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rule is to heat for three to five hours to a temperature not exceeding 300 C. The range of the temperature in which the change takes place is in some cases exceedingly small. Temperature-shrinkage curves are helpful in indicating the behaviour of a clay. Refractoriness is best tested by firing cones of the clay side by side with Seger cones. It must be remembered that these complex silicates have no definite melting point, and that the fineness of the material and the duration of firing are as important as the temperature reached. Other things being equal, the firing point is dependent on the chemical composition. Colour. The chief colouring impurity in a clay is iron oxide, though the depth of colour is dependent on the amount of alumina and lime as well as iron. Clays high in lime or alumina may contain over 1 per cent, of iron without affecting the whiteness of the body. The atmos- phere of the kiln also influences the colour, as a reducing atmosphere forms the green or blue black ferrous colours. Standards of colour should be made for purposes of com- parison. Lovibond states that pure calcium sulphate is the whitest material obtainable. The three primary colours red, green, and violet are matched by carmine, emerald green, and French mauve. With these four substances and lamp-black one can express any colour by the amount of each ingredient. The Co-efficient of Expansion of the fired body should