Brick earths are generally divided into three classes.

1. Plastic or Strong Clays (called by the brickmaker "foul clays"), which are composed of silica and alumina, with but a small proportion of lime, magnesia, soda, or other salts. These are also known as pure clays.

2. Loams or Mild Clays, consisting of clay and sand, and sometimes called sandy clays.

3. Marls or Calcareous Clays, which contain a large proportion of carbonate of lime.

1 Mallet On Brickmaking.

2 Common salt.

Malm is an artificial imitation of natural marl, and is made by-mixing clay and chalk in a wash mill. It is sometimes called washed clay.

It generally happens that a clay as found in nature is unfit for brickmaking by itself.

It will probably turn out to be deficient in some necessary quality which has to be supplied by mixing it with other clays, or by adding the constituent required, such as sand or lime.

A good Brick Earth should contain sufficient flux to fuse its constituents at a furnace heat, but not so much as to make the bricks run together and become vitrified.

Such earths contain from 1/5 to 1/3 alumina, and from 1/2 to 3/5 silica, the remainder consisting of carbonate of lime, carbonate of magnesia, oxide of iron, etc.

The bricks made from such clays are a silicate of alumina and lime or other fluxes.

The following Table gives the analysis of some brick clays: -