Hard-soldering by brazing with spelter is used to a very great extent in the metal industries, especially in the manufacture of cycles and motor-cars. Although several mechanical joints have been tried in cycle manufacture, the greatest number of joints are made by means of brazing solder. A joint made in this manner is almost as strong as a weld, and the steel tube itself will often break under a strain and the brazed joint remain intact. Copper and brass tubes, when well brazed, will stand a pressure of 40 lb. or more per square inch.
=Spelter.=--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.