7. Pottery Technology 1
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7.7 Firing
Firing is effectively a way of turning the clay back into stone, but in the shape dictated by the potter. Water is actually contained at three different “levels” within the structure of the clay used to make pottery, from the previously mentioned free and surface-bound water, loosely holding the clay particles, making it wet and malleable, to that chemically locked up within the crystal structure itself. The earliest stage of firing involves a slow and gradual rise in temperature to drive off any remaining water on particle surfaces at around 1-200 degrees C. The water may be turned into steam and the high-pressure could shatter the pot if the temperature rises too quickly. Simple low-fired pottery is the product achieved when clay is heated to a high enough temperature to drive off the chemically bound “water of crystallization”, requiring between 450 and 760 degrees C. Again the temperature must be controlled or pressurised steam will be generated. The clay goes through an irreversible process and its physical properties are changed. It becomes hard and water resistant, although porous, and its colour is often altered, depending on the amount of oxygen in the kiln gasses. At this stage none of the constituents have melted and so this low-fired earthenware is unvitrified pottery. If the temperature is raised further, to a maximum of around 1,100 degrees C, some of the lower melting temperature material (flux) melts and fills some of the minute gaps and pores, cementing the other constituents together so it is partly vitrified, but too much and the pot might weaken and collapse. Earthenware can therefore come in a range of qualities from low to high-fired. Earthenware cooking vessels can last longer if coarsely fired with less vitrification, as the larger voids allow them to withstand heat changes and cracks may terminate on voids and not propagate to become catastrophic, even though basically it is weaker than a more highly vitrified vessel.
If the clay has the right refractory properties, and heating is continued, the higher temperature melts more of the flux so it coalesces and further wets the remaining components, some of which also partly dissolve, causing the material to become increasingly vitrified. If this vitrification happens at a temperature above about 1,200 degrees C the product becomes stoneware. If sufficient molten liquid is present all the pores are filled and a completely dense, vitrified material is formed. At higher temperatures the molten silicate liquid can start to recrystallise into different forms of crystalline material, eventually becoming porcelain containing mullite crystals. If the firing temperature becomes high enough all the crystal structure will be completely destroyed and the resultant is an amorphous glass, so a ceramic results from a process where the chemical reactions are not allowed to proceed to completion. Conventionally the temperature is raised to the “maturing point” of the ware, that is the temperature aimed at producing the densest structure below the melting point of the body. Firing is also a function of time, with lower temperatures requiring longer times to complete the transformation, so the temperature has to be held for a minimum time to achieve a properly fired pot.
In Neolithic times pottery was fired in primitive bonfires with the pots encased in the fire, or in pits so that the ‘walls” retained some heat or in slightly more sophisticated “clamp kilns”. In the latter the pots were stacked on the ground or in a depression; fuel, including wood, leaves, straw and dung was packed round the pots and over the top of them; and the whole was encased in turves or plastered with mud, apart from a few air holes round the base and a chimney at the top. In these early days it was quite difficult to achieve more than a few hours at 7-800 degrees C, producing crude earthenware. This method was abandoned when the significant advantages were recognised in separating the fire itself from the pots in the kiln chamber. This permitted further fuel to be added as firing progressed, and prolonged firing became possible. Sufficient time at a given temperature is vital to achieve the right end product, and as an example of the demands on the potters, porcelain would require some 8-10 days at around 1,300 degrees C to fire properly. The greater control of firing conditions that can be achieved with such improved kiln structures allows the gaseous atmosphere in the kiln to be varied; it gives more even firing; consistent pot colours; reduces surface blemishes and can achieve higher temperatures.