A transparent glaze, applied to the surface of earthenware, to provide a shiny surface and to protect the body beneath. After an initial biscuit firing, the glaze, using lead oxide as the flux, was applied to the vessel and fired again at a lower temperature, when it melted on to the surface. Lead glaze was widely used in the Middle Ages and could be coloured brown or green with the addition of copper oxide. Later the clear, colourless glaze was used to cover painted decoration. As fumes of lead proved dangerous to workers, lead glaze was rarely used after the early 19th century.