The object of soldering is to unite two portions of the same metal or of different metals by means of a more fusible metal or metallic alloy, applied when melted, and known by the name of solder. As the strength of the soldering depends on the nature of the solder used, the degree of strength required for the joint must be kept in view in choosing a solder. The parts to be joined must be free from oxide and thoroughly clean; this can be secured by filing, scouring, scraping, or pickling with acids. The edges must exactly fit, and be heated to the melting-point of the solder. The latter must have a lower melting-point than either of the portions of metal that require to be joined, and if possible only those metals should be chosen for solder which form alloys with them. The solder should also as far as possible have the same color and approximately the same strength as the article whose edges are to be united.

To remove the layers of oxide which form during the process of soldering, various so-called "fluxes" are employed. These fluxes are melted and applied to the joint, and act partly to keep off the air, thus preventing oxidation, and partly reduce and dissolve the oxides themselves. The choice of a flux depends on the quantity of heat required for soldering.

Solders are classed as soft and hard solders. Soft solder-. also called tin solders or white solders, consist of soft, readily fusible metals or alloys, and do not possess much strength; they are easy to handle on account of their great fusibility. Tin. lead-tin. and alloys of tin, lead, and bismuth are used for soft solders, pure tin being employed only for articles made of the same metal (pure tin).

The addition of some lead makes the solder less fusible but cheaper, while that of bismuth lowers the melting-point. Soft solders are used for soldering easily fusible metals such as Britannia metal, etc., also for soldering tin-plate. To prepare solder, the metals are melted together in a graphite crucible at as low a temperature as possible, well stirred with an iron rod. and cast into ingots in an iron mold. To melt the solder when required for soldering, the soldering iron is used; the latter should be kept as free from oxidation as possible, and the part applied should be tinned over.

The fluxes generally used in the soft-soldering of metals are powdered rosin or a solution of chloride of zinc, alone or combined with sal-ammoniac. A neutral soldering liquid can be prepared by mixing 2.7 parts neutral zinc chloride, 11 parts sal-ammoniac and 62 parts water, or 1 part sugar of milk, 1 part glycerine, and 8 parts water.

A soldering fat for tin-plate, preferable to ordinary rosin, as it can be more easily removed after soldering, is prepared as follows: 150 parts beef-tallow, 250 parts rosin, and 150 parts olive oil are melted together in a crucible and well stirred, 50 parts powdered sal-ammoniac dissolved in as little water as possible being added.

Soldering fat for iron is composed of 50 parts olive oil and 50 parts powdered sal-ammoniac. Soldering fat for aluminium is made by melting together equal parts of rosin and tallow, half the quantity of zinc chloride being added to the mixture.

Soldering paste consists of neutral soldering liquid thickened with starch paste. This paste must he applied more lightly than the soldering liquid.

Soldering salt is prepared by mixing equal parts of neutral zinc chloride, free from iron, and powdered sal-ammoniac. When required for use, 1 part of the salt should be dissolved in 3 or 4 parts water.

Borax is the flux most frequently used for hard-soldering; it should be applied to the soldering seam either dry or stirred to a paste with water. It is advisable to use calcined borax, i. e., borax from which the water of crystallization has been driven out by heat, as it does not become so inflated as ordinary borax. Borax dissolves the metallic oxides forming on the joint.

Finely-powdered cryolite or a mixture of 2 parts powdered cryolite and 1 part phosphoric acid is also used for hard-soldering copper and copper alloys.

Muller's hard-soldering liquid consists of equal parts of phosphoric acid and alcohol (80 per cent).

A mixture of equal parts of cryolite and barium chloride is used as a flux in hard-soldering aluminium bronze.

A flux used in soldering iron with cast iron is composed of equal parts of cast-iron filings and calcined borax. This black. glassy mixture is pulverized, and the powder spread on the seam.

It only remains to mention the so-called cold-soldering, in reality a joining of the edges by means of a copper amalgam. The parts to be joined are well cleaned, and a substance made by triturating 1 part of metallic sodium with 50 to 60 parts of mercury rubbed in. This substance may to some 'extent be used for the same purpose as soldering fluid, as it causes the firm adhesion of the copper amalgam employed as solder.

To make copper amalgam, dissolve copper sulphate in water and add some zinc-plate chips. A fine powder consisting of pure copper is deposited, which should be filtered off, washed, and triturated in a heated porcelain bowl with a double quantity by weight of mercury. The amalgam, which resembles 18-carat gold in color, is formed into little pellets or bars, which are made soft by heating when required for use.