In applying the smoke test, a machine designed for the purpose of producing a heavy volume of black smoke is used. Various materials are used in this machine for producing the smoke, among them being oily cotton waste, tarred paper, and oakum which has been soaked in petroleum. Waste is the best material, as it gives off a dense smoke and is not so inflammable as most other materials. In Fig. C, Plate 25, is shown the manner in which the smoke test is applied. Generally the hose connection from the smoke machine is run through a lead cap which is closed up with putty. The smoke-test plug shown in Fig. B is also used, the smoke passing through the plug.

After the whole system is filled with smoke, an air pressure equal to a one-inch water column is applied. Defects are shown by puffs of smoke escaping through them.

The smoke test appears to be displacing the peppermint test, and for work in general, it appears to be the more reliable of the two.