The most important characteristics of a fireclay are that it shall be able to resist any temperature to which it may be exposed and that the articles into which it is made shall not be affected by rapid changes in temperature. Other characteristics of importance in some industries are the resistance to corrosive action of slags and vapours, to cutting and abrasion by dust in flue-gases or by the implements used in cleaning the fires. For those purposes it is necessary that a fireclay should possess high infusibility (p. 32), a low burning shrinkage (p. 29) and a high degree of refractoriness (p. 34), and before it is used these characteristics should be ascertained by means of definite tests, as they cannot be determined by inspection of a sample or from a study of its chemical analysis. Several grades of fireclay have long been recognized on the Continent and in the United States of America, but the recent Specification of the Institution of Gas Engineers is the only official recognition in Great Britain of definite grades. This specification defines as No. 1 grade a fireclay which shows no signs of fusion when heated to 1670° C. or Cone 30 at the rate of 10° C. per minute, and as No. 2 grade fireclay those which show no signs of fusion when similarly heated to 1580° C. or Cone 26. It is regarded as a sign of fusion if a test piece with sharp angles loses its angularity after heating to a predetermined temperature (see p. 35). It is customary to regard as 'fireclay' all clays which, when formed into the shape of a Seger Cone (fig. 6) do not bend on heating slowly until a temperature of 1580° C. (Cone 26) is reached. Any clays comprised within this definition and yet not sufficiently refractory to be of the No. 2 grade just mentioned may be regarded as No. 3 grade fireclays. Many of the last named are well suited for the manufacture of blocks for domestic fireplaces, for glazed bricks and for firebricks not intended to resist furnace temperatures. To resist sudden changes in temperature the material must be very porous--the article being capable of absorbing at least one-sixth of its volume of water. For this reason it is customary to mix fireclays with a large proportion of non-plastic material of a somewhat coarse texture, the substance most generally employed being fireclay which has been previously burned and then crushed. This material is known as _grog_ or _chamotte_ and has the advantage over other substances of not affecting the composition of the fireclay to which it is added, whilst greatly increasing its technical usefulness. The addition of grog also reduces the shrinkage of the clay during drying and ensures a sounder article being produced.