To produce this glaze upon a fire-clay or stoneware pipe, it is usual to throw salt into the kiln during the tiring, so that when the pipe is completely burnt, it has a highly-glazed surface, which practically renders the pipe impermeable except under extraordinarily-high pressure, and enables it to resist the action of whatever acids are present in the sewage, and also the abrasion caused by the silt and other bodies which find their way into drains.

5. Materials for drain-pipes. - Drain-pipes are most frequently made of earthenware, but cast-iron pipes are now coming into favour, as they possess certain advantages.

Earthenware pipes are now most generally used, and may be divided into stoneware and fire-clay. These materials, though in some respects different, are to a great extent similar both in properties and method of use. It is a common thing for drain-pipes to l>e specified as stoneware without regard to these difler-ences, and for fire-clay pipes to be accepted as stoneware; it is, however, very difficult to define precisely where stoneware ends and fire-clay begins. The well-known Lambeth "stoneware" pipes are made from clays obtained from Dorset and Devon, and their peculiar excellence consists in the vitreous and impermeable body of the material. It is more tenacious than fire-clay, so that an equally strong and durable pipe is obtained with a thinner crust. The pipes are consequently lighter, which effects a small saving in carriage, and in handling the larger pipe. It is a significant fact that this clay, owing to its power of resisting the action of corrosive liquids, is extensively used for chemical apparatus.

More frequently, however, drain-pipes are made from the days obtained from the coal-measures, and known as the "fire-clay beds". These vary very much in their density, porosity, and ductility, all of which qualities are severely tested in the various processes of pipe-manufacture. The raw mixture should be so prepared and pugged, as to be able when dry and fired to retain, with only a slight deviation, the .shape imputed to it in moulding. The most essential features to be obtained in the finished material are toughness, tightness, impeniieability, durability, and strength, and the clay should be so finely ground that the finished pipe shall possess a perfeetly smooth surface throughout, the crust being thoroughly homogeneous and capable of noting intense heat.

Analyses show that the composition of the clays from which the stoneware pipes are made, and that of the Wortley elay from which the Leeds pipes are made, are much the same, but the latter pocesses a mechanical condition which renders the pipes less brittle.

A good pipe should be well burnt throughout its entire body, straight in the barrel, truly cylindrical when cut die-square, highly glazed over the whole surface; the interior should be free from blisters, clinkers, and other defects, and the pipe should ring sound when rapped with a hammer.

It is customary for the crust of stoneware pipes above 6 inches in diameter to be 1/12 of the diameter in thickness, and that of fire-clay pipes to be 1/16. The following table, containing various dimension- of pipes, will be found useful for reference:-

Table XXVIII. Dimensions Of Earthenware Drain-Pipes

Diameter of pipe, .

4 in.

6 in.

9 in.

Thickness of crust

Stoneware, .

9/16 diam

11/15 in

3/4 in.

Firec-clay .

9/16 in

11/16 in.

15/16 in.

Depth of socket,

.

1 3/4 in.

2 1/2 in.

2 1/2 in.

Weight of pipe,

112 diam,.

__

-

591 lbs.

1/10 diam.,.

17 lbs.

30 lbs.

54 lbs.

Number per ton,

1/12 diam.,.

-

-

25 yds.

1/10 diam.,.

88 yds.

50 yds.

23 1/8 yds.

The use of cast iron for the carriage of water naturally suggests its employment for the conveyance of that water when it has becomecomc converted into sewage, and if it is possible to make a joint that will withstand the pressure exerted by the water, the same joint will Ikj effective in preventing any escape of sewage or sewer-gas. Cast-iron pipes used for the purpose of drainage, although not requiring to l>e so heavy or strong as water-pipes, should certainly be finished in the same way, in order to prevent the action of rusting both on the interior and exterior. The mere mention, in a specification, of iron pipes without stating details is not sufficient, as builders would get very light ones, and even rain-water pipes, so that the consequences would be worse than the condition of things it was sought to remedy. There must therefore be rigid regulations with regard to size, weight, and finish. As the pressures in drains, even under ididental circumstances, are not particularly great, it is not necessary to me very heavy cast-iron pipes. They should in all cases be of the following dimensions an«l weight as a minimum: -

Table XXIX. Dimensions Of Cast Iron Drain-Pipes

Length

Diameter

Thickness

Weight

Depth of Lead.

Weight of Lead in lbs

Space in Sockets.

ft.

4in.

3/8in.

1 cwt 1 qr. 3 lbs.

1 \ in.

2-14

3/8 in

It.

6 in

3/8 in

2 cwt

2 in.

5.6

3/8 in

It.

9 in.

1/2 in.

4 cwt

2 in.

10 1/2

3/8 in

Iron pipes must be free from rough projections, and should be coated in some way to prevent oxidation from contact with the sewage. The composition b known and most commonly used is the varnish devised by the late Dr. Angus Smith. It i- a compound of tar, pitch, and oil, into which the iron is dipped, both iron and solution being at a temperature of not less than 300° F. The solution dries into a hard glossy black skin, which resists, for a considerable time at any rate, the ordinary influences to which pipes are exposed.

Another class of pipe which is frequently used is the one known as the glass-enamelled iron drain-pipe. The skin is practically like glass, and of a clear yellow or brown colour. These pipes are made in 6-feet lengths exclusive of socket, and their weights are - for 4 in. 90 lbs., and for 6 in. 150 lbs

The use of cast iron for pipe- permits of much more forcible methods being adopted in making the joints, the plan usually adopted being to make a lead joint on the ordinary socket and spigot pipes, as shown in Fig. 313. The depth of the sockets should be 4 inches, the inner 2 inches being first caulked with white spun yarn, and the outer 2 inches then filled with molten lead, care being taken to use sufficient lead in one running to complete each joint. Bends, traps, junctions, access-holes, and all the usual accessories as made for stoneware drains, can also be obtained for cast-iron drains, and can be fixed and jointed without difficulty.