- THE MAGAZINE
Spain, too, has experienced an increase in demand for porcelanic gres (see Table 1). Production has increased threefold during the past four years-currently accounting for 5% of Spain's total tile output. And manufacturers in Mexico, Latin America, the Far East and even the United States are starting to gear up for production of this increasingly popular tile.
So what is porcelanic gres? And how can manufacturers make their porcelanic gres "stand out from the crowd"?
Characteristics of Porcelanic Gres
The primary characteristics of porcelanic gres are low porosity (absorption rates of less than 0.5%) and color that penetrates the entire body of the tile. Its basic composition is feldspar (40-50%), white clays (30-40%), quartz (0-15%), kaolin (0-15%) and other fusible material (0-15%).
Today, nearly half of the porcelanic gres produced is unglazed, while the other half is glazed. Unglazed porcelanic gres includes the following styles:
In addition to the above styles, several main decorative effects are currently popular with porcelanic gres. These effects can be grouped as double loading, single multi-color loading, rustics or special effects.
The double loading technique involves loading the mold first with body powder of two or three colors without prior mixing. During the second loading, uncolored or salt and pepper is filled in. This second loading can be granulated or micronized, depending on the specific effect that the manufacturer wishes to obtain. (Micronizing produces a smaller particle size.) The cost of producing this product is affected by the low speed of the press, which is 50% that of normal production speed.
The single multi-color loading effect is obtained by loading the mold once with several atomized powder colors and granulate colors. The colors are loaded by moving pipes or rollers. This enables the manufacturer to put colored veins on the tile-creating a natural stone look. In this case, installation is complex but press speeds are normal, and therefore do not negatively impact the cost.
Rustics are obtained by pressing an atomizer-resulting from the mixing of two or more powders of different colors-so that a random "detonification" effect is achieved. In some cases, this technique is coupled with glazing using a semi-transparent glaze.
Other special effects are obtained by using granulated glazes, special compositions, crushed frits, etc., which usually require sophisticated equipment.
While these are some of the most popular techniques, many others are also used to obtain unique looks. For example, some manufacturers choose to double charge or use soluble salts with salt and pepper. Many possibilities exist, but the goal is simple-to create tiles with the greatest aesthetic effects while retaining the performance characteristics of porcelanic gres.
Innovative Techniques for Enhanced AestheticsWith the competition for porcelanic gres increasingly fierce, manufacturers are constantly looking for new ways to differentiate their products from others on the market. To meet those needs, suppliers are developing new technologies such as fast coloring methods and adding frit to the body formulation-technologies that could vastly increase the aesthetic possibilities of porcelanic gres. In some cases, these techniques have yet to be commercialized, and some questions remain regarding various application scenarios. But the decorative potential that they unleash makes them worth evaluating ahead of time.
Fast ColoringSeveral monocolor effects are available using the fast coloring technique, including matte monocolor, polished salt and pepper and polished soluble salts. Others include granulated, double loading powder and sanded.
The goal with fast coloring is to cover all grains with a slight color. This is accomplished by crushing the body materials to an average size of 1 to 5 microns and adding very hygroscopic (moist) additives. When these additives coat the wet grains, they transport the color and distribute it among all of the grains. The final effect is a nearly uniform color. The slight differences in color intensity that can be observed in the tile result from varying humidity among different grain sizes. Coarser grains are wetter than finer grains of powder.
Low-speed mixers must be used to obtain the coloration of the powder externally because the grain must not be broken. This technique requires relatively short mixing times of roughly six minutes. It is important to measure the color in the mixer through a sieve to remove any agglomerate in the container caused by the extreme fineness of the pigment.
Using the fast coloring technique can affect the density and fired shrinkage of the colored body, and these effects must be accounted for in the manufacturing process. Depending upon the color used, a reduction in density (in raw) usually occurs. Generally, adding 1% color will reduce the density by 0.1 g/cm3 and the mechanical resistance (raw) in the mixture by approximately 20%. It is advisable not to decrease tile density to less than 20 Kg/cm2 if the tile will be decorated.
As for fired lineal shrinkage, the temperature increases when the bulk density in raw is lower. The water absorption of the fired tile (in standard conditions) is zero. And the internal porosity is similar to that of an uncolored tile body. It is highly recommended that manufacturers raise specific pressure to avoid high shrinkage. Raising the pressure will increase the bulk density and mechanical resistance, and thus decrease the shrinkage. In fact, a 25% increment increase in pressure makes all the technical parameters of the colored tile body equal to those of an uncolored tile body.
Adding Frit to the BodyAdding frit to the standard porcelanic gres body formulation affects the increment of the liquid phase in the sintering of the tile. This effect provides the ability to achieve new color tones and reduces both the necessary firing temperature and firing cycle.
The new color tones are possible because the frits are above the color saturation point of normal compositions. One example is the ability to attain "super white" shades. When screen printing on a normal porcelanic gres body (the normal body is a grayish white color), tile manufacturers can achieve intense colors by applying large amounts of pigment; however, the design resolution is lacking. To improve resolution, many opt to rotocoat, but this technique often results in a weak color intensity. The super white body enables manufacturers to use the latter technique to achieve high resolution without sacrificing strong color intensity. The result is an optimal decorative look.
Another benefit of adding frit to the porcelanic gres body is the lower firing temperature and reduced firing cycles that can be achieved. With an addition of only 6% frit, perfectly sintered tiles can be obtained at cycles less than 40 minutes with a maximum firing temperature of 1140°C. The technical properties of the tile remain the same, with the added benefit of improved stain resistance. Stain resistance improves because of the lower internal porosity of the tile.