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Ceramic tiles materials

 
 
Ceramic tiles are building materials used as finishing products and designed to cover floors and walls. A very ancient manufactured product, ceramic tiles are still used today in the most modern and advanced applications.
 
Ceramic tiles are slabs of various formats and variable dimensions (sides of lengths from a few centimetres to a metre and beyond, thicknesses from 5 to over 25 mm), made mixtures of clay, sand and other natural substances fired at high temperatures. This mixture of materials establishes the ceramic nature of the tiles. These mixtures are then moulded into the required shapes through special shaping processes, and finally fired in kilns at extremely high temperatures (between 1000 and 1250 °C).
 
Ceramic tiles have a lower environmental impact compared with other materials, thanks to an intense activity of technological, industrial plant, and production innovation conducted by the Italian ceramics industry. In order to establish the environmental impact on a product like ceramic tiles, the life cycle is analysed, including all stages through extraction, production of raw materials, scrapping, and final disposal of waste.
 
The first phase analysed is the extraction and processing of raw materials used in the production of tiles. The environmental impact of quarrying can be calculated on one hand by the modification of the landscape, and on the other by the consumption of resources, the release of dust, and the production of waste.
 
The production stage represents the main segment of the life cycle. The most significant environmental aspects associated with the manufacture of tiles are: gas emissions, water consumption and the disposal of waste water; waste/residues; energy consumption; noise.
Every kilogram of manufactured tile produces gas emissions of from 0.2 to 0.4 g of fluorine compounds; from 30 to 45 g of dust; from 0.01 to 0.1 g of lead compounds. Of these pollutant emissions more than 90% are eliminated by purification plants that reduce the respective emissions to: 0.02 to 0.04 g of fluorine compounds,; 0.2 to 0.3 g of dust; 0.001 to 0.01 g of lead compounds.
The water consumption of the Italian ceramics industry is lower than the basic requirement thanks to the reuse of waste water in the production process, limiting environmental pollution.
The Italian industries in this sector are also capable of recycling the majority of waste products. Ceramic production processes are characterised by considerable energy requirements.

On the strength of innovative technology and installations the entire sector today consumes less than half the energy compared with the 1970s.

The next stage in the life cycle of tiles is laying. The materials used for laying and joint filling have low relevance from a toxicological perspective.
Regarding safety, tiles present limited health and environmental risks associated with laying. The disposal of the tiles completes the life cycle of this ceramic product.
Demolition waste is inert by nature and can be disposed of in the environment without particular hazards. On the scale of useful life, tiles again emerge in top position compared with other finishing materials. A well laid tiled surface can have a very extended life and so generate less waste.

 

 

The market offers a wide range of different types of materials. Before describing each of these materials in detail, it is helpful to understand some basic information in order to choose correctly the products that will best suit your needs.

Surface
Tiles can be either glazed or unglazed. Glazed tiles are coated with a thin layer of glass; whereas unglazed tiles do not have a coated surface and body.
Glazes come in a wide variety of finishes and colours and the aesthetic effects they can produce may vary greatly.
Unglazed tiles, on the other hand, can be specially treated so that their surface appearances are altered, producing tiles that can also be: polished, semi-polished, lapped and satin-finished.

Body

The body is the structural portion of a ceramic tile. In glazed tiles the body is covered by glaze. The mixture from which the tile is derived can obtain its final shape through two different processes: pressing (the powder mixture is pressed and formed by high-pressure presses) or extrusion (the final shape is obtained by forcing the raw material, in the form of a soft paste, into special moulds).
The body of a tile may be either porous or compact, though the difference between a compact or a porous tile cannot be seen at a glance, it is important to bear in mind that tiles with a compact body will be more resistant to water absorption.
Depending on the raw materials used in the mixture, the body of the tiles may be of a wide range of colours: red, white, coloured.

Sizes and thickness

The most common shapes are square and rectangular (but occasionally other shapes can be found as well). Sizes range from just a few centimetres, like in “mosaic” tiles, to slabs of 60 cm per side or more.
Thickness also varies: the average thickness is 8/10 mm, while tiles for special uses may be over 2 cm thick.

Selection

During the firing process different factors (air moisture, kiln temperature, etc) can lead to different results; therefore tiles must undergo accurate checking in order to be classified in homogenous groups before they are finally packaged and shipped.
Firstly, materials are classified according to their defects: 1st choice (perfect tiles), commercial tiles, 2nd choice, 3rd choice (tiles with visible imperfections such as chips or cracks).
Then the tile’s work size and the shade are specified; the former indicates, in exact millimetres, the deviation from the nominal size, while the latter expresses the deviation from the colour originally intended.
 
