Brazing and silver soldering, also known as hard soldering, are basically identical operations; the difference between them is the alloy used to make the joint. They both require more heat than is needed in soft soldering but are similar processes in terms of preparation and technique.
This, strictly speaking, is brass and is known as spelter. Brass is an alloy of copper and zinc; by varying the proportions of these two components, the melting temperature of spelter can also be varied. Spelter may be obtained in sticks or granular form; always state the purpose for which it is intended, to avoid buying one which has too high a melting point for the planned work. For working in steel, a spelter of 65 percent copper and 35 percent zinc is suitable; this will melt at 920°C — steel melts at 1530°C. For work in copper, a spelter of 60 percent copper and 40 percent zinc should be used; the melting points are 900°C and 1080°C respectively.
This alloy contains a substantial proportion of silver. Its melting point is lower than that of spelter, which makes it suitable for forming strong joints in brass or copper. A silver solder containing 60-62 percent silver has a melting range of 690-735°C.
Borax, the basic flux used with brass spelters and alloys melting generally above 850°C, may be applied in powder form or mixed to a paste with water and applied with a brush. With brazing alloys of the low temperature silver solder type, fluxes based on complex fluoborates are required.
Most hard solders have to be heated to 900°C before they begin to melt; this prohibits the use of soldering irons. To provide the extra heat, a blowtorch (or blowpipe — fed by gas and air under slight pressure) is needed. It is not unknown for these processes to produce serious accidents; for example, pieces of work can easily fall from a brazing hearth. Always have water and a pair of tongs near at hand when using these techniques.
Making the joint
Preparation is very important, as it is for soft soldering. In addition to mechanical cleaning methods you can dip the joint in dilute sulphuric acid; wash it in clean water afterwards. Don’t use this method with ferrous metals since these will contaminate the acid and you will not be able to use it again for the same purpose. Brush the cleaned joint surfaces with some flux and assemble the work before placing it on a piece of asbestos or in a brazing hearth, which will provide a fireproof base. You can easily make a brazing hearth from firebricks; it should be as small as possible to conserve and reflect heat.
Heat the work with a blowtorch until it glows red; in due course the flux ‘ will melt and you should continue heating until the work glows cherry red. Dip the spelter stick into the flux and apply to the work while the blowtorch is still heating the joint. The spelter will melt and flow into the joint. Allow the work to cool, removing any traces of flux (which forms a hard coating) before the work cools completely. This can be done with hot water or you can sprinkle salt onto the joint; this will help to wash off traces of flux. If the spelter has taken you can file the joint flat with an old file. If it has not taken to the joint, this might be because of insufficient preparation, in which case you should repeat the soldering process.
A common problem when dealing with metal at these temperatures is establishing exactly when the required melting point has been reached. If possible, practise with scrap pieces of matching material before tackling the main job. There are cones available, made of kaolin and other materials including lime, which collapse at known temperatures; these are known as seeger cones. They are used only once and, although inexpensive, will add to the cost of a job. Place them as close to the work as possible without being in the way. Holding work The temperatures involved in hard soldering prevent the use of a vice, so the work must be fastened before soldering with wire or wood or a screw, rivet or nut and bolt. Any improvized clamps used to hold the work while soldering must be kept away from the joint.
If a bent piece of metal is held with split pins behind the joint to hold the work whole soldering, it must be bent away from the work so it does not draw heat from the joint. Floating This soldering technique involves coating a joint with solder to make it watertight. When the joint is made, heat the metal with a blowtorch and float or flood extra solder over the existing joint. Sweating This technique enables you to make joints in long sheet metal or tubing. You should ensure the surfaces to be joined are as smooth and clean as possible so they may be brought very close together. Tin both surfaces and clamp them together, heat the joint with a blowtorch until the solder remelts and allow the work to cool.
Designing joints in sheet metal
You should not butt-join sheet metal using soldering methods since the surfaces of the metal will not give sufficient contact area for the solder. Instead you should use a lap seam, sunken lap seam, folded seam or a double strap joint. You can seam larger sheet metal together by tacking with solder before the main joint is attempted. Some sheet metal may need to have a piece of metal tube or a pin soldered inside the joint for strength.
Cylindrical joints To make joints in one piece of metal you can hold the two ends together with soft wire. You can make a butt joint in this way if you tighten the wire by twisting a nail underneath and around the wire. Dovetails cut into the metal will give a stronger joint ; cut the tongues before aligning them with their slots. To make a lap joint in a cylinder, bend the metal to a diameter smaller than the required size, leaving an overlap, and insert a piece of hardwood the same width as the required internal diameter. Alternatively use two split pins to hold the work together.