Silvering

Silvering

[sil-ver-ing]
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 aldehydes. 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.

See also

References

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