For some purposes even the soft solders of tin and lead are too difficult of fusion, and in this case alloys of tin, lead, and bismuth are employed. This is a most excellent solder, but its use is limited to very special purposes, on account of the expensiveness of bismuth. For ordinary work, also, there is no need of such an extremely low fusing point.

Tin and bismuth alone are sometimes used, but not often. This alloy is more resonant than pure tin, but less ductile. The fusing points of some alloys of tin and bismuth are given below:

All solders containing bismuth have a very low fusing point; their disadvantage is that they have but little cementing power, and are very brittle, so that if an article soldered in this way is allowed to fall, the whole soldering seam will often give way. We give below the composition of some of the solders of tin, lead, and bismuth, with their respective fusing points:

With a considerable admixture of bismuth, the fusing point can be brought down to a temperature lower than the boiling point of water. Some of these mixtures are here given:

Newton's Metal

Bismuth .................................... 1664

Lead ....................................... 1035

Tin ........................................ 354

Fusing point, 94.5 deg. C.

Rose's Metal

Bismuth ..................................... 416

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

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

Fusing point, 93.75 deg. C.

D'Arcet's Metal

Bismuth .................................... 1664

Lead ....................................... 1656

Tin ........................................ 354

Fusing point, 79 deg. C.

Attention should be called to the fact that these readily fusible metallic mixtures are remarkably well adapted to making reproductions of delicate plaster casts. A cast is made, for example, of a beetle. After drying at 100 deg. C. (212 deg. F.) the insect is carefully removed from the mold, and the latter filled with the fluid alloy. A perfect reproduction will be made.

Bismuth solder is usually made by melting the required quantity of bismuth together with ordinary tin solder, in the proportion of 1 part of bismuth to 8 parts of solder. The tin must be melted first, and heated above the melting point; then the lead is added, the vessel removed from the fire, and the bismuth thrown into the mass, which will at once become thinner. It must be stirred rapidly, and the stirring should be continued until the alloy begins to stiffen, in order to insure an even mixture.

A very good way is to scatter the bismuth, in powdered form, into the melted tin and lead, stirring all the time. Frequent reuniting of bismuth solder is to be avoided, since this is apt to-raise the fusing point.