If the solder obstinately refuses to adhere, the cause is to be sought in the oxidation of the copper, or of the solder, or both--in either case the result of too high a temperature or too prolonged heating.

The simple remedy is to get the iron hot, and then to dress it with an old file, so as to expose a bright surface, which is instantly pa.s.sed over the resin as a means of preserving it from oxidation. If the process above described be now carried out, it will be found that the difficulty disappears.

Before using the iron, wipe off any soot or c.o.ke or burned resin by means of an old rag. An iron tinned in this way is much to be preferred to one tinned by means of chloride of zinc.

A shorter and more usual method is carried out as follows: The solution of chloride of zinc is prepared by adding bits of zinc to some commercial hydrochloric acid diluted with a little (say 25 per cent) of water. The acid may conveniently be placed in a small glazed white jar (a jam pot does excellently), and this should only be filled to about one-quarter of its capacity. An excess of zinc may be added.

It may be fancy, but I prefer a soldering solution made in this way to a solution of chloride of zinc bought as a chemical product. The jar is generally mounted on a heavy leaden base, so as to avoid any danger of its getting knocked over, for nothing is so nasty or bad for tools as a bench on which this noxious liquid has been upset (Fig. 78).

Fig. 78.

To tin a soldering bit, a little of the fluid is dipped out of the jar on to a bit of tin plate bent up at the edges--a few drops is sufficient--and the iron is heated and rubbed about in the liquid with a drop of solder. If the iron is anything like clean it will tin at once and exhibit a very bright surface, but quite dirty copper may be tinned by dipping it for a moment in the liquid in the pot and then working it about over the solder. An iron so tinned remains covered with chloride of zinc, and this must be carefully wiped off if it is intended to use the iron with a resin or tallow flux in lead soldering.

One disadvantage of this process is that the copper bit soon gets eaten into holes and requires to be dressed up afresh. On the other hand, an iron so tinned always presents a nice clean solder surface until the next time it is heated, when it generally becomes very dirty and requires to be carefully wiped before using.

In my experience also an iron so tinned is more easily spoiled as to the state of its surface, "detinned," in fact, by overheating than when the tinning is carried out by resin and friction. When this happens, the shortest way out of the difficulty is the application of the old file so as to obtain a perfectly fresh surface. No one who knows his business ever uses an iron that is not perfectly clean and well tinned.

The iron may be cleaned from time to time by heating it red hot and quenching it in water to get rid of the oxide, which scales off in the process.

-- 95. Soft Soldering.

In the laboratory the chief application of the process is to copper soldering during the construction of electrical apparatus and to zinc soldering for general purposes.

In ninety-nine cases out of every hundred where difficulties occur their origin is to be traced to dirt. There seems to be some inexplicable kink in the human mind which renders it callous to repeated proofs of the necessity for cleaning surfaces which it is intended to solder. The slightest trace of alb.u.minous or gelatinous matter or sh.e.l.lac will prevent solder adhering to most metals and the same remark applies in a measure to the presence of oxides, although these may be removed by chloride of zinc or prevented from forming by resin or tallow. A touch with an ordinarily dirty hand--I refer to a solderer's hand--will often soil work sufficiently to make the adherence of solder difficult.

The fluxes most generally employed are tallow for lead, resin or Venice turpentine for copper, chloride of zinc for anything except lead, which never requires it. The latter flux has the property (also possessed by borax at a red heat) of dissolving any traces of oxide which may be formed, as well as acting as a protecting layer to the metal.

We may now turn to the consideration of a simple case of soldering, say the joining of two copper wires. The wires are first cleaned either by dipping in a bath of sulphuric and nitric acids--a thing no laboratory should be without--or by any suitable mechanical means.

The cleaned wires are then twisted together--there is a regulation way of doing this, but it presents no advantage in laboratory practice--and the joint is sprinkled over with resin, or painted with a solution of resin in alcohol.

The iron, being heated and floated with solder, is held against the joint, the latter being supported on a brick, and the solder is allowed to "sweat" into the joint. Enough solder must be present to penetrate right through the joint. Nothing is gained by rubbing violently with the iron. If the copper is clean it will tin, and if it is dirty it won't, and there the matter ends.

Beginners generally use too small or too cold a bit, and produce a ragged, dirty joint in consequence. If the saving of time be an object, the joint may be twisted together on ordinarily dirty oxidised wires and heated to, say, 200 C. It is then painted with chloride of zinc and soldered with the bit.

