SOLDERING is the union of the surfaces of two metals, generally by the intervention of a third.. In the ordinary mode of soldering, the alloy used as a solder must be more fusible than the metal or metals which are to be united, and must have a strong affinity for them. The solder usually contains a large proportion the metal to which it is to be applied, in combination with some more easily fusible metals. To insure perfect metallic 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, in order to prevent the formation of any oxide while the process is going on. This is effected in various ways, but most commonly by the use of borax, sal ammoniac, or rosin, either mixed with the solder or•applied to the surfaces to be joined.
Various kinds of solders or alloys are used, according to the metal which is to be soldered. Platinum is soldered with gold. Gold is soldered with an alloy of fine gold, silver, and copper. Silver solders usually consist of silver mixed with brass, and sometimes with zinc. Brass, copper, and iron are soldered with an alloy of zinc with copper or brass. Articles of wrought 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. Common plumbers' solder is made )f two parts lead and one part block tin; or of he same metals mixed in nearly equal quant ities; bismuth is added when it is desired to nake the alloy more fusible. Soft solder has
wo parts tin to one lead; and other alloys of In, lead, and bismuth, are used for uniting carious articles of lead, tin, pewter, and other ,oft compounds. Such highly fusible solders tre usually cast in ingots or strips, and melted .
as they are used by means of en insfinmelit called a soldering-iron, which is tipped with copper—that metal being preferred for its greater affinity for tin. In soldering tin plates together, their edges are made to overlap ; but in almost every other case the edges to be joined are made only to meet, the solder being run between their abutting edges.
A kind of soldering, called burning-to, has been long 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 on to the edges to he united, until they fuse together. Somewhat similar to this is the process intro duced some' years ago under the name of antogenous soldering. This process, which is the invention of a French gentleman, M. de Richemont, consists of the union of two pieces of metal without the interposition of any solder, by fusing them at the point of junction by jets Of flame from a gas blowpipe. The apparatus used for the purpose contains a hydrogen gas generator, bellows for atmospheric air, and valves for regulating the proportion in which the gas and air are to be mixed.