Invented in 1782 by the English potter Josiah Wedgwood (1730-1795), who used it to measure the temperature in his kilns, is used to measure temperatures above 580 degrees Celsius and is also known as the clay pyrometer since it is based on the contraction of a block of dry clay when heated to high temperatures caused by changes that take place in the chemical nature of its components.
The exemplar presented is made entirely of brass with a wooden case. It consists of a rectangular platform (17.8 cm x 6 cm) on which there are three rectangular guides of the same dimension (length 6.5 cm, width 1.0 cm, height 0.65 cm). The two guides on the sides are graduated, one from 0 to 120 divisions and the other from 120 to 240 and they are perfectly parallel to each other; the central one is inclined with respect to the other two so that the width of the canal that it forms with them decreases from 1.24 to 1.06 cm in going from 0 to 120 divisions and is reduced to 0.85 cm at division 240.
To make a measurement, a block of clay that fits perfectly in the canal between the guides of the pyrometer is prepared up to zero on the scale and it is dried in an oven at 100 degrees. Once the block thus prepared is placed in a kiln and heated to a high temperature it shrinks and, when left to cool, passes in the canal between the two guides to a certain level.
The reading of the scale in correspondence to the position occupied by the brick, multiplied by 72 and adding 580 to the result, gives the temperature of the kiln in degrees Celsius.
Boutan - D'Almeida (1867), T. I, p. 164
Ganot (1864), p. 210
Turner(1983), p. 116