This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
For brazing, where powdered or grain spelter (a very fusible brass) is used, the borax is mixed as a powder with a spelter, usually with a little water, but sometimes the work to be brazed is made hot and dipped into the dry powder mixture, which partially fuses and adheres. In either case, care is requisite not to burn or oxidize the grains of the spelter with the blowpipe flame, or it will not run or adhere to the surface to be brazed; and for such small work as can be done with the mouth blowpipe, it is better to discard spelter entirely, and use either common silver solder (an alloy of 1 silver and 2 tinned-brass pins), or what is still better an alloy of 13 parts copper and 11 fine silver. If fine silver is not easily to be got, the same alloy can be made by equal weights of copper and coin silver. The solder should be rolled into thin sheets, cut into small bits of the shapes and sizes required, and put into a small saucer, containing a rather thin pasty mixture of powdered borax and water. The surfaces of the joint to be soldered should be brushed with this mixture, using a small camel-hair brush, the bit of solder being put in its position either with the brush or a fine pair of tweezers. The heat of the blowpipe must then be applied very slowly.
The borax dries up and swells enormously, frequently lifting the solder along with it. The borax then sinks down again and begins to fuse. There is now no risk of blowing the solder away, and the full blast can be at once applied, directing the flame principally round the solder so as to heat the body of the work. When hot enough, the solder begins to fuse and adhere to the work, and the flame must now be instantly reduced to a small point, and directed on the solder only, which usually fuses suddenly. The instant the solder runs, the blast must be stopped by the tip of the tongue, or in delicate work mischief may be done which may take hours to make good.
One great difficulty with beginners is in soldering two or more parts in exact positions relatively to each other, these parts being of such a form that they cannot be held in position. The way to overcome the difficulty is this: With a stick of beeswax, the end of which has been melted in a small flame, stick the parts together as required. The wax is sufficiently soft when cold to admit of the most exact adjustment of parts, and it must surround the parts only which are to be soldered. Make a mixture of about equal parts of plaster-of-paris and clean sand, and stir this up in a cup or basin with sufficient water to make a paste, turn it out on to a sheet of paper, and bed the work to be soldered into it, taking care that the part covered with wax shall be freely exposed. When this is set hard, say in about 10 minutes, slowly warm it over a Bunsen flame, or near a fire (if suddenly heated it will break up); wipe the melted wax off with a small ball of wool; apply the borax and solder as before mentioned, and continue the slow heating up until the whole mass is hot enough to complete the soldering with the blowpipe.
If a light bit has only to be carried or held in position after fixing with wax, as before mentioned, a bridge or arm may be made between the pieces with a very stiff paste made of common whiting and water, or a mixture of clay, whiting, and water. This, being only small in bulk, dries much more quickly than the plaster and sand, but it requires also very slow heating at first, so as to drive the moisture out gradually, otherwise it explodes as steam is formed inside, and the whole work has to be recommenced. The Indian jewellers in making filagree work use clay alone for holding the parts together, but it is very slow in drying, and requires much more care in use than either of the forms given.
When soldering, the work has to be supported in such a manner that it can be turned about and its positions altered quickly, more especially when a fixed blowpipe is used, and for this purpose it is common to use either a lump of pumice or a small sheet-iron pan with a handle, and filled with broken pumice, broken charcoal, and plaster-of-paris, or other non-conductor. The best material is willow charcoal, and the best result can be obtained by its use, as, burning with the heat of the blowpipe, it gives off heat and assists the workman, giving a greater power than when any other support is used. Oak charcoal is not admissible, as it crackles and disturbs the work. For a permanent support, which does not burn away to any practical extent, the best is a mixture of finely-powdered willow charcoal and a little china clay, made into a stiff paste with a rice flour starch, and rammed into a mould. These are to be bought in many shapes, and are the most convenient for all purposes.
Speaking generally of the mouth blowpipe, the most practised users, as a maximum feat, might, with gas, soft solder a 3-in. lead pipe, or, with a lamp, do the same with a 1 1/2-in. pipe. In hard soldering (with silver solder or spelter), it is usually as much as can be done to solder properly any work weighing over 3 oz., if gas is used; or about half this weight with a lamp; although in exceptional cases, using a charcoal support, these weights may be exceeded, and more especially if the bulk of the work is heated up by a fire or other means so as to admit of an extra strain being put on the lungs for a short time for finishing only. It is a common practice for heavy or awkwardly-shaped work - where the heat is liable to be conducted away quickly - to support the work on a bed of burning coke or charcoal, using the blowpipe only for running the solder whilst the body of metal is heated by the burning coke. By this assistance the capacity of any blowpipe is doubled, or more than doubled, and when the work to be done is beyond the capacity of the blowpipes available, this remedy is a valuable one.