surprisingly high porosity to produce tile of very good quality. Such bodies are easier to fire flat because they shrink less.
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.
This is the dolomite body recipe L4410P. In the plastic form we have measured it’s drying shrinkage at 6%. It has no firing shrinkage at cone 04. The final size needed is 20.5 cm. Thus I calculated the cut size to be 20.5 / (100 - 0.06) = 21.8 cm (or 20.5 / 0.94 = 21.8 cm). Sure enough, this 21.8 cm square dried and shrunk down to 20.5. To keep these flat we put them between sheets of drywall, the process takes 2-3 days. As noted, during firing no change in size occurs.
Plainsman Snow clay (developed under code number L4410P) makes this QRCode mosaic possible. Each 8mm square porcelain pixel is glaze-glued onto a 21cm square 5mm thick ceramic tile made of Snow (simply rolled and cut and dewatered between sheets of Gyproc). During the bisque firing to cone 04, the Snow tile had zero shrinkage, from dry to fired, and therefore did not curl up at the edges. On refiring to melt the glaze it again stays flat. No other common plastic clay can do this! Snow continues this zero-shrinkage performance for seven more cones of firing (all the way to 4). The secret is the 40% dolomite it contains. We silicone-sealed this work front and back, now it is ready for outdoor installation.
Yes. The body is Plainsman M370 (~ 25 silica, 25 feldspar, 30 kaolin, 20 ball clay + talc to tune maturity), a plastic throwing clay with far too much drying shrinkage to be suitable for tile. It is 3.8 mm thick fired (vs. commercial tiles are 5-7mm) and 33cm (13 in) square. It dried absolutely flat between sheets of plasterboard. We have even achieved total flat drying at this size using Polar Ice. Bisque and glaze firing were on an alumina shelf in an electric pottery kiln (at 300F/hr up through quartz inversion on the glaze firing), a completely unsuitable method for firing tile evenly top and bottom. Cooling on both firings was free-fall in a fairly empty kiln. Yet, it is flat! And flexible enough that I could lay it on the cement floor and stand on it without it breaking! Of course, to produce these consistently, special furniture that sinks minimal heat and a kiln that can evenly apply it front and back are needed. This is doable for custom applications. Of course, to compete in the commercial market, they need to be dust-pressed and there are lots of specifications to meet.
This was done on an affordable RepRap printer. The red plastic templates were drawn in Fusion 360 and sliced and printed using Simplify3D. A wooden block was used to press these cookie cutters into the clay. The plastic wrap made sticking a non-issue (and rounded the corners nicely). Commercial bottled glazes were applied to this low fire talc body by brushing (in three coats) after bisque - the rounded corners make brushing easier. The tiles were fired at cone 03. This is an old classic design that I discovered when researching Damascus tile. The toughest obstacle was learning how to use Fusion 360. It turns out that cookie cutters are a starter project for many 3D software packages, there are lots of videos on making them.
Tile Center Inc website
Saltillo Tile page at Wikipedia
Inkjet Printing for Ceramic Tiles
Product Ferro Corporation
Ceramic Tile Manufacturing
|By Tony Hansen|
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