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.

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.

To braze, also to solder, it is absolutely necessary that the surfaces to be united are clean and free from oxide. The term "clean" is used in brazing and soldering, to mean that there is no "matter in the wrong place" as far as the surfaces to be operated upon are concerned. If the surfaces should be covered with a mixture of plumbago and soap, it is pretty sure that the brass would not adhere, and they could be called "dirty." If, on the contrary, the surfaces were daubed with grease, resin, lime, borax or similar substances, the brazing will not be interfered with; hence, it is better to say that surfaces to be brazed or soldered, should be made bright and free from oxide, finger marks, and all other matter except the proper flux to prevent oxidization of the surfaces when heated. This, and this alone, is the purpose of all the fluxes used either in soldering, brazing or welding. The flux prevents oxidization from contact of the hot metal with the air, or with the gases from the fuel used in heating.

Aside from the proper cleaning and fluxing of metals to be brazed or soldered, it is necessary that they be fitted together as closely as possible. It may seem like a paradox, but is the truth never the less, that when surfaces are united by brazing, the union is stronger the less brass there is between the surfaces. That is: The closer the fitting of the parts, the stronger will the braze be after completion. It is unnecessary to "leave space for the brass," in fitting for a brazed joint. The penetrating power of melted brass may be demonstrated by drilling a hole in a piece of iron or steel. Drive a plug in the drilled hole, and force it in as tightly as possible, then rivet the ends of the plug and proceed to braze around one end of it, when it will be found upon test, that no matter how tightly the plug may have been driven in, the melted brass has found its way through the plate beside and around the riveted plug, and that it has brazed both ends of the plug and its entire length as well. Therefore, fit tight, for brazing, and trust the liquid brass to find its way through the entire joint without fail.

Borax is the flux usually employed for all kinds of brazing. For commercial work on a large scale, boracic acid is used as it is cheaper than borax, being purchased in a granular form, in bulk, by the keg or barrel. For the uniting metal, some alloy of copper and zinc is universally employed. When other substances, such as silver is used, the operation becomes known as "hard soldering," as described elsewhere.


SOLDERING

Soldering is much like brazing in some of its details. In fact, some kinds of soldering are done exactly like some kinds of brazing, but other varieties of soldering are totally unlike any brazing operations. Soldering, therefore, may be taken to mean the uniting of two or more pieces of metal, with fusible alloys of lead and tin. Sometimes, lead areas are united by melting their surfaces without the use of solder, the surfaces being fluxed. This form of soldering differs slightly from welding, and is called "burning" by the trade. The method is usually employed in uniting the edges of sheets of lead used in the lining of acid tanks or similar apparatus.

The particular kind of soldering usually employed is by the use of the so-called "soldering iron," which is really a copper bit placed on the end of an iron handle. An alloy of lead and tin is used which readily adheres to the surface of the bit, which must be clean, free from oxide, etc. The operation of coating a copper bit with solder is known as "tinning," and will be described elsewhere. The theory of soft soldering is: that as the soft metal adheres to, and unites with the surface of the copper bit, so will the soft metal, under certain conditions, adhere to, and unite with the surface of the metals to be soldered. In fact, soft soldering, as well as brazing, consists of welding together two or more pieces of similar or dissimilar metals by means of another metal of lower melting point. That constitutes soldering; all the rest of the operation, is detail, which may be varied' to suit conditions.