The term used to describe the uniting of chemically clean surfaces of suitable metals or alloys, which are not themselves melted, by using heat and a fusible alloy or solder. There are two distinct forms of solder - soft and hard. The more fusible one is usually called 'soft' or 'lead' solder; lead has a relatively low melting point. Ordinary soft solder consists of two parts tin and one part lead, but a range of soft solders is available which contains silver with tin, lead, copper, cadmium or zinc added. Depending on purpose, soft solder is suitable for use on copper, nickel, tin, iron, zinc, lead and numerous alloys. Hard or silver soldering, or 'silver alloy brazing' as it is called to distinguish it from soft soldering, is used for most nonferrous metas and alloys as well as for steel and iron. Hall-marked silver is soldered with silver solder of high silver content, and various hardnesses and melting ranges of solders of this type are available. Hall-marked gold is soldered with a gold solder of high quality in various carats, hardnesses and colors. Solders may be purchased in various forms of wire, rod, sheet or powder-paste. Different fluxes are necessary for different types of work. Soldering fluxes help the metal to flow and prevent oxidation under heat. Although many fluxes are available, soft-solder flux generally consists of resin, or killed sulphuric or hydrochloric acid, or one of a series of patent preparations. Hard-soldering flux is traditionally boracic acid (or borax), made up into a paste with water.