Solder
. Solder, any of several metallic alloys that melt at comparatively low temperatures and are used for the patching or joining of metals. Solders are commonly classified as soft and hard solders, depending upon their melting points and strengths. Soft solders are alloys of lead and tin, sometimes with the addition of bismuth; hard solders are alloys of silver, copper, and zinc (silver solder) or of copper and zinc (brazing spelter). In joining two pieces of metal with solder, the joining surfaces are first cleaned mechanically and then coated with a flux, usually of rosin or borax, that cleans them chemically and assists the solder in making a bond. The surfaces are then heated, either with a hot metal tool called a soldering iron or soldering copper or with some form of alcohol or gas blowtorch. When the surfaces are heated to the melting point of the solder, the solder is applied and runs freely, solidifying as the surfaces cool. In the form of soldering known as sweating, the pieces ...