Nothing is more variable, naturally, than the temperature at which the firing takes place, considering the variety in the composition of the clays used. This temperature usually oscillates between 1000° and 1200° C, but it may rise to 1300° or 1400° C.
In practice there is no advantage in measuring it; the degree of firing is observed by noting the contraction, as pointed out on p. 226.
But if we wish to find out at what temperature the firing is taking place, we must have recourse to the pyrometric indicators. These are mixtures, in variable proportions, of different oxides which soften at different temperatures. They have been specially studied by the German pottery expert, Seger. To make use of these instruments, which are in the shape of a four-sided pyramid, a series of them is placed in the kiln, and when the proper degree of firing is reached, their condition is examined. The number is noted of the one whose point has begun to bend, the next one being still intact. In order to regulate later firings, it will be sufficient to place in the kiln several indicators near on the list to the one noted, and to observe their behaviour.
Indicators can be obtained in the trade (in Paris, from MM. Poulenc freres) fusible at intervals of 30°.
The fusion of metals and even the colour of the fire only will show the temperature reigning in a kiln.
Here are some hints on the subject -
Aluminium melts at
Zinc volatises at
Silver melts at
Light cherry colour,
Gold melts at
Copper melts at
White cast-iron melts at
Grey melts at
Nickel melts at
Palladium melts at
Platinum melts at
A large number of types of pyrometer based on different systems, are in existence, but are seldom used in pottery. The simplest one, Wedgwood's, gives very inaccurate indications; the best one, the thermo-electric pyrometer of M. Le Chatelier, is too delicate for practical use.