It was mentioned in the tabular view, that the several works united with hard solders, receive nearly the same treatment; a few examples will therefore serve to convey a general idea of hard-soldering; a process commonly attended with some risk of partially melting the works, because the fusing points of the metals and their respective solders often approach very nearly together.
Several of the hard-solders contain zinc, which appears to be useful in different ways: first it increases their fusibility; in cases where the solder cannot be seen it serves as an index to denote the completion of the process, for when the solder is melted the zinc volatilizes, and burns with the well-known blue flame; and as at this moment some of the zinc is consumed, the alloy left behind becomes tougher, and more nearly approaches to the condition of the metal which it is desired to unite. The zinc may be therefore considered to act as a flux, and so likewise does the arsenic occasionally introduced into the gold and silver solders, as the arsenic is for the most part lost, between the processes of making and using the solders; but this metal being of a noxious quality, it is but little resorted to, and besides, it renders the other metals very brittle.
In every case of soldering, a general regard to cleanliness in the manipulation is important, and for the most part the edges of the metals are filed or scraped prior to their being soldered, as before observed; in those cases in which the red-heat is employed, filing or scraping are less imperative, as any greasy or combustible matters are burned away, and the borax has the property of combining with nearly all the metallic oxides and earthy bases, thereby cleansing the edges of the metals should that proceeding have been previously omitted.
The works in copper, iron, brass, etc, having been prepared for brazing, (or soldering with a fusible brass,) and the joints secured in position by binding wire where needful, the granulated spelter and pounded borax are mixed in a cup with a very little water, and spread along the joint by a slip of sheet metal or a small spoon.
The work, if sufficiently large, is now placed above the clear fire, first at a small distance so as gradually to evaporate the moisture, and likewise to drive off the water of crystallization of the borax; during this process the latter boils up with the appearance of froth or snow, and if hastily heated it sometimes displaces the solder. The heat is now increased, and when the metal becomes faintly red, the borax fuses quietly like glass; shortly after, that is at a bright red, the solder also fuses, the indication of which is a small blue flame from the ignition of the zinc. Just at this time some works are tapped slightly with the poker to put the whole in vibration, and cause the solder to run through the joint to the lower surface, but generally the solder flushes, or is absorbed in the joint, and nearly disappears without the necessity for tapping the work.
It is of course necessary to apply the heat as uniformly as possible, by moving the work about so as to avoid melting the object as well as the solder; the work is withdrawn from the fire as soon as the solder has flushed, and when the latter is set, the work may be cooled in water without mischief.
Tubes are generally secured by loops of binding-wire twisted together with the pliers; and those soldered upon the open fire are almost always soldered from within, as otherwise the heat would have to be transmitted across the tube with greater risk of melting the work, air being a bad conductor of heat; it is necessary to look through the tube to watch for the melting of the solder. Long tubes are rested upon the flat plate of the brazier's hearth, and portions equal to the extent of the fire are soldered in succession. The common Biramingham tubes for gasworks, bedsteads, and numerous other purposes, are soldered from the outside; but this is done in short furnaces open at both ends and level with the floor, by which the heat is applied more uniformly around the tubes.
Works in iron require much less precaution in point of the heat, as there is little or no risk of fusion; thus in soldering the spiral wires to form the internal screw within the boxes of ordinary tail vices, the work is coated with loam, and strips of sheet brass arc used as solder, the fire is urged until the blue flame appears at the end of the tube, when the fusion is complete; the work is withdrawn from the fire and rolled backwards and forwards on the ground to distribute the solder equally at every part. Other common works in iron, such as locks, arc in like manner covered with loam to prevent the iron from scaling off.*
The finer works in iron and steel, those in the light-coloured metals generally, and also the works in brass which are required to be very neatly done, are soldered with silver-solder. From the superior fusibility of silver-solder, and from its combining so well with the different metals without "gnawing them, or eating them away" or wasting part of the edges of the joints, silver-solder is very desirable for a great many cases; and from the more careful and sparing manner in which it is used, many objects require but little or no finishing subsequently to the soldering, so that the more expensive solder is not only better, but likewise in reality more economical.
The practice of silver-soldering is essentially the same as brazing. The joint is first moistened with borax and water; the solder, (which is generally laminated and cut into little squares with the shears,) is then placed on the joint with forceps. In heating the work additional care is given not to displace the solder; and for which reason some persons boil the borax, or drive off its water of crystallization at the red heat, then pulverize it, and apply it in the dry state along with the solder; others fuse the borax upon the joint before putting on the solder.
Numerous small works united with the hard-solders, such as mathematical and drawing instruments, buttons, and jewelry, aresoldered with the blowpipe; in almost all cases the work is supported upon charcoal, and sometimes for the greater concentration of the heat it is also covered with charcoal. The management of the blowpipe having been explained, it is only necessary to add that the magnitude and shape of the flame are proportioned to those of the works.
* " Sheet iron may be soldered by filings of soft cast iron, applied in the usual way of soldering with borax, which has been gradually dried in a crucible and powdered, and a solution of sal-ammoniac." - London Journal of Arts and Sciences, vol. v. 1823, p.274.
In soldering gold and silver, the borax is rubbed with water upon a slate to the consistence of cream, and is laid upon the work with a camel's-hair pencil,and the solders although generally laminated are also drawn into wire, or filed into dust; but it will be remembered, the more minute the particles of the granulated metals, the greater is the degree of heat required in fusing them. In many of the jewelry works the solder is so delicately applied, that it is not necessary to file or scrape off any portion, none being in excess, and the borax is removed by immersing the works in the various pickling and colouring preparations to be adverted to. See Note A G, Appendix, Vol. II., page 978.