Combustion is a rapid reaction between oxygen in the fuel which generates heat as a result of chemical reactions between a fuel and oxygen. Oxygen from the air or have parts volume by 21%, nitrogen 78%, and the molecules of other gases 1%. Some of the factors that will determine the success of the combustion process, among others:
a. The type of furnace.
b. Stove burner.
c. The operation, and
d. Fuel.
The first change occurred in the clay when it is 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. In the subsequent changes in the chemical content of the water will be lost. 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 of the changes are permanent (conversion) and others are able to turn back (inversion). So that the clay can be transformed into ceramic must go through the combustion process with a temperature exceeding 600ºC. After going through the temperature of the clay will change into a mineral dense, hard, and permanent. These changes are called ceramic tile change or changes that occur at a temperature of 573 o C.
Baked clay of less than 600 ° C has not had a proper maturity despite changes ceramics. Maturity is a vitrified clay or ceramic condition that has reached maturity temperature accurately without changing shape. At combustion temperatures below 800 ° C, free silica minerals such as carbonate minerals will change as well. This is a result of the burning of all the elements of carbon, called calcination process.
Physical changes occur at temperatures above 800 ° C, which is when the ingredients alkali act as ‘flux’ over silica and alumina form a network of crystal (noble) and the glass binding material can not be dissolved into a mass of strong (burning biscuits). When clay was burned at a temperature of 1300 ° C, some changes will occur, for example, the body becomes harder when it cools and becomes impermeable.
The clay has undergone a process of ‘vitrification’, meaning that most of the material, especially silica has menggelas, enter 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 volume shrinkage shows that, the higher the temperature the greater the fuel but the lower shrinkage porosity.
In other words, the objects turn into increasingly dense and impermeable. Clay that has not undergone vitrification process at high temperatures (1300 ° C) can be classified into types of clay ‘fireproof’ (refractory clay).
Figure 33. ceramic objects reach maturity
Each clay can be melted when the fuel temperature is sufficient. Ideally, each type of clay has vitrification point without any change in shape (deformation). In practice, the vitrification is often accompanied by deformation. Maturity (vitrification) is a ceramic condition that has reached maturity precisely without changing shape. This is determined by melting the materials feldspatik and free quartz in a ceramic body, which serves as an adhesive particles of clay, so that after the process of cooling the soil particles as though glued to one another to form a hard ceramic body.
Baked clay has a temperature range between (range) is quite large, usually between 950 ° C-1200 ° C For example clay Earthenware from a particular location has matured a temperature between 950 ° C-1050 ° C, meaning that when burned under a temperature of 950 ° C, the clay ceramics has not changed completely. Conversely, if the temperature exceeds 1050 burned o C, the clay will distort or even melt. This happens because of the stresses on the weakest part of the object as a result of melting clay minerals. Change the color of fire, temperature, and what happens to the clay when the combustion process.