CHAPTER V 
Soldering (continued) Soft solder—Composition of soft solders—Fluxes—Soldering bits—Various hints—Removal of soft solder. 

We now come to the soft solders. They melt at a com paratively low temperature, so they may be used for solder ing almost any metal or alloy. Soft solders consist chiefly of tin and lead, though other metals are sometimes added to lower the melting point. Mixtures of tin and lead melt at a temperature lower than the melting points of either tin or lead. So an alloy, that is to say, a mixture of tin and lead may be used as a solder for either of those metals. It has been found that an alloy containing two parts of tin to one of lead, or, more precisely, 63% of tin to 37% of lead has a lower melting point than alloys made in any other proportion. It fuses at 175° C. It is also nearly the hardest of all the lead-tin alloys. As the two qualities most desirable in a soft solder are (1) low melting point, and (2) strength, this alloy forms one of the best soft solders known. 

There is another alloy of tin and lead much used by plumbers. They employ it for " wiping " joints in lead pipes. It consists of two parts lead to one of tin. Its fusing point is about 250° C. This alloy undergoes a prolonged pasty stage on cooling, and it is on this property that the plumber depends. For, as pointed out by by Mr. Hiorns, at temperatures above 180° C. the mass consists of granules of solid metallic lead, floating in a liquid which is almost identical with the soft solder mentioned above. The mass continues in a pasty state until it has cooled down to about 180° C. The above two are the soft solders in general use, but occasionally one is required with a much lower melting point. For this purpose bismuth, cadmium or mercury is added to the alloy of tin and lead. Thus, an alloy composed of two parts bismuth and one part each of lead and tin, will melt in boiling water. For further parti culars of these fusible alloys see Chapter XXVI. 

In making soft solder it is well to use a deep, narrow vessel, rather than a shallow, wide one, for these alloys oxidise rapidly when heated and exposed to the air. A layer of dross forms on the surface. It is usual, therefore, to put a little fat or resin on the surface of the molten metal to protect it from the action of the oxygen in the air. The metal should be thoroughly stirred with a stick of green wood before pouring. The gases liberated by the charring of the wood assist in bringing the dross to the surface. This should be skimmed off and the metal poured at once. A good mould can be formed from a piece of angle iron, propped up level on the floor. It will form a long, narrow trough, the ends of which can be stopped with a little heap of sand or clay. Pour the solder into the trough to a depth of about \ inch, starting at one end of the trough and moving towards the other. When cool the solder will be in a long triangular stick ready for use. 

As a comparatively low temperature is sufficient to melt soft solder, all that it is necessary to do is to apply the heat so that the parts of the work near the joint to be soldered may be made hot. The solder will flow before the heat has had time to spread. The tools required for soft soldering are few and inexpensive; a soldering bit (or other heating apparatus), some acid and some pieces of solder and zinc, are the only essential ones. The flux employed may be chloride of zinc prepared in the manner described below, sal-ammoniac, resin, or even tallow. Its purpose is to keep the air away from the joint while it is getting hot, and to assist the flow of the solder. For iron or steel, resin or sal-ammoniac is used as a flux. But for cop per and its alloys chloride of zinc is generally employed. 

To make this flux, take 8 oz. of hydrochloric acid (known also as muriatic acid or spirits of salt), and put it in a strong dish in the open air. Add to it pieces of clean zinc cut small. The acid will boil violently, getting quite hot, splashing all around and giving off poisonous fumes. Do not, therefore, add too much zinc at a time, or the acid may get hot enough to crack the dish. Go on adding zinc till no more gas is given off. The acid now is said to be " killed." When quite quiet the liquid, which is now zinc chloride, is ready for use as a flux. Pour the clear liquid into a bottle. You can use a stick, a brush or a chicken's feather to apply it to the joint. A flux for soft soldering can be made in the form of paste in the following manner. Take some petroleum residue, such as vaseline or petro latum. (These are identical in composition, but the latter is cheaper.) To one pound of this add one ounce of cblorids of zinc, prepared as described above. Mix well, and the flux is ready for use. Prepared in this way it has the ad vantage of not spattering. Chloride of zinc should always have some excess of zinc, for any free acid might corrode the work if left on it after soldering. Powdered sal-ammoniac, mixed into a paste with water, also works excellently. A flux which may be used on any metal, except aluminium, is made of the following ingredients : \ pint spirits of salt, zinc enough to kill it, \ oz. sal-ammoniac and 2 oz. water. Powdered resin or resin and sweet oil are largely used by plumbers as a flux for making joints in lead pipes. 

