Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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11.11 Hard-Paste Porcelain

Modern British hard-paste porcelain is made of three constituents. Firstly, China clay (kaolin) that provides the plasticity allowing the clay and water mixture to be modelled into a shape that is retained when dried, and is relatively refractory (has a high melting point) to withstand the extreme heat during firing. Secondly, feldspar that provides the flux to lower the melting-temperature of the mix. The feldspar melts at about 1000 to 1100 degrees C and coalesces around the refractory components, partly dissolving them, and on cooling becomes glassy giving the body translucency. Thirdly, quartz, an appreciable amount of which is dissolved by the molten feldspar at around 1200 degrees C, making the melt more viscous, helping to hold the shape, reducing shrinkage and helping to prevent the body from distorting during firing. The result after firing to 1400 degrees C is the fusion of all the kaolinite and feldspar, the melting of most of the quartz and the recrystallisation as a matrix of needle-shaped “mullite” crystals reinforcing the surrounding glass matrix. (Quartz as sand in the relatively pure form melts at 1728 degrees C and is very viscous).

The usual composition is China clay 50%, feldspar 25% and quartz 25% (which is the classical German formula). The China clay content can be increased, reducing feldspar and quartz, which will improve mechanical strength and heat resistance, but at the expense of reduced translucency. Although hard-paste porcelain is not as strong as bone china, it is highly resistant to shock, so is much more suited to cooking vessels, such as the “oven-to-tableware”. The glaze on hard-paste porcelain is also hard and much more resistant to scratching and rubbing. However, much of the late 20th century commercial earthenware is heat and cold proof and can also be used for cooking or freezing.

The attempt to find the secret of hard-paste porcelain in Europe led to much experimentation with different ingredients, and most factories ended up with their own different formulae. This means that each body can have different characteristics, which also helps to determine where pieces originate and sometimes whether they are fakes or not.

The glazing of hard-paste porcelain that has a low coefficient of expansion can be a particular problem. In China a hard-paste porcelain object was dried in air after forming, then typically blue cobalt was painted on, the glaze thinly applied and the whole finished with one high-temperature firing. To achieve its exceptional qualities and appearance, firing and glazing of hard-paste porcelain in the West is done in two steps. It is firstly fired unglazed at 900-1000 degrees C for 16 hours to harden, which produces a porous body. During this firing the item has to be protected from dirt, damage, smoke and direct heat. If underglaze decoration is used the item is then decorated and dipped in the high temperature glaze and dried. There is then a second firing in a reducing atmosphere at around 1400 degrees C for 32 hours, which completes the firing of the body and fires the glaze. In this case significant further shrinkage takes place in the second rather than as is usual in the first and normally highest firing. This suited the bottle kilns, as the first firing would be on the second level, above the main firing chamber, saving fuel.

Meissen hard paste cup and saucer

Meissen hard paste cup and saucer

The reduction firing is essential to maintain all the iron in the ferrous state (FeO) giving the characteristic bluish colour, if oxidised to ferric (Fe2O3) yellowish stains and blisters would result. Hard-paste porcelain glaze has the same main constituents as the body but far more feldspar and quartz, making its melting temperature lower, but it can also contain white pigment, to reduce the slightly blue colour of the porcelain body. Typically today the glaze ingredients have to be ground for up to 10 days to achieve the necessary consistency. Glaze can affect the shrinkage of the hard-paste porcelain, so if a cup is glazed on the outside only, the rim of the cup can be drawn in on firing. Hard-paste porcelain can be decorated and further fired at between 900 and 1250 degrees C.

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