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Introduction

The origin of tile production is pottery, which is one of the most ancient human arts. The earliest works of this art in Iran date back to about 10,000 BC, which was in non-flowered form, and the first pottery kilns date back to about 6,000 BC. Continued advances in the pottery industry have led to changes in the manufacturing process, including the conversion of kilns, the invention of pottery wheels, as well as the quality of pottery materials such as painting and glazing. The time of glazing began to allow waterproofing, as well as painting and beautification of containers and pottery and the preparation of tiles around 5,000 years ago.

Tile History

The earliest forms of tile date back to pre-historic times when the use of clay as a building material was developed in several early civilizations . Early modern tiles were roughly shaped and lacked the durability of modern tiles. Tile materials were extracted from the riverbeds in the building blocks and dried in the sun. Early tiles were crude, but even 6,000 years ago people used them for decoration by painting and carving delicate tiles.

The Evolution of the Tile

1 – Firing Tile

The ancient Egyptians were the first to discover the clay tiles cooked in the furnace were more durable and water-resistant . Many ancient civilizations used small square clay cooked tiles for decoration in architecture . The buildings of the ancient cities of Mesopotamia were decorated with red glazed pottery and colorful tiles.

2 – Glazing Tile

Iranian tiles were influenced by tiles imported from China. These tiles were used for decorative purposes throughout South Asia, North Africa, Spain and even Europe. Since Islamic art originated from the human imagination and was instrumental in the development of Islamic religion, craftsmen turned to tiles of light color and complex texture or texture. Bold enamel tiles were arranged in patterns of large mosaics and subtle color changes. Muslim craftsmen used metallic oxides such as tin, copper, cobalt, magnesium and antimony to glaze the tile, which made the glaze stronger and firmer. In the 15th century, metal oxide glazed tiles became common in Italy and gradually penetrated among northern Italian craftsmen. Important European business centers paid attention to these local motifs, with some of these tiles still being used, such as Delft (from Delft in the Netherlands) and Magulica (from Mallorca in Spain).

3 – Modern Tile

Most commercial construction companies today use the Press Dust method. The mixture is first pressed to the desired shape and then glazed (may not be glazed as well) and then baked in the oven. Some artisans may produce the tiles in the desired shape by pressing mortar or flattening the dough and cutting it using a mold like pastry. Whatever the method of cutting the tile, it needs to be cooked to harden. Clay purity, cooking time and furnace temperature are factors that influence the price and quality of the tile. Furnace temperature varies from 900 to 2,500 degrees Fahrenheit. The lower the furnace temperature, the higher the porosity of the tile and the softer the glaze. Higher temperatures produce denser tiles and firmer glazing.

Anti Acid Tiles

Anti Acid Tiles are porcelain tiles manufactured using special non ferrous clay, processed at extremely high temperatures. This process leads to fusion of the raw materials and formation of a compact body with extremely low water absorption of less than 0.5%. As a result, formation of a compact porcelain body brings high flexural strength and hardness, low probability of reaction with chemical materials and consequently long life in environments in contact with acid or alkali materials (or generally corrosive materials).
Anti-acid tiles, in Iran, normally are produced according to instructions of Iran National 3051 Standard or relevant ASTM/DIN standards which ensure excellent performance in Acid/Alkali/Chemical environments.

Applications of Anti-Acid Tiles

Anti-acid tiles can be used in different environments which chemical materials are used there. Some examples of these applications are as follow :

• Industrial floors in contact with corrosive chemical materials
• Floor and surrounding area of tanks containing chemical materials
• Floor of food Industry factories
• Floor and walls of alkaline battery rooms and control rooms
• Plants of mineral materials extraction
• Walls and floor of washing and painting areas

Size of Anti-Acid Tiles

Anti-acid tiles can be produced in different sizes, but more common sizes in the global market are: 30×30, 40×40, & 60×60 centimeters. Thicknesses of these tiles usually can be in the range from 8 to 25mm. Anti-acid tiles provide excellent abrasion resistance beside aesthetic appearance.

Factors affecting the creep of porcelain tiles should be related to the microstructure of the tiles. These pores are usually aggregated and consist of decomposition of specific impurities. The origin of the microcracks are varies, though in particular as a result of the discrepancy between the residual quartz particles and the glass matrix and the spray dryer powder granule junctions that did not deform during the full press.

The creep is very high in porcelain tile pieces due to the growth of microcracks in the tile when the tile is exposed to tensile stress. Therefore, the factors that create these microcracks determine the creep in these types of tiles. Thus increasing the amount of quartz, such as particle size, creates microstructures with a larger number of microcracks, which probably increases the tile creep. This can also happen when the compound is not sufficiently milled.

The effect of dry coloring on creep is that the piece over time is subjected to creep, which is very rapid at the beginning and is fixed over time. The piece with pigment has a significantly higher tension than the piece without pigment, indicating that the deformation capacity under pressure is much higher for the colored piece.

The pieces with pigment have a higher creep that can be attributed to the microstructure of this type of tile where particles pigments are concentrated in certain areas of the piece.

