solder , metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i.e., silver solders) have high melting points and are suited for use with ferrous and high-melting-point nonferrous alloys. Areas to be soldered are cleaned and coated with a flux (such as hydrochloric acid or borax) to prevent oxides from separating the solder from the surface. When brass is used in the solder or when brass surfaces are to be joined, the process is known as brazing, though the name is sometimes applied also to other hard soldering.