Soft solder, or tin solder, can be used to solder many different metals, gold, silver, lead, copper, and steel, as well as brass, wrought iron, and zinc. Its principal use, however, is in ordinary tinsmith's work, for which tin plate, zinc, and sheet brass are the materials most frequently employed. Soft solder can be used for any purpose where the soldered articles need not be heated much above the boiling point of water, so that there is no danger of its melting.

In regard to their qualitative composition, soft solders vary but little. Tin and lead, and sometimes bismuth, are the metals combined. But the proportions of these are varied so much as to make quite a large number of different solders. It may be remarked, in general, that the fusing point of the alloy is raised by increasing the content of tin, and lowered by the addition of more lead. Since lead is much cheaper than tin, alloys containing a large proportion of lead are of course used for objects where a high fusing point of the solder is not necessary, as for instance in making toys. For certain other purposes, solders of pure tin, or those containing but a small amount of lead, are required. This is the case in the soldering of metallic utensils for use in chemical laboratories, or in the preparation of drugs and extracts. These utensils are made either of copper, and tinned on the inside, or of pure tin, which is a good resistant of chemical action. If solder containing much lead were used upon them, the lead would be liable to attack by certain chemical agents, and some amount would be dissolved into the contents of the vessel. In such cases, therefore, no regard should be paid to the cost of the solder; the only consideration should be the prevention of any possibility of attack by substances such as vegetable extracts, fruit juices, etc. Pure tin is the best material for the solder, and it is of importance that it should be the best obtainable. English tin and "Banka" tin rank first with respect to purity, and the fusing point of the metal and the appearance of the fracture may be taken as a standard of the quality. Pure tin melts at 235 deg. C. (455 deg. F.) and the fusing point of a good quality of tin will be very near to this. If the fusing point is more than five degrees higher or lower, there is reason to suppose that there are adulterations which will be liable to affect tin-solder injuriously.

If the fusing point is lower than 230 deg. C. (446 deg. F.) it is probable that the metal contains a small quantity of lead, usually added intentionally: a rod of such tin. on being bent, will make but little of the peculiar crackling noise characteristic of pure tin, and if broken, the fractured surface will plainly show the difference.

A fusing point above 240 deg. C. (464 deg. F.) will usually be-due to small quantities of iron; tin containing iron is hard to bend, and has a hackly fracture. The presence of iron, however, is far less dangerous than that of lead.

For ordinary tinsmith's work, where the resistance of the solder to acids, etc., is of less importance, it is customary to use mixtures of tin and lead, in varying proportions according to different purposes and according to the required melting point of the solder. Experts have taken much pains to make accurate determinations in this important matter, and the following table gives the fusing point (Centigrade) of a solder containing a given amount of lead to 100 parts of tin:

It will be seen that the alloys of tin and lead become denser and less readily fusible as the contents of lead are increased.

According to other experiments, the fusing points of the alloys are as given below:

Before the solders really melt, they soften considerably, and the following: table gives the softening point of some alloys:

Alloys used especially for soldiers.

Composition Of Ordinary Soft Solder

Lead ........................................ 207

Tin ......................................... 118

Weak Soft Solder

Lead ........................................ 207

Tin ......................................... 236

Strong Soft Solder

Lead ........................................ 414

Tin ......................................... 118

Fluid Solder

Lead ........................................ 621

Tin ......................................... 590

Fluid solder is prepared by making the given mixture and letting it stand until partially hardened, when the part which is still fluid is poured off. In using this, it is poured into large seams, and works extremely well. The stiffened part can be used as ordinary solder.

If the alloys are to be made in small quantities, it requires very sensitive scales to weigh the metals accurately. The composition of some varieties of tin solder is given below, in round numbers, with the fusing point of each. They are numbered according to their fluidity, No. 1 being the hardest.

1. Lead .................................... 2

Tin ...................................... 1

Fusing point, 240 deg. C.

2. Lead ..................................... 1

Tin ...................................... 1

Fusing point, 200 deg. C.

3. Tin ................................... 2 - 2 1/2

Lead.................................. 1

Fusing point, 18s - 190 deg. C.

4. Lead................................... 10

Tin .................................... 177

Fusing point, about 180 deg. C.