There is a difference of opinion as to the relative merits of chloride of zinc and of resin as a flux in soldering copper. Thus the standing German practice is, or was, to employ the former flux in every case for soldering electric light wires, while in England the custom used to be to specify that soldering should be done by resin, and this custom may still prevail; it lingers in Australia at all events.

However, it is agreed on all hands that when chloride of zinc is used it must be carefully washed off. I have known of an electrical engineer insisting on his workmen "licking" joints with their tongues to ensure the total removal of chloride of zinc; it has a horrible taste; and I have occasionally pursued the same plan myself when the soldering of fine wires was in question.

In any case, it is very certain that chloride of zinc left in a joint will ruin it sooner or later by loosening the contact between copper and solder.

Very often it is requisite to solder together two extensive flat surfaces--for instance, in "chucking" certain kinds of bra.s.s work.

The surfaces to be soldered must be carefully tinned, most conveniently by the help of the blow-pipe and chloride of zinc. After tinning, the surfaces are laid together and heated so as to "sweat"

them together; the phrase, though inelegant, is expressive.

96. Soldering Tin Plate.

If the plate be new and clean, a little resin or its solution in alcohol is all that is necessary as a flux. If the tin plate is rusty the rust must be removed and the clean iron, or rather mild steel, surface exposed. The use of chloride of zinc is practically essential in this case. Tin plate is often spotted with rust long before it becomes rusty as a whole, when, of course, it may be regarded as worn out, and such rust spots are most conveniently removed by means of the plumber's shave-hook. The shave-hook is merely a peculiarly shaped hard steel sc.r.a.ping knife on a handle (Fig. 79).

Fig. 79

With tin plate the soldering of long joints is often necessary. The plate must be temporarily held in position either by binding with iron wire, fastening by clamps, or holding by an a.s.sistant. The flux is applied and the iron run slowly along the joint. Enough solder is used to completely float the tip of the iron. By arranging the joint so that it slopes downward slightly, and commencing at the upper end, the solder may be caused to flow after the iron, and will leave a joint with the minimum permissible amount of solder in it. By regulating the slope, heat of iron, etc, any desired quant.i.ty of solder may be run into the joint.

-- 97. Soldering Zinc.

Zinc alloys with soft solder very easily, and by so doing entirely spoils it, making, it "crumbly," dirty, and preventing it running.

Consequently, in soldering up zinc great care must be taken to prevent the solder becoming appreciably contaminated by the zinc. To this end the zinc surfaces are cleaned by means of a little hydrochloric acid, which is painted on instead of chloride of zinc. Plenty of solder is melted on to the work, and is drawn along over the joint by a single slow motion of the soldering bit. The iron must be just hot enough to make the solder flow freely, and it must never be rubbed violently on the zinc or allowed to linger in one spot; the result of the latter action will be to melt a hole through the zinc, owing to the tendency of this metal to form an easily fusible alloy with the solder.

The art of soldering zinc is a very useful one in the laboratory. The majority of physicists appear to overlook the advantages of zinc considered as a material for apparatus construction. It is light, fairly strong, cheap, easily fusible, and yet hard and elastic when cold. It may be worked as easily as lead at a temperature of, say, 150 to 200 C, and slightly below the melting-point (423 C.) it is brittle and may & powdered. The property of softening at a moderate temperature is invaluable as a means of flattening zinc plate or shaping it in any way. During the work it may be held by means of an old cloth. Zinc sheet which has been heated between iron plates and flattened by pressure retains its flatness very fairly well after cooling.

-- 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.

Bra.s.s.

The same method as described for iron succeeds perfectly. The bra.s.s, 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 bra.s.s 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 c.o.ke may be soldered after coating with copper by an electrolytic process, as will be described.

-- 99. Brazing.

Soldering at a red heat by means of spelter is called brazing.

Spelter is soft bra.s.s, 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 bra.s.s.

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 "gla.s.s borax" is employed. "Gla.s.s" borax is simply ordinary borax which has been fused for the purpose of getting rid of water of crystallisation. The gla.s.s 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 ma.s.s, 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 c.o.ke 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 ma.s.s. 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 gla.s.s-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 circ.u.mstances 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 sc.r.a.ping is the watchmaker's graver, a square rod of hard steel ground to a bevelled point (Fig. 80).