SOLDERING is the process of uniting the surfaces of metals, by the intervention of a more fusible metal, which, being melted upon each surface, serves, partly by chemical attraction, and partly by cohesive force, to bind them together. In the ordinary soldering, the alloy used as a solder must be more fusible than the metals to he united, and must have a strong affinity for them. To insure perfect union between the solder and the surfaces to which it is applied, it is essential that they be made perfectly clean and free from oxide, and that the atmosphere be excluded during the operation. This is effected in various ways, but most commonly by the use of borax, sal ammoniac, or resin, either mixed with the solder or applied to the surfaces to be joined.
The kinds of solder used for the several metals are given under SOLDERS.
Articles of torought-iron, and some qualities of steel also, may be soldered with cast-iron ; the cast-iron being repeatedly heated and quenched in water, by which it becomes sufficiently friable to be beaten to a coarse powder with an iron pestle and mortar. In making fine steel instruments, gold, either alone or with a slight alloy of copper, is often used as solder. Silver solder, being less expensive, and nearer the colour of the steel, is preferred by some for this purpose. In larger articles of iron and steel, a solder consisting of equal parts of tin and iron is sometimes used.
Common plumbers solder is made of two parts lead and one part block tin ; or of the same metals mixed in nearly equal quantities ; bismuth is added when it is desired to make the alloy more fusible. Soft solder has two parts tin to one lead ; and other alloys of tin, lead, and bismuth, are used for uniting various articles of lead, tin, pewter, and other soft compounds. Such highly fusible solders are usually cast in ingots or strips, and melted as they are used by means of an instru ment called a soldering-iron, which is tipped with copper.
A curious tootle of soldering is resorted to in order to fix upon the back of the dial-plate of a watch the small copper studs by which it is attached to the plate that encloses the wheel-work. The heat required for melting spelter soldier would be Injurious to the enamel, and there fore the studs are made of wire plated with silver, and fixed by melt ing the silver on their aides, and causing It to nisi down to their base, where a mixture of borax and water is previously laid. Thus tho
studs are fixed without applying the jet of the blowpipe immediately to the back of the enamelled plate.
A kind of soldering, called Imminy-to, Is practised in some cases with sheet-lead, where it has been desirable to make a vessel entirely of that material; the junction being effected by pouring melted lead upon the edges to be united, until they fuse together. Somewhat similar to this is the process of aulosenous se/drring, This process, the invention of M. de Itichemont, consists in the union of two pieces of metal without the interposition of soy solder, by fusing them at the point of junction by jets of flame from a gas blowpipe. Tho apparatus used for the purpose contains a hydrogen gas generator, bellows for atmo spheric air, and valves for regulating the proportion in which the gas and air are to be mixed. The joints formed in this way are neater and kits liable to flaws than those made by the common process. 31r. Spencer discovered this process about the same time as M. de Itichemont; and his experiments led him to suppose that, by varying the admixture of gases, ajet of flame might be produced of intensity suitable for any metal to which it may be desired to apply this mode of soldering.
The recent investigations concerning the applicability of the metal aluminium to purposes of use and ornament, have necessarily included the preparation of some kind of Bolder. It is found that aluminium cannot be soldered in the ordinary way. N. Mourey, of Paris, has devised the following plan. lie 'puts on each surface a solder of zinc and altuninium, so as to form a thin film. Then, with a solder richer in aluminium, he joins the two pieces together. The first solder thus adheres to the metal, and the second solder to the first. The surfaces of aluminium are in the first instance prepared by a mixture of turpen tine, balsam of copaiva, and lemon-juice, and strongly heated. The solder No. 1 consists of 6 aluminium and 94 zinc; No. 2 of 20 alumi nium and 80 zinc.