Furnaces must be lined with a material which is even more refractory than ordinary brick. The oxide and sulphide of iron, which are so common (and comparatively harmless) in ordinary brick, will ruin a fire brick if they are present to a greater extent than a very few per cent. Fire brick should be made from nearly pure sand and clay. There is comparatively little need for mechanical strength, but the chief requirement is their infusibility, and pure clay and silica fulfil this requirement very perfectly.