Where a gas or liquid is to be retained in a pipe or other vessel without leakage, a tight joint is necessary. The method of grinding valves to their seats has already been explained. In that case, it was shown that a metallic contact between the valve and its seat is all that is required in order to make it a tight joint. Two surfaces that have been scraped to fit will also accomplish the same purpose. This is frequently too expensive an operation to be performed, especially on rough work. In such places a softer material may be interposed between the two surfaces. Where the joint is to be a permanent one and is not to be taken down, the red lead joint is usually employed. This consists in the use of a mixture of red and white lead between the joints. To ordinary white lead ground in oil, add enough dry red lead to make a paste that can be spread without sticking to the blade with which it is applied. After the mixture has been made, it will be improved by pounding it well with the hammer. It may then be laid between the two pieces of metal forming the sides of the joint, and the latter be drawn together. Red lead joints are extensively used in pipe-fitting. The red lead has a tendency to rust the iron with which it is in contact, and thus forms a very tight connection between the two pieces. Where provision is to be made for taking down the joint at a future time, it is better to use a graphite paste made for the purpose. This does not rust the metal and it forms a perfectly tight joint, which may be taken down without difficulty at any time.

Fig. 255. Conuecting Rod Brasses

Fig. 255. Conuecting-Rod Brasses.

Joints that are subject to occasional disconnecting can be best held by a disc of rubber packing. The latter is cut to fit the flanges between which the joint is to be made, and they are then drawn tightly together.

Joints that are to be frequently taken down are usually packed with a piece of copper wire. Such a place is the joint between the steam chest and cylinder of a locomotive engine. A groove is cut in the two surfaces, and a copper wire is laid therein. This wire should be about 1/4 inch in diameter. Its size, however, depends upon the joint to be packed. The ends of the wire are soldered together so that no leakage may occur past the ends.

Another form of joint is the rust joint

This is always permanent in character. The making of such a joint consists in rusting the two surfaces together. The following are the proportions by weight of the rusting material: 100 parts of iron turnings, 1 part of sal ammoniac, and 1/2 part of sulphur. The setting of the joint can be hastened by increasing the amount of sal ammoniac from 15 to

25 per cent. Mix the ingredients thoroughly, and just cover them with water.