The quality of the bricks produced depends to a very great extent on the selection and mixing of the clay.

Pure or Foul Clays are sometimes used for bricks without the addition of other substances. In such a case any sand they contain acts merely to prevent excessive contraction. For want of a flux it does not become fused so as to bind the particles of the brick together.

Bricks made from such clays are rather bated than burned. They are not so well able to resist the action of the weather as those which are partly vitrified through the aid of a flux.

Pure clays are therefore very much improved by the addition of sand or loam, by adding lime to act as a flux, or ashes to provide alkalies for the same purpose.

1 Abney.

2 Knapp.

Loams are so loose and sandy that they require a flux to fuse and bind the particles together, and to take up the excess of sand that would otherwise remain in an uncombined state.

Marls are, of all the clays, the best suited for making bricks without mixture with other substances, though they are often mixed with chalk or lime when there is any deficiency in that constituent.

The Colour of Bricks depends upon the composition of the clay, upon the kind of sand used for moulding, on the state of dryness of the bricks before burning, on the temperature at which they are burnt, and upon the amount of air admitted to the kiln.

Pure clay, free from iron, will burn white, but the colour of white bricks is generally produced by adding chalk to the clay.

The presence of iron produces a tint which varies from light yellow to orange and red; the colour increases in intensity according to the proportion of iron contained in the clay.

To obtain a clear bright red brick the clay should be free from impurities, and should contain a large proportion of oxide of iron, which is converted by burning into the red oxide, but not fused.

When there is from 8 to 10 per cent of oxide of iron, and the brick is raised to an intense heat, the red oxide of iron is converted into the black oxide, combines with the silica, and fuses, producing a dark blue or purple colour.

When a small quantity of manganese is present, with a large proportion of iron, the brick becomes darker still, blue or even black.

A little lime in the presence of a small quantity of iron produces a cream colour; an increase of the iron changes the colour to red, and an increase of the lime produces a brown colour.

Magnesia in the presence of iron makes the brick yellow.

A clay containing alkalies and burnt at a high temperature becomes a bluish green.