Terra cotta, stoneware, and fireclay ware, are all burnt in domed kilns.
The heat is applied gradually, and after it has risen to its height is kept up for a period varying from 24 to 72 hours, according to the size of the kiln and of the articles in it. The kiln is then allowed to cool down gradually.
Terra cotta is burnt at a much lower temperature than stoneware.
In order to protect articles of a delicate nature from direct contact with the fire, which would discolour them, they are placed in large fire-clay jars called seggars, or enclosed in a casing of fire-brick formed within the kiln, and known as a muffle.
Pipes are made from clay, very finely ground, washed, sieved, tempered, pugged, and forced by machinery through a mould, or dod as it is called - dried, and baked in a circular kiln.
Agricultural Drain Pipes are made of various sections, but the circular and shaped are those in most common use.
These pipes are sold in 2-foot lengths, and of diameters varying by hall' inches from 1 inch to 6 inches.
They are 3 or 4 inches in length, and about 1 inch greater in diameter than the pipes they unite.
They are, however, generally omitted altogether.
Sewer Pipes should be of a vitreous imperishable material, of sufficient strength to resist fracture, having toughness enough to withstand shocks, tenacious, hard, homogeneous, impervious, uniform in thickness, true in section, perfectly straight longitudinally, or formed to the proper curve, uniform!)' glazed both inside and outside, free from fire cracks and flaws of all kinds.
When struck they should ring clearly.
Porous substances are not so good as those that are vitreous throughout, and pipes burned at a low temperature are inferior to those that have been subjected to a considerable heat.
Sewage pipes are made both from stoneware and fireclay. The former is the stronger material, and is said better to resist the decomposing effect of sewage and other substances having a chemical action.
Salt-glazed pipes only should be used; if the glaze can be picked off it is proof that the pipes are made out of a clay that would not stand a high temperature; in fact, that the pipes are not stoneware.
Fireclay pipes should be made thicker than those of the same diameter in stoneware.