§ 98 Soldering Other Metals
Iron. —
The iron must be filed clean and then brushed with chloride of zinc solution. Some people add a little sat ammoniac to the chloride of zinc, but the improvement thus made is practically inappreciable. If the iron is clean it tins quite easily, and the process of soldering it is perfectly easy and requires no special comment.
Brass. —
The same method as described for iron succeeds perfectly. The brass, if not exceedingly dirty, may be cleaned by heating to the temperature at which solder melts (below 200° C.), and painting it over with chloride of zinc, or dipping it in the liquor. If now the brass be heated again in the blow-pipe flame, it will be found to tin perfectly well when rubbed over with solder.
German Silver, Platinoid, Silver, and Platinum are treated like iron. With regard to silver and platinum the same precautions as recommended in the case of zinc must be observed, for both these metals form fusible alloys with solder.
Gold
when pure requires no flux. Standard gold, which contains copper, solders better with a little chloride of zinc.
Lead
must be pared absolutely clean and then soldered quickly with a hot iron, using tallow as a flux. Since solder if over hot will adhere to lead almost anywhere, plumbers are in the habit of specially soiling those parts to which it is not intended that solder shall adhere. The “soiling” paint consists of very thin glue, called size, mixed with lampblack; on an emergency a raw potato may be cut in half, and the work to be soiled may be rubbed over with the cut surface of the potato.
Hard Carbon or gas coke may be soldered after coating with copper by an electrolytic process, as will be described.
Soldering at a red heat by means of spelter is called brazing. Spelter is soft brass, and is generally made from zinc one part, copper one part; an alloy easily granulated at a red heat; it is purchased in the granular form.
The art of brazing is applied to metals which will withstand a red heat, and the joints so soldered have the strength of brass.
The pieces to be jointed by this method must be carefully cleaned and held in their proper relative positions by means of iron wire. It is generally necessary to soften iron wire as purchased by heating it red hot and allowing it to cool in the air; if this is not done the wire is usually too hard to be employed satisfactorily for binding.
Very thin wire—i.e. above No. 20 on the Birmingham wire gauge — does not do, for it gets burned through, and perhaps allows the work to fall apart at a critical moment.
The work being securely fastened, the next step is to cover the cleaned parts with flux in order to prevent oxidation. For this purpose “glass borax” is employed. “Glass” borax is simply ordinary borax which has been fused for the purpose of getting rid of water of crystallisation. The glass borax is reduced to powder in an iron mortar, for it is very hard, and is then made up into a cream with a little water. This cream is painted on to the parts of the work which are destined to receive the solder.
The next step is to prepare the spelter, and this is easily done by mixing it with the cream, taking care to stir thoroughly with a flattened iron wire till each particle of spelter is perfectly covered with the borax. The mixture should not be too wet to behave as a granular mass, and may then be lifted on to the work by means of the iron spatula.
Care must be taken to place the spelter on those parts only which are intended to receive it, and when this is done, the joint may be lightly powdered over with the dry borax, and will then be ready for heating.
If the object is of considerable size it is most conveniently heated on the forge; if small the blowpipe is more convenient. In the latter case, place the work on a firebrick, and arrange two other bricks on edge about it, so that it lies more or less in a corner. A few bits of coke may also be placed on and about the work to increase the temperature by their combustion, and to concentrate the flame and prevent radiation. The temperature is gradually raised to a bright red heat, when the spelter will be observed to fuse or “run,” as it is technically said to do.
If the cleaning and distribution of flux has been successful, the spelter will “run” along the joint very freely, and the work should be tapped gently to make sure that the spelter has really run into the joint. The heating may be interrupted when the spelter is observed to have melted into a continuous mass. As soon as the work has fallen below a red heat it may be plunged into water, a process which has the effect of cracking off the glass-like layer of borax.
There is, however, some risk of causing the work to buckle by this violent treatment, which must of course be modified so as to suit the circumstances of the case. If the joint is in such a position that the borax cannot be filed off, a very convenient instrument for its removal by scraping is the watchmaker’s graver, a square rod of hard steel ground to a bevelled point (Fig. 80).
Several precautions require to be mentioned. In the first place, spelter is merely rather soft brass, and consequently it often cannot be fused without endangering the rest of the work. A good protection is a layer of fireclay laid upon the more delicate parts, such for instance as any screwed part.
Gun-metal and tap-metal do not lend themselves to brazing so readily as iron or yellow brass, and are usually more conveniently treated by means of silver solder.
Spelter tends to run very freely when it melts, and if the brass surface in the neighbourhood of the joint is at all clean, may run where it is not wanted. Of course some control may be exercised by “soiling” with fireclay or using an oxidising flame; but the erratic behaviour of spelter in this respect is the greatest drawback to its use in apparatus construction. The secret of success in brazing lies in properly cleaning up the work to begin with, and in disposing the borax so as to prevent subsequent oxidation.