Every mechanic who is a worker in any kind of metals should at least be able to make a simple soldered or a brazed joint. To acquire a. knowledge of the operations is not at all difficult, a working acquaintance being readily obtained after a few hours' practice.

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The operations of soldering and brazing are not analogous to those of gluing, gumming, or cementing, as it is not simply a question of inserting some adhesive substance in between the two surfaces of the joint, and thus sticking the metals together. When two edges or surfaces of metal are soldered or brazed together, the solder or spelter actually alloys with the metal to be soldered for some small distance beneath the surface; hence the solder or spelter penetrates into the pores of the metal, and thus obtains a firm grip. If a joint be cut through and the section examined under the microscope, no clear line of demarcation between the solder and the metal can be observed. For instance, if the metal soldered is copper, it will be noticed that the bottom layers are yellow, the solder having combined with the copper and formed a bronze. In a brazed joint the spelter will have alloyed with the copper and thus formed a brass.

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Considerations such as above will lead to the conclusion that for a joint to be properly made the temperature of the melting solder or spelter and of the joint to be soldered is of some importance.

Before proceeding to describe the methods of making the above kind of joints, it will be as well to first consider the subject of solders and fluxes.

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In making or choosing a solder the requirements of a good solder should be kept in mind. They are as follows: -

1. The melting-point must be below the melting-point of the metals to be soldered.

2. The solder must flow readily.

3. The solder must firmly unite with the metals to be soldered.

4. The solder must be strong.

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Let us consider the above requisite properties of a good solder or spelter. In the first place, it would manifestly be foolish to attempt to solder a metal with a solder whose melting temperature was higher than that of the metal to be soldered, as before the solder commenced to run the sheet itself would have a hole melted in it. So that, in soldering the softer metals, such as block tin and pewter, care must be taken to choose the proper solder.

For the solder to properly permeate every part of the joint it is, of course, necessary that it should become liquid or thin, so as to flow readily. To obtain this property all foreign substances must be kept out of the solder. Thus, to give an illustration, if a small quantity of zinc gets into a soft solder composed of lead and tin, it makes it become thick or pasty in use.

From what has been said at the commencement it will be readily understood that the solder must be of such a nature as to alloy with the metals to be soldered, or else it will be impossible to make a firm joint.

For iron, copper, or brass work that is to be subjected to pressure, it is essential that the joint shall be as strong as possible. Hence, in making joints for this kind of work a brazing spelter must be chosen that will give the best results.

The following is a table of a few of the soft solders in ordinary use: -

Lead

Tin

Melting-point

Coarse plumber's solder

...

3

1

480°

Fahr

Ordinary " "

....

2

1

440

"

Blowpipe "

....

1

2

340

"

Fine tinman's „

...

2

3

335

"

Ordinary " "

...

1

1

320

"

Pewterer's solder

...

1

1

201

"

Bismuth

2

It is interesting to notice the change in the melting-points of the solders from that of the metals which form them.

Thus, lead melts at 620° Fahr., and tin at 440° Fahr., yet when these are alloyed together in equal proportions to form ordinary tinman's solder, the melting-point drops to 320° Fahr. This is one of the advantages that is derived from the alloying of metals.

It is generally the best plan to make one's own solder, as much of that which can be bought is unreliable. Besides which, without some guarantee that the solder contains the required proportions of lead and tin, there is no knowing whether or not there is more lead in the solder than has been bargained for. Tin being about ten times the price of lead, a small reduction in the quantity of tin makes a considerable difference in the value of the solder. A rough test of the quantity of tin in a solder is by listening to the characteristic "cry" of the tin when the solder is bent.

In making solders, the lead and tin are melted together, the metals properly mixed, and the scum or oxide skimmed off the surface. And before pouring into the mould, it is a good plan to dust a little resin on the surface of the solder, and let it burn away. In lieu of a cast-iron mould, a bar of small angle-iron can conveniently be used for running the sticks of solder.

It will be noticed that pewterer's solder melts some degrees below the boiling-point of water; but it does not of necessity follow that boiling-water will melt away the solder from the joint on a pewter vessel, as the solder, by virtue of alloying with the pewter, will, in this case, have its melting-point raised.

The following table gives the composition of the ordinary hard solders or spelters: -

Copper

Zinc

Silver

Iron work... ... ... ...

2

1

0

Copper and thin iron work

3

2

0

Brass work.........

1

1

0

Thin brass work.........

8

8

1

The term "spelter" should not be confused with the same name that is applied to ingot zinc, as a hard solder is essentially a brass, whilst, of course, ingot zinc is almost pure zinc, and is principally used in galvanising. It will be seen that the first spelter has the same composition as ordinary brass, and it might be here said that sheet brass is often used, instead of brazing spelter, as it is sometimes found to be more convenient to put along the joints. In bent joints, such as that in a kettle spout, a strip of brass can be cut that will lie along the whole length of the joint.

In practice there is really very little need to trouble about the composition of brazing solders, as they are usually sold in a graded form, numbers 1, 2, 3, etc., the coarse being used for iron and the finest for thin brass work. Silver solders, mostly composed of copper and silver, are used principally in jeweller's work, with which we are not here concerned.

The fluxes used in soft-soldering are "killed spirits," resin, resin and oil, tallow, and for pewter Gallipoli oil. Soldering fluids are sold ready made-up, and these are probably composed of crude chloride of zinc, with some salammoniac in solution. A lump of salammoniac is sometimes used for cleaning the point of the soldering-bit, and powdered salammoniac is used as a flux in various tinning operations.

Borax is almost generally used as the flux for brazing. There are, however, several advertised substitutes; but the principal ingredient in these is probably borax in some form or other.

It may be noticed in passing that the object of using a flux is to assist the solder to flow, and to keep the part of the joint which is being soldered from contact with the atmosphere. The air being kept from contact with the surface of the joint, no oxides can form, consequently the melted solder is free to unite with the heated metal. In many cases, too, the flux has a cleaning action, removing any thin film of oxide that may have formed on the surface of the sheet previous to soldering.