Pipes and other articles made in clay are practically divided under four heads.

1. Unglazed Earthenware, made from ordinary clays, similar to those used for common bricks and tiles.

Earthenware of this description is weak, porous, liable to the attacks of frost, and is not adapted to resist the atmosphere or other destroying agents.

2. Fireclay Ware, made from the fireclays of the coal-measures (see p. 120).

This material has a very open grain, is porous, except where protected by glazing; and is weak when compared with terra cotta or stoneware.

It is much used for common varieties of the different articles about to be described, especially in the localities where the fireclay is found; but it is inferior to stoneware or to terra cotta for nearly every purpose.

3. Stoneware is made, as before stated, from clays of the Lias formation, mixed with sand and ground pottery, to prevent shrinkage.

The characteristics of this material have already been pointed out (see p. 128).

Its strength, durability, imperviousness, and resistance to destructive influences make it an admirable material for sanitary ware, sewer pipes, ornamental works exposed to the atmosphere, and for vessels to contain chemical compounds.

4. Terra Cotta is often used for pipes and other miscellaneous articles.

Its composition, and the mode in which it is manufactured, have already been described.

It is inferior to stoneware, inasmuch as it is more absorbent and less dense in grain. It is burnt at the same heat as fireclay goods, but is superior to them in strength and durability.