Ceramic materials
Since the VI millennium BC, handmade ceramics have took part in history of human settlement. For quality and artistic value of products and decorations, ceramics are one of the most reliable indicator of material civilization and cultural evolution development. For its constant presence inhabited place, they have great relevance for archaeology, ethnography and anthropology studies, for art and technology history. Ceramic production which can be found, with partiality similar phases, in Middle East, in Meso-Americans and Andean cultures, in Japan and in Europe since Neolithic to the Modern Age, until the coming of industrial production, adding artistic valued handmade to quotidian use tolls.

Ceramics (from the Greek kéramos, clay, vaseclay) are products obtained through a mix of clays, water and eventual additives, that are dried and fired at extremely high temperatures, in order that they couldn’t come back to the plastic state.

The fundamental  raw material of ceramic production is the clay, sedimentary rock with aluminium silicate base. Its fundamental feature is the plasticity: if the clay is drenched with water, we will mould it in various shapes (it is used a lot by sculptors) that are maintained after the drying process.

There are many kind of clays that are different for the sand and impurity contents (impurity are responsible of the colours).  

Ceramics material are distinguishable in porous body ceramics and non-porous body ceramics.
 
Porous body ceramics are constituted by a tender and absorbent body that they let thyself be scratched easily by a steel point. They are made waterproof by a layer called (ceramic) glaze. Pottery, earthenware, terracotta and majolica are  porous body ceramics.
 
Non-porous body ceramics presented low porosity and good impermeability features to gases and liquids; they don’t let thyself be scratched by a steel point. Non-porous body ceramics are porcelain and grès or stoneware.

The main materials available on the market are:

  • Monocottura (single-fired)

    Single fired tiles are glazed, formed by pressing and are fired only once (both body and glaze are fired at the same time).

  • White body
    White body tiles are glazed, formed by pressing and are fired twice (first the body and then the glaze).

  • Majolica / Cottoforte
    Glazed double fired tiles with a porous and coloured body, formed by pressing.

  • Red Stoneware
    Unglazed tiles with a red and compact body, shaped by pressing.

  • Porcelain stoneware

    Unglazed tiles with a clear or coloured body, shaped by pressing. These tiles are extremely compact and contain almost no porosity. Porcelain stoneware is almost completely absorbent and thanks to its technical features they are now a common choice for all applications.

  • Klinker
    Klinker is the material used for the body of unglazed tiles. Klinker tiles have a coloured body obtained by extrusion.

  • Cotto
    Tiles with a red and porous body, obtained by extrusion.

     

The general properties of ceramic materials.

The hardness is a result of the reactions that occur during firing, creating a compact structure and high internal cohesion, while the nature of the chemical bonds ensure ceramic tiles an excellent breaking strength. They are capable of supporting very high loads without deforming or bending: they are therefore rigid. Another property associated with the ceramic nature of tiles is fragility, which defines their behaviour in relation to impact. Moderately impact resistant, ceramic tiles do not deform, unlike pliable materials. Furthermore, the high temperatures of the ceramic production process create a stable compound with almost zero reactivity to other substances. Ceramic products are therefore defined as inert, in other words they are insoluble and unalterable in contact with water and with the majority of chemical substances. Even the heat of a fire is unable to alter their composition. A final word of advice: it is important to remember that the undeniable qualities of ceramic tiles, in particular their mechanical and chemical resistance, are fully exploited and enhanced only if the tiled surface, the floors or walls, are correctly designed and realized.

Anti-static nature and fire resistance

Ceramic tiles are classed as electrical insulators: their electrical conductance can be extremely low. This property is very important for safety reasons. For applications with particularly strict anti-static requirements, like the flooring of operating theatres, laboratories, or chemical plants, special tiles are necessary characterised by their higher electrical conductance compared with conventional tiles. Ceramic products can also be considered safe even in the case of fire. The response of a material to fire is defined on the basis of certain characteristics, classified into three groups: resistance to the destructive action of the flames, any contribution that the material might give to feeding the fire, and the release of smoke or toxic substances in response to fire. In all three cases ceramic tiles emerge as one of the safest materials.

