Stoneware pipes have been used since antiquity and, because of their excellent resistance to corrosive action, they are still widely used for nonpressure drainage purposes in diameters up to 40 in. (1000 mm).
Clay is ground to a powder and brought to a plastic state with water and other additives. It is then passed through a vacuum chamber to remove trapped air, and fed into extruders in which the pipe is formed. The extruded pipes are trimmed and dried, and then passed into kilns in which the temperature is raised to about 2000° F (1100° C) to cause fusion of the clay particles, or vitrification. Salt is sometimes added to produce glazed surfaces.
Aluminum pipes are either extruded or fusion welded, either longitudinally or spirally. Copper pipes are usually extruded, though various other seamless production processes (as for steel) are used for larger diameters. Lead pipes, now becoming obsolete, are extruded.