—Hard brazing solder (spelter) is somewhat difficult to make. The metals have to be melted in a crucible and cast at a proper heat, and while in a certain condition have to be pounded or punned in a mortar. This disintegrates the materials and forms crystals of various sizes, some being as coarse as wheat grains, varying in fineness down to that known as 0 0, which is very fine dust indeed, and used only on very particular work, such as tubing 1⁄8 in. or 3⁄16 in. in diameter. The quantity of 0 0 from 1 cwt. of solder is very small, the corresponding quantity of coarse grains being much larger; so that unless a quantity is required, it is cheaper to buy than to make. It is necessary to employ the purest materials, and in purchasing hard solder it is advisable to state the purposes for which it is to be used.
As a general rule, a solder should melt at a heat just under the melting point of the metals to be united. Now, in ironwork, or with the steel used in cycle work, this is impossible, for the melting points of these two metals are too high to be effected by the bunsen blowpipe or brazing hearth; but to join two metals in which the melting point approximates very closely to that of the solder requires great care in order that the metals may not be fused and the join spoiled. The reason why the melting point of solder should be about the same as that of the metals being joined is apparent when it is remembered that heat and cold, vibration and concussion, tension and compression, have very considerable effect on metals, and that if the expansion and contraction of these under working conditions is not nearly alike, disruption or opening of the joint will follow.
Hard solders or spelters are mainly composed of copper and zinc—that is, they are brass alloy—the quality most extensively used consisting of equal parts of copper and zinc. As the quantity of copper in the solder is increased, so the fusing or melting point is raised.
Ordinary copper melts at about 2,000° F. and zinc at about 840° F., and a solder composed of equal parts of each metal has therefore a high melting-point.
A very hard solder consists of equal parts of silver and copper. Generally, a spelter of different composition is required for iron, copper, and brass work, that for the latter being required more readily fusible than that for the former. A suitable spelter for ironwork is one composed of 2 parts copper and 1 part zinc; a spelter for copper consists of 3 parts copper and 2 parts zinc; while equal parts of copper and zinc make a suitable spelter for ordinary brass work. If a very low melting point be required, a little silver should be added to the last-given spelter.
The Flux.
—Borax is the best-known flux for brazing. It is beneficial, however, to have the borax calcined (fused), as it settles down to its work immediately when applied to the hot metal, whereas uncalcined borax has a tendency to swell and fall off the work.
Spelter is in the form of filings, a thin stick, or wire. Filings are apt to be blown from the work. Brazing is a very useful, and, if properly done, reliable, method of joining two pieces of iron. A brazed joint is considerably stronger than a soft-soldered one, and easily resists temperatures that would cause ordinary solder to run.
The process is not at all difficult if there is sufficient heat, and, for those who have no gas laid on, the purchase of a paraffin Ætna brazing lamp can be recommended. With this lamp, of course, the bellows is not used, and only an iron hearth with asbestos cubes is wanted; but gas should be used if available.