User Contributed Dictionary
Verb
silvering- present participle of silver
Noun
Extensive Definition
Silvering is the chemical process of coating
glass with a reflective
substance. Glass mirrors were first coated by molten metal. Later,
tin amalgam was used. The mercury
contamination by this process made it necessary to switch to the
more expensive silver
coating, in order to create a mirror. Today, sputtering, powdered aluminium or other compounds
are more often used for this purpose, although the process
maintains the name "silvering".
History
In the early 10th
century, the Iranian scientist
al-Razi
described ways of silvering and gilding in a book on alchemy, but this was not done
for the purpose of making mirrors, which were then produced by
backings using lead, tin and mercury in different mixtures. Using
silver came in 1835, when a German chemist named Justus
von Liebig developed a process for silvering mirrors that
gained wide acceptance after improvement by Liebig in 1856. The
process was further refined and made easier by French chemist
Petitjean
(1857). This reaction is a variation of the Tollens'
reagent for aldehdes. A
diamminesilver(I) solution is mixed with a sugar and sprayed
onto the glass surface. The sugar is oxidized by silver(I), which
is itself reduced to silver(0), i.e. elemental silver, and deposited onto the
glass. Then in 1880, American
astronomer John
Brashear improved the process in order to make more powerful
and accurate telescopes. These techniques
soon became standard for technical equipment.
Potassium
sodium tartrate and mercury
have historically been used in the silvering process.
Modern silvering process
In modern aluminum silvering, a sheet of glass is
placed in a vacuum
chamber with electrically heated nichrome coils that can sublime
aluminum. In a vacuum, the hot aluminum atoms travel in straight
lines. When they hit the surface of the mirror, they cool and
stick. Some mirror makers evaporate a layer of quartz on the mirror; others
expose it to pure oxygen
or air in an oven so that it will form a tough, clear layer of
aluminum
oxide.
Mirrors made by this method are classified as
either back-silvered, with the silvered layer viewed through the
glass; or front-silvered, with the layer viewed from the other
side. Most common mirrors are back-silvered, since this protects
the fragile reflective layer from corrosion, scratches, and other
damage. However, extraneous reflections from the front surface of
the glass make these mirrors unsuitable for high-precision optical work.