Ordinary
soda-lime glass appears
colorless to the naked eye when it is thin, although iron oxide impurities produce a
green tint which can be viewed in thick pieces or with the aid of scientific instruments. Further
metals and metal
oxides 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 with
chromium it gives a richer green color, used for
wine bottles.
Sulfur, together with
carbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black. In
borosilicate glasses rich in boron, sulfur imparts a blue color. With
calcium it yields a deep yellow color.[4]
Manganese can be added in small amounts to remove the
green tint given by iron, or in higher concentrations to give glass an
amethyst color. Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
Manganese dioxide, which is
black, is used to remove the green color from the glass; in a very slow process this is converted to
sodium permanganate, a dark
purple compound. 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. 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 of
cobalt (0.025 to 0.1%) yield
blue glass. The best results are achieved when using glass containing
potash. Very small amounts can be used for decolorizing.
Nickel, depending on the concentration, produces blue, or
violet, or even
black glass.
Lead crystal with added nickel acquires purplish color. Nickel together with a small amount of cobalt was used for decolorizing of
lead 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 yields
emerald green glass. 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.
Cadmium together with sulphur forms
cadmium 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]
Adding
titanium produces
yellowish-
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 or
green color.[8]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. 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]
Striking glasses
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. It is a very important agent to make pink and red glass. When used together with
cadmium sulfide,[9] it yields a brilliant red color known as "Selenium Ruby".
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.
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". The color is caused by the size and dispersion of gold particles. Ruby gold glass is usually made of
lead glass with added
tin.
Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride.
Coloring added to glass
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. 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.
Glass containing two or more
phases with different
refractive indices shows coloring based on the
Tyndall effect and explained by the
Mie theory, if the dimensions of the phases are similar or larger than the
wavelength 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 glass
dichroic optical properties. Also the blue appearance of some automobile
windshields is caused by dichroism.
^
abBernard H. W. S. De Jong, Ruud G. C. Beerkens, Peter A. van Nijnatten: "Glass", in: "Ullmann's Encyclopedia of Industrial Chemistry"; Wiley-VCH Verlag GmbH & Co. KGaA, 2002,
doi:
10.1002/14356007.a12_365
^
abcWerner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994),
ISBN3-540-57572-3