Glass appears colorless when thin, but often has a distinctly green hue when thick or when viewed on end. The green color cast is typically due to the presence of small amounts of iron in the silica that is the principal component of glass. A great deal of effort, and expense, goes into making glass which is optically pure, but glassmakers will purposely add various minerals to give glass color. Special art glass ranging in color from yellow to red to green to blue can be made this way. Iron oxide, although generally considered a common impurity in high-end glass, is added to make glass green for beer bottles. The addition of both iron oxide and chromium makes for the richer green color used for wine bottles. The deep, rich blue of cobalt blue glass comes, unsurprisingly, from the addition of cobalt, in concentrations as small as .025% to .1%.