Glass coloring and color marking may be obtained in several ways.

1. by the addition ofcoloring ions,^[1][2]
2. by precipitation of nanometer-sized colloids (so-calledstriking glasses^[1] such as "gold ruby"^[3] or red "selenium ruby"),^[2]

   

3. bycolored inclusions (as inmilk glass andsmoked glass)
4. bylightscattering (as inphase separated glass)^[2]
5. bydichroic coatings (seedichroic glass), or
6. bycolored coatings

Ordinarysoda-lime glass appearscolorless to the naked eye when it is thin, although iron oxide impurities produce agreen tint which can be viewed in thick pieces or with the aid of scientific instruments. Furthermetals and metaloxides can be added to glass during its manufacture to change its color which can enhance its aesthetic appeal. Examples of these additives are listed below:

• Iron(II) oxide may be added to glass resulting in bluish-green glass which is frequently used in beer bottles. Together withchromium it gives a richer green color, used forwine bottles.
• Sulfur, together withcarbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black. Inborosilicate glasses rich in boron, sulfur imparts a blue color. Withcalcium it yields a deep yellow color.^[4]
• Manganese can be added in small amounts to remove thegreen tint given by iron, or in higher concentrations to give glass anamethyst color. Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
• Manganese dioxide, which isblack, is used to remove the green color from the glass; in a very slow process this is converted tosodium permanganate, a darkpurple compound. InNew England some houses built more than 300 years ago have window glass which is lightly tintedviolet because of this chemical change, and such glass panes are prized as antiques. 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. 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.^[5]
• Small concentrations ofcobalt (0.025 to 0.1%) yieldblue glass. The best results are achieved when using glass containingpotash. Very small amounts can be used for decolorizing.
• 2 to 3% ofcopper oxide produces aturquoise color.
• Nickel, depending on the concentration, produces blue, orviolet, or evenblack glass.Lead crystal with added nickel acquires purplish color. Nickel together with a small amount of cobalt was used for decolorizing oflead glass.
• Chromium is a very powerful colorizing agent, yielding dark green^[6] or in higher concentrations even black color. Together with tin oxide and arsenic it yieldsemerald green glass. Chromiumaventurine, in whichaventurescence is achieved by growth of large parallelchromium(III) oxide plates during cooling, is made from glass with added chromium oxide in amount above its solubility limit in glass.
• Cadmium together with sulphur formscadmium sulfide and results in deep yellow color, often used in glazes. However, cadmium is toxic. Together with selenium and sulphur it yields shades of bright red and orange.^[7]
• Addingtitanium producesyellowish-brown glass. Titanium, rarely used on its own, is more often employed to intensify and brighten other colorizing additives.
• Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow orgreen color.^[8]Uranium glass is typically notradioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can becarcinogenic. When used with lead glass with very high proportion of lead, produces a deep red color.
• Didymium gives green color (used in UV filters) or lilac red.^[7]

• Selenium, like manganese, can be used in small concentrations to decolorize glass, or in higher concentrations to impart areddish color, caused by seleniumnanoparticles dispersed in glass. It is a very important agent to make pink and red glass. When used together withcadmium sulfide,^[9] it yields a brilliant red color known as "Selenium Ruby".
• Pure metalliccopper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production ofruby-colored glass.
• Metallicgold, 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, oftenmarketed as "cranberry". The color is caused by the size and dispersion of gold particles. Ruby gold glass is usually made oflead glass with addedtin.
• Silver compounds such assilver nitrate andsilver halides can produce a range of colors fromorange-red to yellow. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. Alsophotochromic lenses andphotosensitive glass are based on silver.
• Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride.

The principal methods of this areenamelled glass, essentially a technique for painting patterns or images, used for both glass vessels and on stained glass, and glass paint, typically in black, andsilver stain, giving yellows to oranges on stained glass. All of these are fired in a kiln or furnace to fix them, and can be extremely durable when properly applied. This is not true of "cold-painted" glass, using oil paint or other mixtures, which rarely last more than a few centuries.

Tin oxide withantimony andarsenic oxides produce an opaquewhite glass (milk glass), first used inVenice to produce an imitationporcelain, very often thenpainted with enamels. Similarly, somesmoked glasses may be based on dark-colored inclusions, but with ionic coloring it is also possible to produce dark colors (see above).

Glass containing two or morephases with differentrefractive indices shows coloring based on theTyndall effect and explained by theMie theory, if the dimensions of the phases are similar or larger than thewavelength of visible light. The scattered light is blue and violet as seen in the image, while the transmitted light is yellow and red.

Dichroic glass has one or several coatings in the nanometer-range (for example metals, metal oxides, or nitrides) which give the glassdichroic optical properties. Also the blue appearance of some automobilewindshields is caused by dichroism.

• Crystal field theory - physical explanation coloring
• Color of medieval stained glass
• Hydrogen darkening
• Hydroxyl ion absorption
• Transparent materials

• Glass engineering and science
• Glass chemistry

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