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Apart from the use of rivets, screws, etc., metal is commonly joined by soldering, brazing, or welding, three groups of processes that have one thing in common—the use of heat to fuse either the metals themselves or an alloy which is interposed to consolidate the joint. The word “solder” is derived through the French from a Latin word meaning “solid.”

Soldering may be “soft” or “hard.” Soft-soldering uses lead-tin alloys which are easily melted in a bunsen gas flame or with a hot iron or bit; while hard-soldering employs a silver-copper alloy, to melt which a mouth blowpipe at least is necessary. Brazing is hard-soldering with spelter (brass), and a forge or a heavy blowlamp or a powerful blowpipe must be employed to provide the heat.

Welding is a fusion process which in the past was almost entirely confined to wrought-iron and steel, these metals possessing the property of weldability to an extent unknown in the case of any other metals. The blacksmith’s process of welding is to heat the iron or steel until the surface of the metal becomes pasty, and then to bring the two pieces into intimate contact by hammering on the anvil. Of late years the welding of iron, steel, copper and some other metals has been rendered possible by the use of certain electrical and chemical methods and—most important of all—by the use of the oxy-acetylene blowpipe, the process being known as “fusion welding” or “autogenous soldering,” the word autogenous implying that the process is complete in itself and independent of the use of any extraneous substance such as solder. The thermit process, of which so much has been heard, and which is briefly dealt with later, is the fusion welding of iron and steel by means of the intense heat produced by the combustion of a special chemical compound. Perhaps the oldest of the autogenous soldering processes is “lead-burning,” in which the flame of an airo-hydrogen blowpipe is brought to bear upon the lead, the joint being fed with a strip of the same metal.

Soft-soldering is an operation that the beginner will not find nearly so difficult as hard-soldering or brazing, and although the strength of joints made by it is not nearly equal to that produced by the methods named, it fills a useful place within its scope. It is purely a surface union—that is, the solder adheres to the faces in contact in much the same manner as an adhesive sticks to metal; but with the assistance of fluxes, the contact is made so intimate that some force is necessary to break the joint. Soft-soldering is also of use where brazing would simply mean the ruin or destruction of the metals, as in the cases of lead, poor-quality brass, pewter, tin, zinc, and in tinplate and galvanised iron.

In silver-soldering and brazing, the silver or spelter that fuses to form the joint alloys itself so intimately with the copper or brass that it actually becomes part of the piece itself, and for all practical purposes cannot be distinguished from it. But soft-soldering is not always inferior to hard-soldering. Indeed, the surface nature of the soldering often constitutes its value.

The strongest joints of all are produced by fusion welding, as will be duly understood from later chapters.