Solder is an alloy used to stick metals together. It is a mixture of lead and tin usually in the proportion of half and half. Many other solders are used in soldering gold, silver, German silver, and aluminum. Only pure metals can be soldered together.

When the two metals are heated a scale, called an oxide, forms on their surface. To prevent the formation of this while soldering, a substance or liquid called a flux is applied to the surfaces to be soldered. The flux acts by dissolving the oxide formed and at the same time forms a coating which prevents oxidation.

There are a number of fluxes, such as rosin, zinc chloride, muriatic acid, borax, and sal ammoniac, each of which has a particular use. Rosin is used on sheet tin and seems to act more effectively than zinc chloride, which is used on tin, brass, copper, iron, and steel. Sal ammoniac is used in tinning.