Everything about Silvering totally explained
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 wasn't 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), for example 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'll 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.
Further Information
Get more info on 'Silvering'.
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