The bending of a test-piece in this manner is the result of the action of all fluxes[6] in the clay, and as this depends on the size of grain and the duration of the heating above incipient fusion and does not give a direct measure of temperature, nor is the softening effect under one rate of rise in temperature the same as that at another rate. Nevertheless a study of the behaviour of various clays heated simultaneously is valuable and the method forms a convenient means of comparing different materials. [Footnote 6: For fluxing materials see p. 8.] The temperature may be measured by means of a pyrometer, but for the reason just stated it is more convenient and in some respects more accurate to use standard mixtures known as Seger Cones (fig. 6), and to state the softening point in terms of the 'cone' which behaves like the clay being tested. A medium fireclay will not soften below Seger Cone 26 (1650° C.) and a really good one will have a softening point of cone 34 or 35 (1750° to 1800° C.). [Illustration: Fig. 6. Seger Cones indicating a temperature of 1250° C.] The _refractoriness_ of a clay, or its resistance to high temperatures, is an important requirement in bricks required for furnace linings, in crucibles, gas retorts and other articles used in the metallurgical and other industries. The term is much abused and is frequently understood to mean resistance to the cutting action of flue gases and flame, the corrosive action of slags, and the strains set up by the repeated changes in temperature. This is unfortunate, for the term refractoriness has a perfectly definite meaning and should be employed exclusively to denote that a given clay is capable of retaining its shape at a given temperature or under given conditions when heated alone and without being subjected to any pressure. In Great Britain there is no officially recognized standard of minimum refractoriness[7], but where one is required the suggested minimum of Seger Cone 26 (1650° C.) made by E. Cramer (8) is usually employed. This is the recognized minimum in Germany for fireclays, and though objections may be urged against the use of Seger Cones as a standard, equally forcible ones may be brought against making a temperature-scale the basis of measurement. Under present circumstances, however, it is necessary to adopt one or other of these. [Footnote 7: See _Refractory Clays_, Chapter V.]