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In all what follows, I am talking only about clay manufactured tiles. Will not talk about any natural stone ones.

First, lets clasify a bit the tiles. From manufacturing point of view, they are: hand made; extruded; ram (wet)pressed and dry pressed tiles.

From the texture, properties point of view: porous (wall) tiles; low porosity (semi-vitrified) gress tile; advanced vitrified floor tile (as the Italian Porcelanatto); fully vitrified porcelain tile.

Any of the above, are manufactured of a mixture of 50-65% clay, 20-25% Silica, 10-15% Feldspar; Calcite and additives.
All will go through a firing process of at least 1050 dgr.C. or higher.

During the firing process, series of physical and chemical reactions take place. All the physical modifications are reversible, but most of the chemical reactions are unreversibil.

Plasticity of the clay: is given mainly by the finesse of the clay\’s granules; nature of the clay; impurities. Ball Clay is an impurified kaolin. Kaolin is a variety of white clay. Is whitter and cleaner if remained on its forming site. If transported away (generally from mountains in valleys) by rain waters, will become a ball clay by grinding to finer in tranport and gaining impurities (mineral salts, iron and other metal salts). A pure kaolin will be always less plastic than a ball clay. As higher the alumina content of a clay, the clay will be a higher firing one. The pure chemical component of a clay, better to say, its activ component is called kaolinit:
Al2O3.2SiO2.2H2O

The wet formed tile, have a water content made up by two categories of water: when drying, eliminates its water content which is replaced (as volume) by shrinkage of the tile. After a wile, water still is eliminated but the body does not shrink any more, the water\’s place is taken over by pors forming porosity. Dry pressed tiles have practically no drying shrinkage. Water content of the formed tiles. is 5-6.5% which will generate porosity by drying process.

During firing, function to temperature and heat work, are taking place the above mentioned physical and chemical modifications, as follows (just a short sketch):
The heat is on and the temperature is rising in the kiln. Up till about 200 dgr.C ceramic items are eliminating all of their physical remaining moisture;
200-450 dgr.C no reactions take place. At this stage, if the firing is stoped, the clay mixed with water, will be plastic again, as it was before;
450-573dgr.C starts elimination (desintegration) of organic compounds. They burn and will eliminate in form of carbon dioxide and water vapors. Same, from 450 starts the chemical reaction of eliminating the chemically bound water content in the clay particles. Kaolinit, by eliminating its two water molecules, will transform in so called metakaolinite:
Al2O3.2SiO2.2H2O —> Al2O3.2SiO2 + 2H2O

Metakaolinit is no more plastic and conferes more strength to the clay, but not much and makes it brittle at this stage.

The 573 dgr.C temperature is called \”Quartz Point\”. It is a very important stage of the firing, due to structural modifications of the silicon content. The modification is from the variety of \”crystobalit\” to \”tridimit\”. This modification, takes place with a quite large volume modification as well, so, items are in danger of cracking. To avoid this happen, this point will be past very slowly, specially if the used clays are rich in crystobalit content.
573-900 dgr.C are taking place reactions with no great importance.

From 900dgr.C and up, the ceramic mixture starts to form melting points first, areas later. Acting flux for this is the feldspar content wich is a flux mineralizator. That means, will melt itself and then will disolve other components in its liquide mass. During the reactions, some born components are in gas form and will be eliminated as firing gases. Other components, melting together by forming eutectics (melting points of 2-3-4 component mixture) will form the final ceramic body. Melted litle areas of the ceramic body, will hold the tile together as a whole and will generate strength. By all these reactions, volume modifications take place. Formed gases by elimination, will leave a free space. If these spaces remain empty, this will form the porosity of the body. More spaces are filled with melted mass, more vitrified the body is and its volume will decrease acordingly (firing shrinkage).

Porcelain tile, have a total vitrified body, their porosity being under 0.1%. This fact does not mean that the whole tile has been melted, but it means that during the firing, was generated enough melted mass, to be able to fill all the empty spaces in between the particles.

Guys, in this telegraphic manner I hope to clarify majority of your questions. Will answer with pleasure to any further inquires. Just ask John to pick me up.

All the best to all of you and excuse my English
Uri Schwartzberger

P.S. The tile saw dust feels muddy due to its finesse. No plastic as clay (does not bond) due to the metakaolinit (see above).
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This was a real fiddly one, mosaic travertine floor and walls with boxing as well.

and a natural terracotta floor, and i tell ya i aint doing another for a while. Needed 6 coats of sealant then waxing.

By Tiler from

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