Tin glazing creates a white, opaque surface for painting. The colours are applied as metallic oxides or as fritted underglazes to the unfired glaze, which absorbs pigment like fresco, making errors impossible to fix, but preserving the brilliant colors. Sometimes the surface is covered with a second glaze (called "coperta" by the Italians) that lends greater shine and brilliance to the wares. In the case of lustred wares, a further oxygen-starved firing at a lower temperature is required. Kilns required wood as well as suitable clay. Glaze was made from sand, wine lees, lead compounds and tin compounds. Tin-glazed earthenware is frequently prone to flaking and somewhat delicate.