It is not always an easy matter in burning a red brick to obtain two kilns full of the same tint, even in the same yard. When the employment of pyrometers becomes more general, that will be considerably simplified; but it is a difficult matter to get a reliable instrument, none of the forms hitherto invented being altogether suitable. That by Professor Roberts-Austen is as good as any. Many manufacturers, we are sorry to say, place colour before everything else; they even sacrifice durability to attain a certain tint. And there is much excuse for them so long as they find a ready sale for the material. When colours are made from artificially introduced mineral matter (which is not so often the case as some appear to think) the mineral introduced is, most commonly, iron; though it will be understood, from what we have previously said, that it must be used very sparingly. The ultimate tint assumed by the brick cannot always be judged beforehand from the colour of the brick-earth. In brickmakers’ language, a red clay is one that produces a red brick, a blue clay a blue brick, and so on. For the most part, colour depends on the proportion of hydrated oxide of iron in the clay; if iron is present in an earth that contains no lime, or similar mineral substance, the colour produced in the brick at a moderate red heat will be red, and at the same temperature, with the same brick-earth, the more iron present the deeper the tint. In an ordinary brick-earth, when more than 10 per cent. of iron is present, the clay is apt to burn bluish, however, and, in certain cases, almost black. With a smaller proportion of iron, and the application of intense heat, the same tint may result, and the brick become vitrified. A brown colour may frequently be obtained when the brick-earth has from 2.75 to 4 per cent. of magnesia, or a similar proportion may be artificially added to the earth. To obtain a white brick, so that it shall also be of excellent quality, the pure white clays of Devon and Cornwall are the best, though the so-called “white” is, in the majority of cases, a light cream colour, unless, of course, the brick is glazed. In the neighbourhood of London, a whitish brick results from a mixture of chalk (carbonate of lime) with clay or loam, and is known as a “malm.” In parts of Yorkshire, white pressed bricks are manufactured from common red clay mixed with magnesian lime (made from magnesian limestone) in a slacked condition. The latter ingredient, on introduction, immediately absorbs about 40 per cent. of the moisture present in the clay. Yellow bricks can easily be manufactured from the more impure kaolins; also from certain clays in Cambridgeshire, Huntingdonshire, Kent, &c. (gault bricks); “malms” are mostly yellow, though called white.