Stoneware porosity

Stoneware Porosity 0.5-3% Firing temperature 1100-1300°C Glassy surface Crucible, labware, pipe ... 

Many of these properties depend upon others which may themselves be governed by yet other factors. Thus, as mentioned above, increased porosity usually gives better thermal shock resistance, but it may be necessary for reasons of watertightness to employ a body with a very low porosity. The size of an article is also closely related to the degree of thermal shock which it will withstand. For this reason it is very difficult to give accurate figures for the thermal shock resistance of stoneware bodies. In practice, if precautions are taken to heat up any stoneware articles slowly and evenly no trouble will be experienced. This is a matter on which the ceramic manufacturer should be consulted. 

Stoneware and porcelain are attacked only by hydrofluoric acid and hot concentrated caustic alkalis and are almost completely unaffected by all concentrations of mineral and organic acids, acid salts and weak alkalis. The degree of susceptibility depends upon composition, firing temperature and porosity. 

Ceramics can be classified by considering the firing temperature and the resulting porosity thus high-fired stoneware (produced at above 1000°C) has porosities less than 2% and low-fired earthenware (firing between 600 and 900°C) with far more than 10% porosity are at the upper and lower ends of the scale. Porcelain (defined as white and translucent ceramic, fired up to 1400°C) can exhibit an extremely low porosity, whereas terracotta or raku (both fired below 1000°C) would be examples of high porosity. 

Ceramics are inorganic solids, usually oxides, which contain ionic and covalent bonds. The material, formed by sintering at high temperatures, ranges from amorphous glass-like material to highly crystalline solids, from insulators to conductors or semiconductors. They include earthenware, which is fired at 1,100-1,300 K and a porosity of about 8% fine china or bone china, fired at 1,400-1,500 K with a porosity of less than 1% stoneware, fired at over 1,500 K with a porosity of about 1% before glazing and porcelain which is fired at over 1,600 K and has a much finer microstructure than either stoneware or bone china. 

We have talked so far about relatively porous ceramics, fired at about 1,000-1,050°C maximum. With stoneware and porcelains, porosity decreases (less than 5%... 

The term vitreous china denotes dense products obtained from pastes close to those used to manufacture feldspathic earthenwares. The feldspar content of these pastes is increased in order to prodnce, during the firing, a sufficient quantity of liquid to eliminate open porosity (< 0.5%). Used more particularly to manufacture sanitary articles and very robust crockery (wash basin, crockery for communities), vitreous materials are in the middle between white paste stonewares and porcelains. 

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