These areas can act as microtransfer defects when higher percentages of pigments are used, those growth under certain stresses can create more creep. There are, of course, color patterns that show no delayed curvature. However, despite the asymmetric residual stresses on the tiles, and other equal factors, the tiles that exhibit the highest creep are likely to withstand time changes.

The evolution of curvature over the time for glazed porcelain tiles, when the minimum curvature of the glazing tile was observed (24 h, 0.37 mm) practically is simultaneous with the maximum curvature in the tile without glaze detected in the (23 h, 1.02 mm). This behavior may be due to the different expansion rates between the top and the bottom of the tile, since the tiles are firing with the same cycle of the kiln and therefore the residual stresses due to cooling must be similar. The same behavior has been observed in porcelain tiles without coating, which seems to indicate a very high effect of glaze evolution of curvature over the time.

The cooling stage in industrial roller kiln has rapid initial cooling by injecting air into the kiln at ambient temperature. This type of cooling produces a high temperature gradient between the surface and the inner part of the tile, resulting in residual stresses that can cause delayed curvature In symmetric cooling (segment in vertical possition), both sides of the segment cool equally.

And in asymmetric cooling (the piece is placed horizontally on a refractory board) in this case, cooling is basically done from above. In both cases, the two surfaces of the parts are subjected to compressive stress, while the inner part of the part is under pressure, which is the same situation in ceramic materials. Cooling also has a significant effect on the stress diagram. Therefore, when the test piece is cooled equally on both layers (the cooled piece in a vertical position), the stress diagram is symmetric, while the piece is on a horizontal position on the refractory slab (asymmetric cooling) displays asymmetric diagram. This results in a difference in the amount of cooling through the top and the bottom of the segment, which can lead to asymmetric diagram.

Delayed Curvature in Porcelain Tiles – Part 1

A significant percentage of porcelain tiles exhibit a phenomenon called “Delayed Curvature”. That includes a change in tile curvature after leaving the kiln. This phenomenon becomes problematic with increasing the size of the tile. Factors affecting delayed curvature can be categorized as direct and indirect factors. Direct factors include expansion of the body and release of residual stress.

The expansion of the porcelain tile body begins as the tile exits the industrial kiln. Expansion is very fast at first and reaches about 0.18% after 96 hours; however, the result depends on the composition used and the highest kiln temperature. If there is an overlap between the expansion of the top and the bottom of the tile, for example, a 0.1% difference in 410*410 mm tile expansion can cause a curvature of about 0.3 mm .

تاب تاخیری در کاشی های پرسلان بخش اول

Even if the overall expansion of both surfaces is the same, differences in the expansion kinetics can cause changes in the tile curvature . Another direct factor affecting tile curvature are residual stresses in the tile that can be due to two reasons

1- Stresses caused by the rapid cooling of the tile in the industrial kiln, which causes thermal gradients in the tile .

2 – the stresses produced by the glazed body. Because the body is thicker than glaze and it has a relatively high elasticity .

There must be a mechanism to release the tile from these stresses, which is known as creep.

Among the factors that indirectly influence the delayed curvature are :

1 – Materials (Sprayer Powder, Particle Size, Mineralogy, glaze and chemical compounds)

2 – Process (kiln cycle, the highest kiln temperature, residence time in kiln) that this leads to changes in the ceramic tile (Microstructure, existence of different phases, modulus of elasticity, temperature expansion, environmental conditions, relative humidity and temperature).

تاب تاخیری در کاشی های پرسلان بخش اول

Expansion of ceramic bodies due to moisture is a well-known feature. Moisture expansion of ceramic bodies is due to the physical and chemical absorption of water molecules at free capacities in the hydrated phases present in the samples removed from the kiln. For this reason, the expansion depends largely on the porous structure of the segment (which determines its access to water more or less) and the nature and amount of phases in the part removed from the kiln.

These properties are greatly influenced by the mineralogical composition , Particle size and firing schedule. Therefore, as the melting temperature of the body composition and the firing temperature or the residence time increases, the moisture expansion of these bodies due to porosity and the amount of hydrate phases reduced .

When the maximum kiln temperature is reached, the porcelain tile is composed of a large amount of liquid phase, quartz and residual albite, and sometimes mullite .

تاب تاخیری در کاشی های پرسلان بخش اول

In this case, the piece is able to release any tension applied to it, as it is highly deformable. In the cooling phase, the residual stresses in the tiles, either due to a mismatch between the body layers and the glaze, or due to different shrinkage resulting from the higher cooling velocity in outer regions relative to the center of the piece. Therefore, the factors that determine the residual stresses of porcelain tiles are in principle the thermal expansion and modulus of elasticity of the body and the glaze relative thickness and the cooling rate of the tile. It has been observed that mismatches than thermal expansion of both layers increase the residual stress. In addition, as the cooling time get faster, the temperature gradient inside the tile, increase resulting in different contraction rates at the tile cross-section, which creates a stress profile within the tile that increases with cooling velocity increases .

In addition, when the cooling rate on both sides of the tile is not the same (common situation in the furnace rollers), the resulting stress profile is not symmetric.


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