Which Is The REAL Porcelain?
September 23, 2004
In recent years, porcelain tiles have grown tremendously in popularity, in particular within the retail sector. In many cases, end-users request porcelain tiles, but can't answer why they want porcelain nor do they know there are several different porcelains to consider. Hopefully, the following information will help clarify what can be a rather confusing matter.
Porcelain tile is a ceramic tile with some very distinct qualities. Before addressing the different types of porcelain, the positive characteristics that make porcelain an excellent choice should be noted. The term PORCELAIN TILE refers to an extremely compact, dry-pressed (dust pressed) tile produced with high quality raw materials and specialized production techniques. Porcelain tiles are pressed under very high pressure so the body is stronger than traditional ceramic and is fired at higher temperatures for longer periods of time than regular ceramic. The resulting tile has a dense, impervious body with extremely low porosity that provides the following advantages: • High resistance to bending • High resistance to rapid climate changes - Freeze/Thaw stable for exterior use • High level of precision in calibration and flatness • Extremely low porosity (always .5 percent or less water absorption) which in turn means the material is frost proof and more resistant to staining • Greater chip resistance due to the strength the porcelain body provides (porcelain is 30 percent more dense than granite!)
Unfortunately, as porcelain's popularity has increased, many designers and consumers mistakenly assume that all porcelain tiles are the same. This is not the case. Manufacturing methods, prices, installation applications and looks vary depending on the type of porcelain being produced. For instance, glazed porcelain does not typically offer the high levels of slip resistance, deep abrasion resistance and resistance to chemicals for which unglazed porcelains are famous. Because of these differences, it is important for us to take a deeper look at the different types of porcelains: Glazed porcelains: In this form of porcelain, the body itself is porcelain, but it has a glazed surface finish, so the color does not go all the way through. The porcelain bisques undergo a glazing and screen printing process (same as those used on traditional ceramics) which provides excellent aesthetic qualities along with outstanding technical characteristics. The addition of the surface glazing process to the porcelain family has somewhat confused the previously understood concept of what was a porcelain tile. With glazed porcelain, the hardness or abrasion resistance of the glaze, not the tile body, determines a tile's suitability to specific environments. Consequently, some glazed porcelains are only appropriate for light to moderate traffic areas. Customers must be referred to the ratings on the glaze for wear resistance (PEI, MOHS, etc).
Glazed Through Body porcelain takes the glazed porcelain and adds an additional advantage. In this case, the manufacturer adds pigments to the porcelain mixture prior to glazing so that the body of the tile will match the color of the glaze surface to be later applied. This provides the distinct advantage that if the glaze surface is chipped or worn, the color of the body beneath closely resembles the color of the glazed surface and therefore makes such damage unnoticeable. This added feature makes this type of porcelain often suitable for high traffic commercial applications.
Unglazed porcelain is a porcelain product with no surface glaze therefore having the same composition throughout the entire body; this is often referred to as "through-body". This is the originally-conceived form of porcelain tile and might be called the "real" porcelain tile. Through-body porcelain is manufactured as one monolithic tile without surface glazes or finishes. It is characterized by the color pigments being consistent throughout the body of the tile. Originally, the aesthetic look was limited to speckled, "salt & pepper" effects. Recent developments in the production process have made it possible to mix various color pigments so that random swirls and veins of color, similar to natural effects, can be produced. Unglazed porcelain provides the highest abrasion resistance due to the fact that the complete body of the tile is the wear layer; the tile exhibits the same properties throughout their thickness. Basically, the aesthetic features of unglazed porcelain can be achieved primarily by three different methods:
Applying soluble chemical salts to a through-body porcelain bisque prior to firing. Applied using multiple silk screens, spray machines or brushes, the overall aesthetic effect can be quite varied. Suspended in an aqueous solution, these metal salts penetrate into the unfired tile body. After the tile body dries, the tile is fired during which time the metal oxide salts react with the porcelain body resulting in various colors. This type of porcelain is classified as unglazed since the salts are not considered a monolithic glaze. Single powder (dry powder) charging is the production process by which charging of colored powders in the press cavity create the aesthetic beauty.
Double-charging (or double-loading) process is similar to the single powder except that a second loading of very fine colored powders is applied to the surface creating the aesthetic beauty. Double-loaded porcelain is manufactured with two layers of porcelain pressed together and then fired. The top layer is approximately 1mm thick and is pressed to the base porcelain layer under high pressure. This top layer is comprised of porcelain clays randomly infused with various color pigments that create an infinite range of color combinations and patterns emulating that of natural stones. This bonded porcelain ‘sandwich' is then fired.
Other methods will no doubt be developed and introduced as porcelain production continues to mature. On all unglazed porcelains, chipping or high abrasion does not change the visual surface of the tile. Additionally, unglazed porcelains will usually provide a high slip resistance, especially when wet, that glazed porcelains often do not. Just about all unglazed porcelains will exceed ADA slip resistance requirements. Through-body or unglazed porcelains are perfect for areas where abusive heavy traffic is anticipated. They will stand up to traffic in an airport, shopping mall, hospital, country club, downtown sidewalk, etc. for many, many years. Other processes that can be applied to porcelain tiles include:
Polished or semi-polished - After the production stages, the tiles undergo a polishing operation similar to the process for polishing marble. A few tenths of a millimeter of material are removed by polishing wheels producing a surface with a mirror-like finish identical to marble or granite. In a semi-polished product, the tiles undergo the same process, however the highly textured surface means that only the "high" areas are polished.
Rectification - In this manufacturing process, the finished tiles are taken out of the box and re-cut by a machine that makes each tile exactly the same size. In this case, the sides of the tile are shaved and calibrated to allow installation with a very narrow grout joint 1/8-inch (often referred to as "credit card" joints) that could not normally be achieved with non-rectified tiles. Some people mistakenly believe that rectified tiles can be butted together with no grout joint. This, however, is not recommended since most floors have imperfections requiring at least a minor grout joint to absorb.
When selecting porcelain tile, it is essential to specify the correct installation materials and methods. By ISO definition, glazed and unglazed porcelain must achieve 0.5 percent or less water absorption. This low porosity rate challenges a bonding material's performance. so Therefore, a premium latex modified mortar system is strongly suggested.
Bottom line - porcelain is so dense and tightly pressed that it will withstand much more pressure, loading and impact than any other product installed on the floor today. Porcelain tile is produced using the finest natural ingredients combined with a rigidly controlled manufacturing process that utilizes the most advanced processes and technology. Porcelain outperforms even granite for years of low-maintenance looks that last.