Soldering and brazing are terms often used to denote the same operation, that of joining similar or dissimilar metals by means of molten metal which may be of the same kind, but which usually has a lower melting point than the metals to be joined. The term "brazing" is usually employed to denote the soldering with an alloy of copper or zinc. "Soldering" is usually taken to represent the joining of surfaces by means of an alloy of lead and tin, and "hard-soldering" is understood to mean the process of uniting as above described with silver and its alloys used as a uniting metal. Hard soldering and brazing are practically the same, and are both done in about the same way.<br><br>The theory of brazing is the melting of a low fusing metal against the metals to be united while they are in such a condition of cleanliness and temperature that the metal welds itself to them. Soft brass, when melted, will weld itself to iron, copper, and a number of other metals, while the temperature of the metals in question is at a considerable number of degrees below their several melting points. In fact, only heat enough need be employed to fairly melt the uniting metal and to render it fluid enough to flow, or to "run," as the mechanic aptly states it.<br><br>To braze, also to solder, it is absolutely necessary that the surfaces to be united are clean and free from oxide.