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.

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http://www.tilingnews.co.uk/2007/01/23/vitrified-or-fully-vitrified-what-does-it-mean.html