The first change that occurs in the primary and secondary clay when burned, is the loss of free water. Especially for secondary clay will be followed by the burning of other organic materials, such as humus, leaves, and twigs contained in the clay. The next change is the loss of the water content of the chemical. Primary and secondary clay containing free silica in the form of sand, quartz, flint and crystal.
Silica is subject to change the shape and volume of clay at a certain temperature. Some changes are permanent and can not return (conversion) and the other is to change back (inversion). So that the clay can be transformed into ceramic must go through a combustion process with temperatures exceeding 600oC. After going through the temperature clay will change into a solid mineral, hard, and permanent.
These changes are called ceramic change or alteration occurs at a temperature ceramic and clay burnt 573oC.Tanah less than 600oC yet have proper maturity despite being unchanged keramik.Kematangan vitrified clay or ceramic is a condition that has reached the proper temperature of ripeness without changing form. At combustion temperatures below 800ºC, free silica minerals such as carbonate minerals will change as well.
This is a result of the burning of all the elements carbon, and the process is called calcination process. Physical changes occur above a temperature of 800ºC, which is when the alkaline ingredients that act as ‘flux’ over silica and alumina that form a crystal network (noble) and the glass binding material can not be dissolved into a solid mass (burning biscuits). When the clay burnt at a temperature of 1300 ° C, some changes will occur, for example, the body becomes harder when it cools and becomes impermeable.
Clay has been undergoing a process of ‘vitrification’, meaning that most of the material, especially silica has menggelas, enters the pores and bind all the particles of clay to form a bond that is known to bond ‘silica alumina hydroxide’. The vitrification process can be accompanied by a volume shrinkage, where the higher the temperature the greater the fuel but the lower shrinkage porosity or in other words objects more dense and impermeable.
Clay that does not undergo a process of ‘vitrification’ at high temperature (1300ºC) can be classified into types of clay ‘refractory’ (refractory clay).
Figure 17. incandescent ceramic objects in the furnace to reach maturity
Each clay can be melted when the fuel temperature cukup.Idealnya every type of clay has the vitrification point without a change of shape (deformation) .In practice, the vitrification is often followed by bentuk.Hal change is due to the stresses on the body the weakest result melting of clay minerals. Change the color of fire, temperature, and the condition that occurs in clay when the combustion process, as shown in the table below.
Things that need to be considered in the combustion:
a. Do not burn too fast clay because the clay will explode into pieces or cracked. This is due to not enough time for the water to evaporate plasticity.
b. The cooling process should not be done quickly, clay volume changes that are often very sudden. Causes a sudden cooling of the surface will be hotter than the other surfaces, so that when the volume change another volume has not changed. It is this factor which causes the fired clay to be broken. Therefore, the cooling process should be done later and as evenly as possible to prevent the outbreak of the goods. These errors will rarely occur when the furnace is not opened before the temperature in the furnace reaches 100 o C.
Table Heatworkchart: Changes in the ceramic material by heat.
