Slag is a by-product from the smelting of metals or the making of glass. It is often glassy or metallic in appearance and has a very irregular form, sometimes a globular mass. When found it can denote metal or glassworking in the vicinity.
The ores of metals such as iron, copper and lead, and others are found in impure states, often oxidized and mixed in with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. The collection of compounds that is removed is the slag.