The joining of metals by the use of solders, alloys that flow at temperatures lower than the metals being joined and achieve a joint by wetting and adhering to the metal substrates. Solders can be divided into two distinct classes, soft solders and hard solders. Soft solders are basically lead-tin alloys that usually melt at a relatively low temperature (below 300C). Hard solders for gold and silver alloys contain mixtures of those metals, commonly with the addition of copper, to give a solder which has a melting point close to that of the metals being joined. For the soldering of granulation, where normal solder would flood the interstices between the grains, a technique known as colloidal hard soldering is often used- a metal salt, traditionally of copper, is mixed with an aqueous organic binder. This mixture initially holds the grains to the substrate. As the work is progressively heated, the binder burns off, the metal salt is reduced to metal and mixes with the substrate metal and grain to form a local alloy of lower melting point- this alloy joins the grain to the substrate.