SOLDER, sixl'er, a fusible alloy of metals employed to join or unite metals. The principle involved is that the molten alloy applied shall partly fuse the metal surfaces, thus creating a metallic union. As a rule, the solder should be easier of fusion than the metal to which it is applied. The solder should also be of or near the same color as the metal. For fine steel, copper and brass work gold or silver may be employed. But, broadly speaking, iron is soldered with copper, and copper and brass with tin. The most usual solders are the com pound, which are distinguished into two prin cipal classes, hard and soft. Any solder that requires a red heat for melting is termed a hard solder; these are ductile, will bear hammering and are commonly prepared of the same metal as that which is to be soldered, with the addi tion of some other, by which a greater degree of fusibility is obtained, though the addition is not always required to be itself easier of fusion. Under this head comes the hard solder for gold, which is prepared from gold and silver, or gold and copper, or gold, silver and copper. The hard solder for silver is pre pared from equal parts of silver and brass, but is made better for fusion by the admixture of 1-16 of zinc. The hard solder for brass is ob tained from brass mixed with a sixth or an eighth, or even one-half of zinc, which may also be used for the hard solder of copper. The soft solders, melting below a red heat, are much used in plumbing and tinning; they melt easily, but are usually brittle and, therefore, cannot be hammered. Tin and lead in equal parts make a good soft solder. Of easier fusion is that consisting of bismuth, tin and lead in equal parts. In the operation of soldering, the surfaces of the metal to be joined must be made very clean and applied to each other. It is
usual to secure them by a ligature of wire or other device. The solder is laid upon the joint, together with sal-ammoniac and borax or com mon glass, according to the degrees of heat intended. These additions defend the metal from oxidation. Glaziers use resin, and pitch is sometimes employed. Tin-foil, applied be tween the joints of fine brass work, first moist ened with a strong solution of sal-ammoniac, makes an excellent juncture, care being taken to avoid too much heat. It was at first found very difficult to solder aluminum, but a satis factory solder was found in an alloy of gold, silver and copper, to which some add a little zinc. Soft solder is applied with a soldering iron, or in some cases with a machine as in closing fruit cans. The regular soldering iron has a pointed copper head and a wooden handle. In use the copper head is heated almost to redness and having been carefully cleaned and scraped, it is used to pick up some of the molten solder and apply it to the work. The surface or edges to be soldered must be quite clean and bright and the process is assisted by a flux, as of resin or chloride of zinc. Hard solder is essential for joining steel, iron gun metal, brass, etc., and permits a much stronger joint than soft solder. The process of applying hard solder is often called brazing, from the similar ity of the alloy used to brass. Common mix tures are copper 60 and zinc 40, for soldering iron; copper 40 and zinc 60, for copper and hard brass; silver 30, copper 50 and zinc 20, for soldering silver, brass, gold and some irons. See ALLOYS ; WELDING.