Unglazed tiles consist of one layer - a ceramic shard. The color is obtained from the raw material or is colored by metal oxides throughout the tile body (tile). The surface can remain untreated (natural) or be processed, taking on various surfaces: polished, honed, rusticated, structured, etc. Unglazed tiles include:
- gres red;
- cotto (terracotta);
- clinker;
- gres porcellanato (ceramic granite).
Next, let's take a closer look at each of the listed types of tiles.
Gres red
Unglazed, red-burning tile with compacted shards, produced by pressing. The raw material is a mixture of red clays with quartz, spar and fireclay. A material widely used in Italy in the 1960s and 70s in the Sassuolo region, as the raw materials for the production of this type of tile are found there. Initially, red gres was not glazed. Typical standard sizes are 10x20 and 7.5x15 (imitation brick) centimeters, but other sizes are possible. Water absorption is less than 3%. It has high strength and abrasion resistance (abrasion resistance). Strength indicators are higher than those of natural stone. It is used for paving terraces and street spaces, mechanical workshops and other industrial areas with heavy traffic. Production, which was strong in the 1980s, is declining. There is a trend to replace this type with porcelain tiles for exteriors and single-fired tiles for interiors. This type of Italian tile corresponds to the category of tiles with stone-ceramic compacted shards, which are widespread in other countries.
Cotto (terracotta)
Most often, unglazed tiles with a red and porous base, produced by extrusion. A mixture of different types of natural clay without special sorting and mixing. The surface can be rusticated, smooth, polished. Cotto is the most ancient type of tile. The names “rusticated terracotta”, “Tuscan terracotta”, “Florentine terracotta”, “cotto Veneto” - they all define the same type of tile. Cotto is an Italian name, generally accepted and well known to us - terracotta.
The most typical sizes are 25x25, 30x30, 20x40, 40x60 cm. Unlike other traditional types, these tiles are unglazed. However, recently a new type of glazed terracotta has appeared, which refers to one-time fired tiles, produced not by extrusion, but by pressing. They are used in interiors to cover floors.
To give terracotta tiles greater color intensity and improve performance, they are impregnated with a special composition. Outdoor terracotta, on the contrary, is not subject to additional processing, as this reduces its frost resistance. The ratio to water absorption is determined by the raw material composition and process conditions and varies from 3 to 15%. Strictly speaking, terracotta occupies a middle position between ordinary brick and tiles such as cottoforte or red gres. Terracotta tiles create a rusticated, hand-crafted surface that ranges in color from pink-orange to red-brown.
Cotto tiles have a significant thickness of 15 to 25 mm and are produced by extrusion. Often, after drying (before firing), the tiles are subjected to the so-called “rusticature”, that is, special treatment to give the tiles an aged appearance. Tiles are fired at a temperature of 1000-1050 degrees Celsius.
Clinker
Unglazed and single-fired glazed tiles with compacted shards, produced by extrusion. The raw material is a mixture of various types of refractory (fireproof) clays, quartz, feldspar, fireclay. Clinker belongs to the class of tiles with enhanced physical and mechanical characteristics associated with high-quality raw materials and production technology. This type of tile can also be called extruded gres.
In the production of unglazed clinker, as a rule, natural ingredients are used without chemical additives and dyes. The color scheme is natural terracotta colors: all shades of red, yellow and brown. Glazed clinker is a one-time fired tile. A layer of glaze (up to 2 mm) is applied to the molded tile, then fired simultaneously with the shard (body, body) in tunnel kilns at a temperature of 1200-1250 degrees Celsius for 24 to 36 hours. Due to the high temperature and duration of firing, the body of the tile has very low porosity and, as a result, high strength indicators, resistance to abrasion and chemical agents. Main properties of clinker:
- Low porosity – low water absorption, stain resistance;
- High strength - the characteristic increases with increasing tile thickness, which varies from 12 to 30 mm;
- Abrasion resistance – resistance to abrasion, scratching, deep damage;
- Frost resistance - achieved due to low porosity;
- Chemical resistance – achieved due to low porosity.
In addition to ordinary flat base tiles, there are also three-dimensional clinker elements. Tiles of complex geometric shapes can be obtained using the extrusion method. These include steps with a “beak”, profile corners, architraves and other shaped parts, which make it possible to smoothly articulate horizontal and vertical planes, seal edges, etc. Thanks to this, when facing, it is possible to completely eliminate the use of homemade parts from cut tiles. A complete set of swimming pools is made from clinker: various corners, plinths, spillways, connecting and other structural elements. The peculiarity of clinker tiles is that they are laid with a wide seam due to dimensional fluctuations associated with production technology.
Gres Porcellanato
Tiles with a light or colored base, plain (monocolor) or fine-grained with a compacted, vitrified shard, produced by pressing. There is a wide variety of names for this product. The tracing-paper translation of this name into Russian is porcelain clay (tile). The technical and commercial name for this type of tile in some countries is as follows:
- Italy – Gres Porcellanato;
- Spain - Gres Porcelanico;
- England – Fully Vitrified Porcelain Tiles or Vitrified Stoneware;
- Germany – Porzellan Steinzeugfliesen;
- France - Gres ceramic;
- Russia – ceramic granite or porcelain stoneware.
Gres is a name for ceramic materials that are characterized by a compact crystalline structure of a shard with partial vitrification (the process of transformation into a glassy mass), therefore tiles of this type are typologically classified as stone-ceramic tiles. However, the significant difference between these two types lies in the water absorption rates. Stone-ceramic tiles have a water absorption of <3% (BI group), ceramic granite - <0.5%. The beginning of porcelain stoneware production dates back to the 1970s. It unfolded in Italy in the Sasuollo region. Ceramic granite arose as a simultaneous modification of clinker, gres and single-fired white-burning tiles. Since the 1990s, active growth in the production of this product began.
The distinctive features of porcelain stoneware are strength, resistance to abrasion, chemical and biological influences, frost resistance, and high bending strength. At the same time, the tiles are not subject to aging and retain their original properties and are not affected by the environment.