Soldering is the art of uniting the surfaces of metals by partial fusion, and the insertion of an alloy between the edges, which is called solder, it being more fusible than the metals which it unites. Solders are distinguished as hard and soft, according to their difficulty of fusion. Hard solders usually melt only at a red heat, but soft solders fuse at lower temperatures. In order to join metals, it is obvious that a solder must be used that melts at a lower temperature than the metals to bo joined; but it may also be necessary that it approach as nearly as possible to them in point of hardness; and occasionally, as is especially the case with jewelry, similarity of color is an object. The heat requisite for soldering small articles, such as jewelry, etc., is usually obtained by employing a common blow pipe; as by its use a sudden heat may be concentrated on a small point. Where a larger surface has to be heated, the flame of a spirit lamp is used. For brazing, or uniting larger objects with hard solder, a furnace, or, if necessary, a forge, may be employed. In working tin plates, the solder is applied and fused by a heated copper tool called a soldering-iron. The surfaces of parts to be joined by soldering must be perfectly clean; and in order to ensure this, as well as to counteract the oxidization which most metals undergo when heated, a flux is used (sec No. 3479), which neutralizes or removes these otherwise serious impediments, securing a firm joint.

3473. To Make Soldering Fluid for Soft Solder

3473. To Make Soldering Fluid for Soft Solder. Into muriatic acid put small pieces of zinc until all bubbling ceases; some

I add 1 ounce sal-ammoniac to each pound of the liquid.

3474. Neutral Soldering Fluid

3474.    Neutral Soldering Fluid. Dissolve zinc in muriatic acid as above, then warm the solution and add sufficient oxide or carbonate of tin in powder to neutralize it. This prevents the fluid from corroding the seams.

3475. Soldering Liquid

3475.      Soldering Liquid. Soldering liquid is made by taking hydrochloric acid, 1/4 pint; granulated tin, 11/2 ounce; dissolve and add some common solder and hydrochlo-rate of ammonia.

3476. Flux for Soldering

3476.    Flux for Soldering. For common purposes powdered resin is generally used. Stearic acid, obtained from the candle factories, makes a good flux for fine tin work.

3477. Flux for Soldering Iron or Steel

3477.     Flux for Soldering Iron or Steel. Dissolve chloride of zinc in alcohol.

3478. Flux for Soldering Steel

3478.    Flux for Soldering Steel. This answers perfectly when the fracture is an old one. To a saturated solution of zinc in 1 pint muriatic acid, add 4 ounces pulverized sal-ammoniac; boil it for 10 minutes; put it, when cold, in a well corked bottle. The boiling must be done in a copper vessel.