The term soldering is generally applied when fusible alloys of lead and tin are employed for uniting metals. When hard metals, such as copper, brass or silver are used, the terra brazing (derived from brass) is more appropriate.
In uniting tin, copper, brass, etc., with any of the soft solders, a copper soldering-iron is generally used. This tool and the manner of using it are too well known to need description. In many cases, however, the work may be done more neatly without the soldering-iron, by filing or turning the joints so that they fit closely, moistening them with the Boldering fluid described hereafter, placing a piece of smooth tin-foil between them, tying them together with binding wire, and heating the whole in a lamp or fire till the tin-foil melts. We have often joined pieces of brass in this way so that the joints were quite invisible. Indeed, with good soft solder almost all work may be done over a spirit lamp or even a candle, without the use of a soldering-iron.
More minute directions may be found in the Young Scientist, Vol. I, page 56.
Advantage may be taken of the varying degrees of fusibility of solders to make several joints in the same piece of work. Thus, if the first joint has been made with fine tinner's solder, there would be no danger of melting it in making a joint near it with bismuth solder, composed of lead, 4; tin, 4; and bismuth, 1; and the melting point of both is far enough removed from that of a solder composed of lead, 2; tin, 1; and bismuth, 2; to be in no danger of fusion during the use of the latter.
Soft solders do not make malleable joints. To join brass, copper or iron so as to have the joint very strong and malleable, hard solder must be used. For this purpose equal parts of silver and brass will be found excellent, though for iron, copper, or very infusible brass, nothing is better than silver coin rolled out thin, which may be done by any silversmith or dentist. This makes decidedly the toughest of all joints, and as a little silver goes a long way, it is not very expensive.
For most hard solders borax is the best flux. It dissolves any oxides which may exist on the surface of the metal, and protects the latter from the further action of the air, so that the solder is enabled to come into actual contact with the surfaces which are to be joined. For soft solders the best flux is a soldering fluid which may be prepared by saturating hydrochloric acid (spirit of salt) with zinc. The addition of a little sal ammoniac improves in It is said that a solution of phosphoric acid in alcohol makes an excellent soldering fluid, which has some advantages over chloride of zinc.
In using ordinary tinner's solder for uniting surfaces that are already tinned - such as tinned plate and tinned copper - resin is the best and cheapest flux, but when surfaces of iron, brass or copper that have not been tinned are to be joined by soft solder, the soldering fluid is by far the most convenient. Resin possesses this important advantage over soldering fluid, that it does not induce subsequent corrosion of the article to which it is applied. "When acid fluxes have been applied to anything that is liable to rust, it is necessary to see that they are thoroughly washed off with clean warm water and the articles carefully and thoroughly dried.
Oil and powdered resin mixed together make a good flux for tinned articles. The mixture can be applied with a small brush or a swab tied to the end of a stick.
In preparing solders, whether hard or soft, great care is requisite to avoid two faults - a want of uniformity in the melted mass, and a change in the proportions of the constituents by the loss of volatile or oxidable ingredients. Thus, where copper, silver, and similar metals are to be mixed with tin, zinc, etc., it is necessary to melt the more infusible metal first. When copper and zinc are heated together, a large portion of the zinc passes off in fumes. In preparing soft solders, the material should be melted under tallow, to prevent waste by oxidation; and in melting hard solders, the same object is accomplished by covering them with a thick layer of powdered charcoal.
To obtain hard solders of uniform composition, they are generally granulated by pouring them into water through a wet broom. Sometimes they are cast in solid masses and reduced to powder by filing. Silver solders for jewelers are generally rolled into thin plates, and sometimes the soft solders, especially those that are very fusible, are rolled into sheets and cut into narrow strips, which are very convenient for small work that is to be heated by a lamp.
The following simple mode of making solder wire, which is very handy for small work, will be found useful. Take a sheet of stiff writing or drawing paper, and roll it in a conical form, rather broad in comparison with its length. Make a ring of stiff wire, to hold it in, attaching a suitable handle to the ring. The point of the cone may first of all be cut off to leave an orifice of the size required. When filled with molten solder it should be held above a pail of cold water, and the stream of solder flowing from the cone will congeal as it runs, and form the wire. If held a little higher, so that the stream of solder breaks into drops, before striking the water, it will form handy, elongated "tears" of metal; but, by holding it still higher, each drop forms a thin concave cup or shell, and, as each of these forms have their own peculiar uses in business, many a mechanic will find this hint very useful.
Hard solders are usually reduced to powder either by granulation or filing, and then spread along the joints after being mixed with borax, which has been fused and powdered. It is not necessary that the grains of solder should be placed between the pieces to be joined, as with the aid of the borax they will " sweat" into the joint as soon as fusion takes place. The same is true of soft solder applied with soldering fluid. One of the essential requisites of success, however, is that the surfaces be clean, bright, and free from all rust.
The best solder for platinum is fine gold. The joint is not only very infusible, but it is not easily acted upon by common agents. For German-silver joints, an excellent solder is composed of equal parts of silver, brass, and zinc. The proper flux is borax.