The soldering bit, Fig. 15, has a copper head and an iron shank. The head is formed from a bar of copper, either pointed or flattened out to a chisel shape. The shape shown in Fig. 16, with the head fixed at right angles to the shank, is a most convenient form. In buying or making a soldering bit be sure that the copper head is not less than a pound in weight. Smaller ones do not hold sufficient heat to warm up any but the smallest pieces of work. Before using the bit its edge must be " tinned," that is to say, coated with a thin layer of solder. 

To do this, heat the bit in the flame of a gas-ring or in a red fire—not a smoky one, for that would make it dirty and unusable. It must not be allowed to get red hot, though. It should feel nice and warm when held four inches from the cheek. Put a small piece of soft solder and a lump of sal-ammoniac on a piece cut from a tin canister. By the way, a tin canister is made of thin sheet iron with a coating of tin on each side. Rub the soldering bit on the tin. Some of the solder will melt and stick on to the bit wherever the sal-ammoniac also touches it. Bub the bit about till a coating of solder is spread all over its working edge—the part farthest from the handle. The bit is now ready for use. It is not, however, an essential tool, for the blowpipe can be, and frequently is, employed to melt the solder. 

The work must first be prepared by scraping the parts to be joined quite bright and clean; any loose pieces being tied into their places, if necessary, with wire. Put some flux along the joints. Heat the bit, then holding it in the right hand rub the edge on the piece of tin canister before mentioned. Then take a stick of solder in the left hand and dip the end of it into the flux. Slide that end up against the edge of the bit. In a second some of the solder will melt and hold to the bit. At once rest the bit against the joint, taking care that the metal on both sides gets equally heated. The solder will run off the bit into the joint as soon as the work is hot enough. If it does not flow easily, reheat the bit and add more flux to the joint. Any pieces of solder which fall on the bench should bo picked up by touching them with the hot bit. The solder turns dull as it sets, therefore the work should not be moved till this change has taken place, or the liquid solder may run out. When all the joint is soldered, reheat the bit and run it along the joint as quickly as possible while melting the solder. This is to spread the solder quite evenly along. It will be much easier to scrape off any superfluous solder now than if it had been left in uneven patches. 

To soft-solder flat surfaces together. Clean them very thoroughly, apply flux, then lay a sheet of tinfoil between them. Tie together in place and heat until the solder (tinfoil) runs. The metal lining of tea-chests makes a good soft solder. In joining together pieces of pewter or tin it is possible to do without solder altogether, or rather to use some of the metal itself as solder. The bit should be used while very hot and the work done quickly. For if the tool were alloAved to rest for more than a second or so a hole might be burnt right through the metal. Should such an accident occur, the hole, if large, may be plugged with fresh metal, and the joint soldered up while the plug is held in place by a pad of cloth or a piece of wood. Flux must be applied just as though solder were being used. Any superfluous metal can be filed or scraped off after wards. In making lead tanks for some purposes, where chemical action might be set up between the lead and any solder used, all the joints are made or " burnt " in this way. The work is done with a blowpipe, and a strip of clean lead is held against the joint in such a manner that it fuses together with the edges of the work. 

Soft solder, if heated to much above its ordinary melting point, when in contact with any gold, silver, copper or brass object, will penetrate deeply into the metal of which it is composed. It forms a dark, spongy alloy with the gold, silver or copper, as the case may be, which can be got rid of only by cutting the whole affected piece out. It will be clear then that no hard soldering can be done on any piece of work on which there is any soft solder whatever. You must either remove every trace of it first, or abandon your intention of using hard solder. 

Soft solder may be removed by boiling the article upon which it is in a solution made as follows. Take— 
Green copperas  2 oz.  
Saltpetre  1 oz.  
Water  10 oz.  

Powder the first two, and boil them in the water in a cast-iron saucepan. When cool, the solution will crystallise. If it refuses to do eo completely, reheat the liquid which remains. When all has crystallised, dissolve the crystals in hydrochloric acid in the proportion of 8 oz. of acid to 1 oz. of the crystals. Dilute this liquid in four times its weight of boiling water when required. Boil the articles in this diluted solution. Another method of removing soft solder is given on page 66.