A process akin to Soldering, -but provides a much stronger union, Briefly described, it consists of the following operations : (I) The pieces that are to be brazed must be made scrupulously clean, as in Soldering. (ii) The cleaned parts are coated with a flux, usually borax, (iii) The parts are bound up so that they are unable to move, relatively, during the application of heat. (iv) Heat is applied, (v) When red hot, the parts are withdrawn from the heat, and the brazing-solder is applied, (vi) More heat is applied, and this is continued until the solder flows into the required positions, (vii) The article is withdrawn from the heat ; it is allowed to cool down, and is, then, finally cleaned.
The difficulty is to provide a suitable heating apparatus. If the article to be brazed is small, it will be possible to arrange the kitchen fire so that the article may be dropped into the glowing coals ; but when brazing something as large as a bicycle frame, a great deal of space is necessary, and a proper blow-lamp or gas blow-pipe will be required. Whatever the flame, there must be no smoke, and the flame must be bluish, not yellow. Some braziers surround the flame with asbestos cubes, in order to keep the heat directed.
Before the article is placed in heat, it must be bound up—with wire or clips—so that it temporarily assumes its permanent shape. But first, the runs for solder must be painted with a paste made of borax and water—only where thjs is applied, will the solder adhere.
When the heat has turned the article a dull red, turn down the flame or remove the article from the fire with a pair of tongs, and drop the solder—usually spelter— into position, together with some dry powdered borax. Then return it to the heat, when it will require careful watching. The spelter will soon begin to flow ; but its course must be directed into the required channels. A darning needle, the tip of which has been coated with borax, will serve admirably (hold this with a cloth : it will soon grow very hot). A long piece of stout wire with a small triangle bent at one end, will be useful in dropping small pieces of fresh spelter in place, where the initial supply refuses to flow. When all the joins have been properly supplied with solder, and the latter has become fully molten, the job is finished ; all that remains is to leave it to cool and give it a final cleaning.
Occasionally borax swells under heat, and pushes the joints asunder, so that the solder has no chance of performing its functions. No amount of extra binding is of any avail, and there seems to be no remedy. It has been suggested, however, that, instead of ordinary borax, the anhydrous or fused variety should be used and that it should be ground in ordinary paraffin oil, in which case, all trouble from this cause will be obviated. Another plan is to try Boron Compo.
An obvious difficulty in brazing is choosing suitable solder. If an article is made of spelter, spelter cannot be used as the solder, since the two fuse at the same moment. There must be a considerable interval between the melting points of the two metals, and the solder must fuse first. Therefore, when the amateur is brazing a metal he does not know, he will be wise to experiment a little. He should take a piece of the metal, and an equal piece of the solder, on an old plate and then apply heat; very small portions are sufficient. If the solder does not melt a good deal earlier than the metal, it will be useless for his purpose.