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The role heat plays in the process

Processes requiring heat are shaded in in the previous diagrams. Now they will be discussed in greater detail. There are other processes that occasionally fall into this category. Wet milling, for example, can be done with hot water. Since these other processes are less important, they will not be discussed here.

Baking

This is the ceramic production process with the most thermal requirements (nearly 70% of the entire energy requirements). It involves heating the pieces up to between 900 and 1,400 °C (depending on the material and the use it will be put to). The physicochemical changes that take place during this process give the ceramic material certain characteristics.

Ceramics can be baked in the following ways:

• Baking in continuous and discontinuous kilns (by batches)
• Baking by direct or isolated exposure to flames
• Baking in kilns with controlled temperatures    

Heat transference in kilns takes place mostly by convection between the piece and the atmosphere of the kiln, or by radiation between the piece and the flame. Within the piece, heat transference takes place by conduction from the surface towards the centre. Baking demands rigorous control of the temperature and the time: heating and cooling speeds are critical to avoid damaging the piece.

Gases coming out of the kiln (in the case of continuous processes) can be directed towards the drying area, and therefore heat is not wasted. With tunnel kilns- the continuous type- pieces undergo temperature changes as they advance through the kiln. The first stages of heating are done using the hot gases coming from the high temperature zone, so burners are not usually installed. Burners are located only in the high temperature zones. After going through that zone, the pieces go through a monitored cool down process, using air from outside. The continuous kiln is efficient because the process is uninterrupted. When the kiln has to be turned off for operating problems or maintenance work, problems with the quality of the pieces could arise. These problems are more serious with gas oil, and could produce unburnt pieces. With discontinuous kilns, heating begins as soon as the pieces are inside the kiln. The increasing of temperatures must follow a certain curve (like in continuous kilns) to prevent the pieces from deteriorating. Once baking temperature is reached, it must be maintained long enough for the entire piece to be heated to that temperature and for the physicochemical processes to transform the piece into ceramic. The last step is controlled cooling, which does not involve any heat. When this is finished, the kiln is opened and the pieces removed. The temperature reached during baking determines what type of ceramic is created, as shown in the table below:

The previous temperatures are for the biscuit or bisque. Baking for glazing and decoration requires slightly lower temperatures, so as to avoid damaging the glaze or colours.  

Controlling the temperature inside baking kilns is usually important. Bisque baking takes place in an oxidizing atmosphere (an excess of air) to eliminate all organic material that may accompany the body. Baking for glazing and decoration may require a reductant atmosphere (very little air), such as when trying to produce pieces with metallic reflections or certain colours. With continuous kilns it is possible to change the atmosphere throughout the tunnel, by allowing air to enter it.

Drying

After moulding the piece, as much water as possible must be eliminated. This is done to prevent deformations or fractures during baking. Drying is done by heating the piece. This contribution of heat can happen by basically two processes:

• Hot air drying, 50-80 °C
  
• Infrared drying   

A third drying method involves drying out the piece at a low temperature. Humidity is removed from the air current through cooling with condensation of the water in order to, later, re-heat the air. Finally, there are industries that simply let the pieces dry naturally in their environment. The drying process can be carried out by a continuous process (the more customary system) or in batches (done when producing only small quantities). It is important to mention that in some processes in which atomisation has eliminated a lot of water, drying is not necessary. The pieces can go directly from dry moulding to baking (final drying takes place during the first stages of baking). 

Atomisation

The atomisation process consists of removing a large amount of water from the piece that resulted from the milling the humid method (barbotine). Humidity is reduced from 50% to 20- 25% (approximately). Removing water is done by exposing the barbotine that has been divided in little particles to a hot air current. There are 2 methods for spraying the barbotine:

• High pressure pumping and spraying
  
• Projecting it onto a rapidly spinning disc     

The hot air current is generally created by air vein burners, steam batteries, or direct exchange in hot air generators.

Systems of co-generation with gases at 500 °C can also be used. 

Frit fusion

Frit fusion kilns are similar to glass fusion kilns. A mixture of raw materials is put into a container. The temperature is raised, usually directly with a flame. The process may be continuous, with the raw materials put in at one end of the kiln and the frit taken out of the other end. The high temperatures reached (around 1,400 °C) require the use of oxygen enriched flames in the burners. 

Conclusion

Which fuel is used for ceramics production depends on the baking kiln’s needs. The rest of the heat requirements are lower. However, on certain occasions, these requirements may justify using fuels that are different from those used in the kiln. Different fuels may improve results or quality, such as the case with air vein burners used for drying or atomization. They also may be used to reduce expenses in processes that are not critical, such as drying with firewood or orujillo (an olive oil derivative) in red ceramic factories.