A spelter suitable for working with copper is made from equal parts of copper and zinc, melting at about 1400° Fahrenheit, 500° below the melting point of the copper itself. A still softer brazing metal is composed of half copper, three-eighths zinc and one-eighth tin. This grade is used for fastening brass to iron and copper and for working with large pieces of brass to brass. For brazing thin sheet brass and light brass castings, a metal is used which contains two-thirds tin and one-third antimony. The low melting point of this last composition makes it very easy to work with and the danger of melting the work is very slight. However, as might be expected, a comparatively weak joint is secured, which will not stand any great strain.
All of the above brazing metals are used in powder form so that they may be applied with the spatula where the joint is exposed on the outside of the work. In case it is necessary to braze on the inside of a tube or any deep recess, the spelter may be placed on a flat rod long enough to reach to the farthest point. By distributing the spelter at the proper points along the rod it may be placed at the right points by turning the rod over after inserting into the recess.
Flux.--In order to remove the oxides produced under brazing heat and to allow the brazing metal to flow freely into place, a flux of some kind must be used. The commonest flux is simply a pure calcined borax powder, that is, a borax powder that has been heated until practically all the water has been driven off.
Calcined borax may also be mixed with about 15 per cent of sal ammoniac to make a satisfactory fluxing powder. It is absolutely necessary to use flux of some kind and a part of whatever is used should be made into a paste with water so that it can be applied to the joint to be brazed before heating. The remainder of the powder should be kept dry for use during the operation and after the heat has been applied.
Preparing the Work.--The surfaces to be brazed are first thoroughly cleaned with files, emery cloth or sand paper. If the work is greasy, it should be dipped into a bath of lye or hot soda water so that all trace of oil is removed. The parts are then placed in the relation to each other that they are to occupy when the work has been completed. The edges to be joined should make a secure and tight fit, and should match each other at all points so that the smallest possible space is left between them. This fit should not be so tight that it is necessary to force the work into place, neither should it be loose enough to allow any considerable space between the surfaces. The molten spelter will penetrate between surfaces that water will flow between when the work and spelter have both been brought to the proper heat. It is, of course, necessary that the two parts have a sufficient number of points of contact so that they will remain in the proper relative position.
The work is placed on the surface of the brazing table in such a position that the flame from the torches will strike the parts to be heated, and with the joint in such a position that the melted spelter will flow down through it and fill every possible part of the space between the surfaces under the action of gravity. That means that the edge of the joint must be uppermost and the crack to be filled must not lie horizontal, but at the greatest slant possible. Better than any degree of slant would be to have the line of the joint vertical.
The work is braced up or clamped in the proper position before commencing to braze, and it is best to place fire brick in such positions that it will be impossible for cooling draughts of air to reach the heated metal should the flame be removed temporarily during the process. In case there is a large body of iron, steel or copper to be handled, it is often advisable to place charcoal around the work, igniting this with the flame of the torch before starting to braze so that the metal will be maintained at the correct heat without depending entirely on the torch.
When handling brass pieces having thin sections there is danger of melting the brass and causing it to flow away from under the flame, with the result that the work is ruined. If, in the judgment of the workman, this may happen with the particular job in hand, it is well to build up a mould of fire clay back of the thin parts or preferably back of the whole piece, so that the metal will have the necessary support. This mould may be made by mixing the fire clay into a stiff paste with water and then packing it against the piece to be supported tightly enough so that the form will be retained even if the metal softens.
Brazing.--With the work in place, it should be well covered with the paste of flux and water, then heated until this flux boils up and runs over the surfaces. Spelter is then placed in such a position that it will run into the joint and the heat is continued or increased until the spelter melts and flows in between the two surfaces. The flame should surround the work during the heating so that outside air is excluded as far as is possible to prevent excessive oxidization.
When handling brass or copper, the flame should not be directed so that its center strikes the metal squarely, but so that it glances from one side or the other. Directing the flame straight against the work is often the cause of melting the pieces before the operation is completed. When brazing two different metals, the flame should play only on the one that melts at the higher temperature, the lower melting part receiving its heat from the other. This avoids the danger of melting one before the other reaches the brazing point.