Solder (Lat. solidus, solid), a metal or alloy used for joining together different pieces of metal, whether of the same or of different kinds. Solders are divided into hard and soft. The soft solders may be used for joining all kinds of metals, but usually those having low melting points. The hard solders are better adapted for the less fusible metals, especially where strength is required. Practically the solder must be more fusible than either of the metals to be united, but the more nearly these points coincide the stronger will be the union. Gold in the form of leaf or fine shreds is used for soldering platinum vessels; it may be slightly alloyed with copper. Silver is considered the best solder for German silver. Copper in shreds is often used for iron when welding is not permissible, sometimes slightly alloyed with zinc. Soft solders have tin for a basis, generally alloyed with lead. Those containing much lead are sometimes ranked with hard solders. Pewter may be used for a solder, and by the addition of bismuth, antimony, or cadmium its fusing point may be lowered so that it can be used as a solder for pewter. The following are some of the more important solders.

For gold: gold (18 carats) 66'6, silver 16.7, copper 16.7. A good gold solder for general purposes is 100 parts of gold, 40 of silver, and 30 of copper (Makins). For silver: silver 66'6, copper 30, brass 3.4; or silver 65, copper 24, zinc 11. It is better to add the metals separately than to use brass, which may have an uncertain composition. Pewterer's solder: coarse - tin 3, lead 4, bismuth 2; fine - tin 2, lead 1, bismuth 1. Plumber's solder: tin 1, lead 3; a finer kind has the same composition as fine pewterer's solder. Hard spelter solder, used for soldering copper, is made of copper 16, zinc 12. Soft spelter solder, for brass, is made of equal parts of copper and zinc. Fluxes are used to preserve the cleanness of the surfaces of the metals and free them from oxide while the operation of soldering is going on. The solder is applied in various ways. The surfaces, sometimes previously cleaned with a file or with muriatic acid or an acid solution of chloride of zinc, are brought together, and the solder in strips or grains laid on. Then a flux composed of borax or sal ammoniac, sometimes mixed with a little common rosin, is applied, and the parts are heated with a blowpipe or a stream of intensely heated air.

But it is more common to use a soldering iron, an instrument consisting of a heavy square, pyramidal, or conical piece of copper, riveted in a fork of wrought iron, to which a wooden handle is attached. This "iron," being heated above the fusing point of the solder, is applied to it, and a few adhering drops of the melted alloy are carried to the parts to be joined, which are then held in position until the solder hardens. Aluminum cannot be soldered in the ordinary way, but must first be tinned. A good general solder for aluminum is composed of zinc 90, aluminum 6, copper 4.