Drying and Firing

After it has been moulded, the green product must be allowed to dry. Before the firing process can take place the green product must be 

Why these compounds produce these specific colours will be explained elsewhere in this section. 

The so-called terra sigillata (pottery with small decorations) from Greece and later also from the Roman Empire is an example of ceramics with engobes. Later people also started to use oxides and mixtures of oxides, the glazes. 

Tg is the so-called glass temperature and this varies according to the cooling speed. Ts (Tolid) is the temperature at which the solid is formed. 

UNDER WHICH CIRCUMSTANCES IS A MIXTURE CAPABLE OF FORMING A GLASS  

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Cordierite [12182-53-5] Mg Al Si O g, is a ceramic made from talc (25%), kaolin (65%), and Al O (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and mote recently for automotive exhaust catalyst substrates. In the latter, the cordierite taw materials ate mixed as a wet paste, extmded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

Cera.mic, The ceramic substrate is made from a mixture of siUcon dioxide, talc, and kaolin to make the compound cordierite [12182-53-5]. Cordierite possesses a very low coefficient of thermal expansion and is thermal-shock resistant. The manufacturing process involves extmding the starting mixture (which is mixed with water and kneaded into a sort of dough) through a complex die to form the honeycomb stmcture. The extmded piece is dried and fired in a kiln to form the cordierite. The outside or circumferential dimension is formed by the die, and the length is cut later with a ceramic saw. 

A silver replica of a holly leaf is to be made by investment casting. (A natural leaf is coated with ceramic slurry which is then dried and fired. During firing the leaf burns away, leaving a mould cavity.) The thickness of the leaf is 0.4 mm. Calculate the liquid head needed to force the molten silver into the mould cavity. It can be assumed that molten silver does not wet the mould walls. 

Vitreous ceramics are different. Clay, when wet, is hydroplastie the water is drawn between the clay particles, lubricating their sliding, and allowing the clay to be formed by hand or with simple machinery. When the shaped clay is dried and fired, one component in it melts and spreads round the other components, bonding them together. 

For technical purposes (as well as in the laboratory) RuOz and Ru based thin film electrodes are prepared by thermal decomposition techniques. Chlorides or other salts of the respective metals are dissolved in an aqueous or alcoholic solution, painted onto a valve metal substrate, dried and fired in the presence of air or oxygen. In order to achieve reasonable thicknesses the procedure has to be applied repetitively with a final firing for usually 1 hour at temperatures of around 450°C. A survey of the various processes can be found in Trasatti s book [44], For such thermal decomposition processes it is dangerous to assume that the bulk composition of the final sample is the same as the composition of the starting products. This is especially true for the surface composition. The knowledge of these parameters, however, is of vital importance for a better understanding of the electrochemical performance including stability of the electrode material. 

Catalyst and adsorbent finishing procedures are outlined including preparation of binder-free forms, zeolite stabilization procedures, ion exchange and metal loading procedures, and drying and firing operations. 

The manufacture of bricks may be classified under five heads—Preparation of Brick-earth, Tempering the Mats, Moulding, Drying, and Firing. 

The loss in weight of a brick by drying and firing will ba further illustrated by the following table —... 

When the results of the yellow limestone clay and the red field clay analyses were processed by the potstat routine (10), an average of data from the two clays yielded a pattern that matched (except for sodium) the analytical pattern of pottery made of a mix of the two clays. In the first case, the two raw clays were simply ground and analyzed separately in the second case, the two clays were mixed in a water bath, sand and Dead Sea salt were added, a vessel was formed, dried, and fired, and this finished product was analyzed. The sand temper did not contribute significantly to the relative test element concentrations, but the salt addition did, of course, raise the sodium concentration. These results are graphed in Figure 2. 

These analyses are mainly performed on natural raw materials, notably on clay. The user of this clay is always interested in the mass percentage Fe, Ca and humus and in the mass percentage loam, fine sand and coarse sand. The contents of iron and calcium affect the baking colour and other contents are of influence on the properties of the mass as far as moulding and drying and firing behaviours are concerned. Furthermore we have learnt from the chapter on clay that this material may contain many more components which sometimes also need to be determined, for instance for environmental reasons. 

The technique is used to study drying and firing behaviours, but it can also be applied to identify clays. 

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