Linear thermal expansion is expressed by dimensional changes in any material, including ceramics, due to changes in temperature. Almost all known materials expand as temperature increases and contract as temperature decreases. Moisture expansion refers to the expansion of the tile due to the absorption of moisture. The consequences of such swelling are similar to the expansion of tiles due to an increase in temperature (linear thermal expansion) and are due to the porous structure of the material.
The coefficient of thermal expansion for floor and wall ceramic tiles varies from 4.1•10 -6 °C -1 to 8.1•10 -6 °C -1 . This means that elongation ranges from 40 to 80 thousandths of a millimeter per meter of ceramic tile and per degree rise in temperature.
Methods for determining moisture expansion and temperature coefficient of linear expansion are given in the standards EN ISO 10545-10 and EN ISO 10545-8, respectively.
The recommended upper limit for moisture expansion of ceramic tiles and slabs is 0.06% when testing according to ISO 10545-10 is applied. This means that the upper limit of moisture expansion of ceramic tiles and slabs should not exceed 6 mm/m.
A moisture expansion test is required for tiles with a water absorption value greater than 6%.
The thermal coefficient of linear expansion α for ceramic tiles is calculated with an accuracy of 0.1•10 -6 °C -1 using the formula: α = dL/(L 0 •dT), where L 0 is the length of the test sample at room temperature; dL is the linear expansion of the test sample during the period of temperature change from room temperature to 100 °C; dT – temperature increase.
Material on this issue is presented in the article Linear thermal expansion and moisture expansion .
The quality and value of the cladding directly depend on the symmetry and uniformity of its individual components, i.e. individual tiles in relation to the quality of the surface and its dimensions. That is why, any dimensional differences and asymmetries of the tiles, deviations of the surface from flatness, which violate the harmony of the cladding and worsen its appearance, will be considered defects. The quality of the surface is determined by the presence of the following defects: gaps, cracks, lack of glaze, unevenness, depressions, pits, damage to the vitrified surface, specks and stains, body defects, decorative defects, darkening, chipped edges and corners.
If defective tiles do not exceed 5%, then the batch is assigned FIRST GRADE, and if they exceed 5%, then the batch is assigned SECOND GRADE.
The appearance characteristics, as well as the dimensional characteristics, are checked randomly on the tiles before they are packaged. Control is carried out visually by specially trained personnel (MANUAL SORTING) or by machine (AUTOMATIC SORTING).
The difference in size and dimensional deviations are expressed as a percentage of the working size of the tile.
The method for controlling size and appearance characteristics is described in EN ISO 10545-2. To control the appearance, a minimum of 30 samples of tiles are selected, which must form a surface area of at least 1 m². The appearance of the tiles is checked visually (with the naked eye) at an illumination of 300 lux from a distance of 1 m from the observer’s eyes. According to the standard, preparation of the sample surface and visual assessment of the surface are carried out by different persons. Surface quality is expressed as a percentage of the number of tiles without defects.
The following type characteristics are important: length of the sides and thickness, straightness of the sides, orthogonality, flatness of the surface.
Information on this issue is presented in the article Dimensional and type characteristics .
The term craquelure itself refers to the crevices and cracks that form on the surface of the glaze. The pattern of these cracks is often circular, although they may be scattered across the surface of the glaze. The reason for the appearance of craquelure is either a difference in the coefficient of thermal expansion of the shard and the glaze, or deformation of the tile due to the impact of mechanical load on it.
This defect can appear immediately after the end of the production cycle (in this case they speak of “immediate craquelure”) or some time after laying the tiles (in this case they speak of “late craquelure”).
“Late craquelure” occurs under the influence of the external environment during operation. The reasons for its appearance are: thermal shock, insufficient drying of the cement base, excessive cement content in the layer, excessive thickness of the mortar layer.
Glazed tiles with an "immediate crackle effect" are not considered defective, although manufacturers sometimes deliberately create collections of tiles with a "craquelure effect" for aesthetic purposes.
When craquelure appears on polished ceramic tiles and slabs, the term "polished craquelure" is used.
The test method for determining the resistance to cracking of glazes (craquelure) of ceramic tiles and slabs is given in the EN ISO 10545-18 standard. To determine the resistance to cracking of glazes, tiles and slabs are subjected to high pressure steam in an autoclave. Then the tiles and slabs, after applying the dye to the glazed surfaces, are examined for the presence of cracks in the glaze.
The material on this issue is presented in the article “ Resistance to craquelure ” and “ Cracking of craquelure glaze ”.
Bending strength is an important mechanical property of ceramic tiles, according to which its quality is controlled. In this case, the resistance of the material is measured in relation to the maximum specific load, with constantly increasing pressure on the surface. Flexural strength is measured in Newtons per square millimeter (N/mm2). In order to fully appreciate the significance of this tile property and correctly apply the test results, you must first check your own understanding of this issue. Please indicate the correct conclusions in your opinion:
The tensile strength of the tile, measured in accordance with the standards, in fact, as a rule, is inferior to the real load-bearing capacity of the tile as part of a multilayer structure, i.e. after installation.
Bending strength is an indicator that does not require additional calculations. It is measured in KG (maximum load leading to destruction of the sample), per surface area (in mm2) to which the force was applied.
In the applied aspect, the tensile strength of the tile, measured in accordance with the standards, is somewhat overestimated relative to the real load-bearing capacity of the tile as part of a multilayer structure, i.e. after installation. This is due to an increase in the area under pressure.
Flexural strength is a property of the material, not the tile. This indicator is used to measure the internal cohesive properties of the material that form the tile, rather than to measure a specific mechanical characteristic of the tile itself. In other words, if we take two tiles from the same material, but of different shapes and sizes, for example, one tile is twice as thick as the other, their bending strength will be the same, although the tensile strength will be different. Thus, the characteristics of the tiles differ, despite the fact that they have the same flexural strength.
The bending strength is determined by an equation that includes such variables as: breaking force, distance between support rods, width of the tested sample and the smallest thickness of the tested samples along the fracture line.
Flexural strength is a characteristic that determines the load-bearing capacity of a tile. In addition to the density of the material, it is also affected by the linear dimensions of the tile: length, width and thickness. So, for example, if one tile is twice as thick as another, and they are made of the same material, then its bending strength will be twice as high.
Material on this issue is presented in the article Flexural strength .
The surface hardness of ceramic tiles is the ability of the cladding surface to withstand the mechanical stress of other materials. For ceramic facing materials or natural stones, this property is usually indicated in accordance with the mineralogical scale of hardness, the so-called Mohs scale, named after the German mineralogist Friedrich Mohs, who proposed his test method in 1811. Please indicate the correct statements in your opinion.
The Mohs scale is a method of rough comparative assessment of the hardness of materials according to the “harder - softer” system, where the material being tested is scratched by a reference mineral and its surface hardness on the Mohs scale is lower, or it is scratched by a reference mineral and its hardness is higher. Thus, the values of the Mohs scale can be considered indicators of the absolute hardness of minerals.
Glazed ceramic tiles are relatively hard, and scratches affect the aesthetic properties of the cladding, while also damaging its functional qualities.
The Mohs scale (mineralogical hardness scale) is a set of reference minerals for determining relative hardness using the scratching method. 10 minerals, arranged in order of increasing hardness, were taken as standards.
Mohs scale - determined by which of ten standard minerals scratches the material being tested, and which of ten standard minerals scratches the material being tested.
Unglazed ceramic tiles are relatively hard, and scratches only affect the aesthetic properties of the cladding, without damaging its functional qualities.
Material on this issue is presented in the article Surface hardness .
