Ceramic tiles are the result of a production cycle that is generally typical for most ceramic products. This process consists of several stages, during which the processing and processing of raw materials occurs and the finished product is obtained, that is, ceramic tiles. The tiles can be glazed or unglazed. In glazed tiles, the upper, relatively thin layer has a glassy structure, that is, the surface of the tile differs from its base and provides a visual effect (color, gloss, ornament, etc.), as well as a number of mechanical properties (water resistance, hardness, etc. .), which the tile base cannot provide. Thus, glazed tiles contain two layers with different structures: the glaze on the surface and the base located below. Unglazed tiles, on the contrary, have a uniform structure throughout their entire thickness. The manufacturing technology includes a number of production stages depending on the type of tile (glazed or unglazed). To begin with, we can distinguish three main technological cycles that cover the production of tiles of all types. The first cycle concerns the production of unglazed tiles . The second is the production of double-fired glazed tiles . This term means that the tile is subjected to two types of heat treatment: to strengthen the base and to stabilize the glaze and decoration that are applied to the fired base. The third technological cycle is the production of single-firing glazed tiles , in which the glaze and ornament are applied to a dried (but not fired) base, which then undergoes only one stage of heat treatment. That is, only a “single” firing is performed, during which the hardening of the base and the stabilization of the glaze occur simultaneously.
Raw materials
Mixtures of various materials are used as raw materials for the production of ceramic tiles, in particular:
- clay materials that provide the plasticity of the wet mass necessary for molding tile blanks, which already in their raw form have characteristics that make it possible to perform various operations with them (in particular, transport);
- quartz raw materials, mainly quartz sand, which forms the “skeleton” of the ceramic product, that is, it performs the structural function necessary to limit and control the change in the size of the product, which is inevitable during drying and firing;
- feldspathic and/or carbonate, that is, materials containing feldspars (aluminosilicates of sodium, potassium, calcium, etc.) or carbonates (in particular calcium), due to which, during firing, the desired viscosity is achieved, which ensures the glassy and dense structure of the finished product products.
Preparing the mixture
The preparation of the mixture consists of several operations that ensure the production of a homogeneous material, the desired grain size and water content necessary for subsequent molding. At this stage of production, the following is obtained: powder with a water content of 4-7% for compression molding or a mass with a water content of 15-20% for extruded tiles. In any case, there are three main operations at this stage: grinding , mixing, homogenization , and moistening . Some clarifications should be made regarding the preparation of powder for pressing (more than 95% of Italian tiles are produced using the pressing method). In this case, two technologies can be used: dry grinding of raw materials, followed by bringing the humidity level to the required values using humidifiers, and wet technology, when raw materials are crushed in water, followed by drying the slip by spraying. The choice of one technology or another depends on the type of mass and the characteristics of the product itself.
Molding
In Italy, most tiles are produced using the pressing method. When pressing, a powdery mass with a moisture content of 4-7% is compressed in two directions, usually under a pressure of about 200-400 kg/cm2. Under pressure, the granules move and partially deform, due to which even unfired tiles have the appropriate density and strength. Other types of products (burnt products, clinker) are produced primarily by extrusion, with the initial mass containing moisture from 15 to 20% depending on the type of product. The strip coming out of the extruder is then cut into appropriate sizes.
Drying
Drying plays an important role, since at this stage water is removed from the product, which is necessary for molding. Drying conditions are critical to product integrity and the process must be carefully controlled to avoid warping, cracking or other defects. In the production of ceramic tiles today, the most common drying units are hot air dryers. This installation ensures the diffusion of moisture, that is, its release to the surface of the product and its further evaporation and removal. The speed of the installation (the drying process lasts several tens of minutes) is ensured by good heat exchange, effective ventilation and the relatively high air temperature at which drying is performed.
Glazes and glazing
Glaze is a mixture of various minerals and compounds (frits, kaolin, sand, various oxides, coloring pigments that are applied to the surface of the product and melted). Upon subsequent cooling, the molten mass hardens, forming glass, which gives the top layer of the tile special properties. Making the glaze involves dosing various materials and grinding them in water. This produces a suspension with a water content of 40-50%, ready for use. As mentioned above, the glaze can be applied either to a fired surface (double firing) or to a dried surface as in single firing. Various types of equipment are used to apply the glaze (cone or spinneret machines, bucket or tubular dispensers, disc sprayers, airbrushes). They are included in fully automated lines, which also include machines for applying ornaments (for example, silk-screen printing machines). Some types of ornament can be applied after firing the glaze; in this case, further heat treatment (third firing) is required.
Burning
Through firing, tiles, like other ceramic products, acquire mechanical characteristics that make them suitable for various uses, as well as chemical inertness properties. These characteristics are a consequence of chemical reactions and physical changes that occur both in the tile itself and in the glaze (in the case of glazed tiles). Firing is carried out in continuous kilns, which are a tunnel through which the tiles move on special conveyors, being first preheated and then heated to the firing temperature, which, depending on the type of product, can be from 900 to 1250 ° C and more. After being in the firing area for a certain time, the tiles move further along the tunnel, successively cooling to a temperature that ensures its safe unloading from the kiln. During firing, various reactions occur, the results of which, in fact, determine the characteristics of the product. At high temperatures, a molten layer is formed, due to which the particles sinter. After cooling, the tile acquires a structure with high mechanical strength. Such structural changes, which play an important role from the point of view of the microstructure and consumer properties of the tile, are also accompanied by shrinkage, which is greater, the lower the porosity that needs to be ensured. In the last 20-30 years, firing technology has undergone the most significant changes. In the 70s, tunnel-type kilns were used almost everywhere for firing ceramic tiles, in which the tiles were placed in stacks or mounted in special fire-resistant cells. In kilns of this type, firing lasts from 12 to 24 hours, depending on the type of product and the degree of loading. At the end of the seventies, the technology of rapid firing with laying tiles in one layer using kilns in which the tiles move along a roller table was introduced and subsequently became widespread. In kilns of this type, firing lasts 40-70 minutes, depending on the type of product. This offers important benefits in terms of reduced energy consumption, uniform firing, production flexibility and automation.
Sorting
Firing completes the technological cycle of ceramic tile production. At the exit from the furnace, a finished product is obtained, with the exception of certain cases of special surface treatment, which may be used for certain products. An example of such a special treatment is, for example, the polishing of porcelain stoneware. Before entering the packaging area and then to the warehouse, the tiles are carefully sorted. This process is designed to solve three problems:
- reject defective products;
- separate tiles of the first grade from tiles of lower grades;
- group tiles of each grade into trading lots in terms of size (or, as is often said, “caliber”) and color (so-called “tone”).
The article was based on the following materials:
“How and Why” by Timellini Giorgio and Palmonari Carlo.