Biscuit firing – the pre firing in the production of porcelain
There are two firing methods in porcelain production. On the one hand, the mono fire (single firing) and on the other hand a dual firing process consisting of two firings. The dual firing method is divided into the first firing – also called biscuit firing, glow firing, biscuit firing, rough firing or glow firing – and the second firing, also called glost firing, which is the main firing. The so-called biscuit firing is thus the preliminary stage of the glost firing (main firing). The name describes the state of the porcelain after the first firing, which is then in a biscuit-hard state.
In glost firing, the dehydration of the kaolin begins at approx. 450 °C. This separation process is completed with a maximum at 580 °C to 650 °C. As the reaction to the amorphous metakaolinite is endothermic, energy must be supplied. The change in mass occurring in this process immediately follows that of the debinding process. The quartz transformation taking place at about 570 °C is of no importance in the annealing firing, since in the unsintered green body, the volume expansion due to the quartz transformation is compensated by the plastic porous mass.
When heated further, a new phase begins at about 925 °C to 950 °C. The pore space in the body becomes smaller and the mass begins to condense considerably. In order to still guarantee the efficiency for the absorption of the glaze, the biscuit firing is stopped at this point. In the biscuit firing process, special attention must be paid to the temperature ranges in which diffusion processes take place, which, when there is excessively pressure in the body, lead to stress fields and thus to the fracture of the article.
If the biscuit firing is ended too early, there is a risk that residual carbon contents in case of insufficient burnout will only cause quality defects in the subsequent glost firing. High stresses in the ceramic body, which are caused by volume changes, which in turn are caused by crystal lattice transformations, can be avoided in their negative effects by a slow increase or decrease of the temperature in the corresponding temperature ranges. Here again, the time factor plays an important role.
This pre-firing is an invention of the European porcelain industry and originally owed to thin household porcelain. The thinner the shape of a green body is, the more delicate and difficult it is to process further. A thin porcelain body can hardly be glazed without a preliminary firing, and the scrap caused by swelling and cracking is considerable. Thus porcelain factories came up with the idea of solving this problem by pre-firing as early as around 1820.
In this firing phase, the arrangement of the articles in the kiln is not yet critical. The pieces can even be placed on top of or inside each other while the mass is fired up into the biscuit-like state.
After the biscuit firing, the moisture has almost completely disappeared from the raw mass and the shrinking process is largely complete. The mass has lost its plasticity and has reached the intended shape. The mass, which was mouldable before the sponge cake firing, has now become the sponge cake-hard shard, which has a porosity of about 20 – 22%. After the biscuit firing, the porcelain can be freed from unwanted residues, seams, bonnets, etc. This biscuit state of the porcelain represents the preliminary stage to the glaze.
The recommended temperature increase for a biscuit firing of the porcelain should be about 50 °C per hour. For hard porcelain the peak temperature is about 800 °C – 1,000 °C. It gives the body its strength for further processing.
——————– As far as the textbooks ——————–
Unnoticed by the manufacturers (meaning both the machine and porcelain manufacturers in this country) is the fact that today’s pre-fire – as a negative development of industrialised porcelain production – has now become inevitable. This is true for a thin household shard as well as for the semi-strong hotel porcelain. Modern technology has virtually overtaken itself in its efforts to produce ever larger quantities in ever shorter times. Today, modern fast firing kilns can no longer operate without an upstream biscuit firing, since the glaze – as a covering layer on the green body – melts first and would thus no longer release the degassing reactions of the different raw materials in the sintering process.
The European porcelain industry has thus not only scored an own goal in terms of sustainability and environmental protection, but also afforded itself higher production costs. In comparison with the Asian mono firing process, the European porcelain industry is wrongly complaining about a disadvantage in this respect.
In practice, biscuit firing is carried out in very different ways. Unfortunately it is not possible to derive a rule here. It is known that the clays of the stoneware require a glowing temperature of 950 °C to 1,300 °C and are therefore considerably higher in the first firing than the qualitatively better porcelain. Despite the higher annealing temperature, the body of the stoneware is significantly more porous than that of the porcelain. This explains why the colours of the glaze in earthenware are not only more brilliant than in porcelain (lower firing temperature), but also denser and thicker (higher absorption efficiency).
In the following table we present some of the extreme differences and variations of biscuit firing known to us.
|Criteria||Medium||Production facilities||Example factory 1||Example factory 2|
|Increase in temperature / h||30- 70 °C||many||X|
|50 – 100 °C||many|
|Firing time||6 – 8 hours||many||X|
|9 – 12 hours||many|
|13 -15 hours||very few|
|> 16 hours||2 factories||X|
|Highest temperature||up to 800 °C||many||X|
|up to 1.000 °C||some|
|> 1.000 °C||some||X|
|Decrease in temperature||3 – 5 hours||many||X||X|
|> 6 hours||some|
From the manufacturers’ point of view there are many reasons for or against a sponge cake firing. Many reasons in favour of biscuit firing result from a characteristic of industrial porcelain production: The amount of production output of the machines – e.g. those of large cycle lines – is so large that the time actually needed for the porcelain to mature must be reduced by the use of technology.