This is not true of "cold-painted" glass, using oil paint or other mixtures, which rarely last more than a few centuries. All of these are fired in a kiln or furnace to fix them, and can be extremely durable when properly applied.
The principal methods of this are enamelled glass, essentially a technique for painting patterns or images, used for both glass vessels and on stained glass, and glass paint, typically in black, and silver stain, giving yellows to oranges on stained glass.
The way the glass is heated and cooled can significantly affect the colors produced by these compounds. Silver compounds such as silver nitrate and silver halides can produce a range of colors from orange-red to yellow.Ruby gold glass is usually made of lead glass with added tin. The color is caused by the size and dispersion of gold particles. Metallic gold, in very small concentrations (around 0.001%, or 10 ppm), produces a rich ruby-colored glass ("Ruby Gold" or "Rubino Oro"), while lower concentrations produces a less intense red, often marketed as " cranberry".Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-colored glass.When used together with cadmium sulfide, it yields a brilliant red color known as "Selenium Ruby". It is a very important agent to make pink and red glass. Selenium, like manganese, can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color, caused by selenium nanoparticles dispersed in glass.The coloring is caused by silver nanoparticles. Didymium gives green color (used in UV filters) or lilac red.When used with lead glass with very high proportion of lead, produces a deep red color. Uranium glass is typically not radioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic. Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color.Titanium, rarely used on its own, is more often employed to intensify and brighten other colorizing additives. Adding titanium produces yellowish- brown glass.Together with selenium and sulphur it yields shades of bright red and orange. Cadmium together with sulphur forms cadmium sulfide and results in deep yellow color, often used in glazes.Chromium aventurine, in which aventurescence is achieved by growth of large parallel chromium(III) oxide plates during cooling, is made from glass with added chromium oxide in amount above its solubility limit in glass. Together with tin oxide and arsenic it yields emerald green glass. Chromium is a very powerful colorizing agent, yielding dark green or in higher concentrations even black color.Nickel together with a small amount of cobalt was used for decolorizing of lead glass. Lead crystal with added nickel acquires purplish color. Nickel, depending on the concentration, produces blue, or violet, or even black glass.2 to 3% of copper oxide produces a turquoise color.
Very small amounts can be used for decolorizing. The best results are achieved when using glass containing potash. Small concentrations of cobalt (0.025 to 0.1%) yield blue glass.
Details of the process and the composition of the glass vary and so do the results, because it is not a simple matter to obtain or produce properly controlled specimens. This process is widely confused with the formation of "desert amethyst glass", in which glass exposed to desert sunshine with a high ultraviolet component develops a delicate violet tint. In New England some houses built more than 300 years ago have window glass which is lightly tinted violet because of this chemical change and such glass panes are prized as antiques.
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