There are chemical and physical methods
of glass
discoloration. In the chemical method, they aim to transfer all the iron into Fe3 +. To
do this, oxidizers are introduced into the mixture - alkali metal nitrates,
cerium dioxide CeO2, as well as arsenic (III) oxide As2O3
and antimony (III) oxide Sb2O3. Chemically discolored
glass is only slightly colored (due to Fe3+ ions) in a yellowish-greenish
color, but it has good light transmission.
 |
| Discolored and Translucent Glass |
When physically
bleaching, “dyes” are introduced into the glass composition, i.e. ions that
paint it in additional tones to the color produced by iron ions. These are
oxides of nickel, cobalt, rare earth elements, and also selenium. Manganese
dioxide MnO2 has the properties of both chemical and physical
bleaching. As a result of the double absorption of light, the glass becomes
colorless, but it's light transmission decreases. Thus, it is necessary to
distinguish translucent and discolored glass, since these concepts are
different.
Transparency is
one of the most important properties of glass. However, in some cases, the
glass is specially made opaque by its “damping”. This is the process by which
glass becomes opaque. Substances that contribute to the clouding of glass are called
silencers. The damping occurs due to the distribution of the smallest
crystalline particles over the entire glass mass. They are undissolved silencer
particles or particles precipitated from the liquid mass upon cooling the
glass. These particles are usually transparent, but their index of refraction
differs from the index of refraction of glass. Therefore, the beam incident on
them deviates from the rectilinear direction and the glass ceases to be transparent.
In the distant past, a bone meal containing calcium phosphate Ca3(PO4)2,
as well as SnO tin, As2O3 arsenic, and Sb2O3
antimony oxides were used as glass silencers. Currently, cryolite Na3[AlF6],
fluorspar CaF2 and other fluoride compounds are used for this
purpose.
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