Functionality and care

Two essential properties of finishing materials are: durability, this being the capacity to maintain their technical and aesthetic functions through time, and functionality which is defined on the basis of the mode of application and replacement.
On tiled floors or walls it is important that the ceramic tiles are strongly and durably fixed to a suitably dimensioned and prepared support. The installation requires qualified personnel and the removal or replacement of tiles is difficult. Thus ceramic tiles are not among the most functional materials from the point of view of installation.
On the other hand, the toughness and the durability of the tile-base union ensures durable installation, placing ceramic tiles in top position compared to other materials like, for example, softer substances. Durability includes the capacity to resist attack from ambient conditions: abrasive wear, attack from chemically aggressive substances or liquids and environmentally contaminating substances.
Thanks to a high chemical inertia a tiled surface is not only less exposed to the risk of deterioration compared with other materials, but is also resistant to dirt and more easily cleaned. In general any form of staining can be removed from a ceramic tile. As regards maintenance, ceramic tiles are among the most functional materials.
 
Structural characteristics and regularity
The regularity of a batch of tiles is based on the measurements of the sides and flatness. The sides are checked to ensure that they are not curved and are perpendicular to each other. Flatness requires a more detailed analysis of the curvature of the centre, curvature of the corners, and warping. In addition to the dimensions the issue of regularity also involves the appearance of the tiles, which can have the following defects: fractures, crazing, shrinkage of glaze, unevenness, voids, devetrification of the glaze, dots and stains, sub-glaze faults, decolouration, flaking, blisters, irregularity and accumulations of glaze along the edges.
The characteristic used to quantify the structure of ceramic tiles is water absorption. This absorption occurs through pores in the material that are in communication with the exterior, providing an immediate indication of the structure of the material itself: porous when there is a high absorption of water and compact when absorption is low.

Mass and superficial mechanical characteristics
One of the main mass mechanical characteristics of tiles is the breakage modulus under flexion, defined by the maximum tension that the material can bear before breaking. This property can be correlated to the structure of the material.  In addition to modulus, the breaking load under flexion is also measured, determined not only by the structure of the material but also by the dimensions of the tile (in particular the thickness), establishing the breaking strength of the tile subjected to testing.
Another important property is impact strength. This measures the resistance of flooring materials to breakage as a result of impact from falling bodies. Tiles, like all ceramic materials, are not resilient but fragile, which means they have relatively limited impact resistance.
The principal mechanical surface characteristic of tiles is resistance to abrasion or wear associated with the movement of bodies. These actions generate two effects: the material is progressively worn and/or the surface undergoes alterations, with a loss of shine, variations in colour shade, a possible increase in surface porosity, etc.
 
Thermo-hygrometric and chemical characteristics
Frost resistance is a characteristic of certain types of ceramic tiles. This atmospheric phenomenon acts through a two stage process: first there is penetration of water, encouraged by the presence and size of the pores in the tiles, then a second phase involves the solidification of the absorbed water under the influence of sub-zero temperatures. As is well known, when water solidifies to become ice it increases in volume. In this way the frozen water in the tile pores can produce tensions high enough to cause breakage and the detachment of parts of non frost resistant materials. There is clearly a close correlation between frost resistance and water absorption in the sense that the more difficult it is for water to penetrate, the higher the frost resistance.
Resistance to sudden temperature changes.
Sudden temperature changes can damage tiles. Resistance to these changes indicates the capacity of the tiles to endure such events without damage. Thermal expansion is the characteristic that produces dilation when temperatures increase and contraction when temperatures decrease. Crazing resistance. Crazing is occasionally encountered on glazed tiles and consists in the formation of fine cracks in the thickness of glaze. The causes of crazing can be traced to a poor dilatometric match between glaze and support. Resistance to the occurrence of these micro fractures is a further characteristic of glazed ceramic tiles.

Chemical characteristics
The characteristics in this category include resistance to attack from aggressive chemical agents which could corrode the surface and so alter the aesthetic appearance of tiles. Closely associated with this characteristic is stain resistance which indicates the behaviour of the ceramic surface in contact with staining agents, with an assessment of the effectiveness with which stains can be removed and thus the cleanability of the surface.
 
Finally, there are a number of technical standards valid across Europe and even worldwide that lay down the acceptance requirements that each material must have in order to fulfil its intended purpose.
Nearly all tile-manufacturing companies provide their catalogues with technical sheets describing the characteristics of the materials detected by their lab technicians.
 
Another important factor to take into consideration is the quantity of tiles to order. Do not forget that some tiles may be wasted during the laying process (tiles may need to be cut or they may break) so it could be useful to keep a stock of extra tiles in case of future repairs.
 
The journey through of the world of ceramic tiles does not end with the choice of the most suitable type for the consumer's requirements. Coloured, glazed or decorated, the tile does not guarantee a successful covering of a kitchen, square or building on its own.