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Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal.




Process


Most common household mirrors are "back-silvered" or "second-surface", meaning that the light reaches the reflective layer after passing through the glass. A protective layer of paint is usually applied to protect the back side of the reflective surface . This arrangement protects the fragile reflective layer from corrosion, scratches, and other damage. However, the glass layer may absorb some of the light and cause distortions and optical aberrations due to refraction at the front surface, and multiple additional reflections on it, giving rise to "ghost images" (although some optical mirrors such as Mangins take advantage of it).
Therefore, precision optical mirrors normally are "front-silvered" or "first-surface", meaning that the reflective layer is on the surface towards the incoming light. The substrate normally provides only physical support, and need not be transparent. A hard, protective, transparent overcoat may be applied to prevent oxidation of the reflective layer and scratching of the metal. Front-coated mirrors achieve reflectivities of 90–95% when new.




History



Ptolemaic Egypt had manufactured small glass mirrors backed by lead, tin, or antimony. In the early 10th century, the Persian 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.
Tin-coated mirrors were first made in Europe in the 15th century. The thin tinfoil used to silver mirrors was known as "tain". When glass mirrors first gained widespread usage in Europe during the 16th century, most were silvered with an amalgam of tin and mercury,
In 1835 German chemist Justus von Liebig developed a process for depositing silver on the rear surface of a piece of glass; this technique gained wide acceptance after Liebig improved it in 1856.
The process was further refined and made easier by the chemist Tony Petitjean (1856). 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.
In 1856-1857 Karl August von Steinheil and Léon Foucault introduced the process of depositing an ultra-thin layer of silver on the front surface of a piece of glass, making the first optical-quality first surface glass mirrors, replacing the use of speculum metal mirrors in reflecting telescopes. These techniques soon became standard for technical equipment.
An aluminum vacuum-deposition process invented in 1930 by Caltech physicist and astronomer John Strong, led to most reflecting telescopes shifting to aluminum. Nevertheless, some modern telescopes use silver, such as the Kepler Space Telescope. The Kepler mirror's silver was deposited using ion assisted evaporation.




Modern silvering processes






= General processes

=
Silvering aims to produce a non-crystalline coating of amorphous metal (metallic glass), with no visible artifacts from grain boundaries. The most common methods in current use are electroplating, chemical "wet process" deposition, and vacuum deposition.
Electroplating of a substrate of glass or other non-conductive material requires the deposition of a thin layer of conductive but transparent material, such as carbon. This layer tends to reduce the adhesion between the metal and the substrate.(pp 3 & 107) Chemical deposition can result in better adhesion, directly or by pre-treatment of the surface.
Vacuum deposition can produce very uniform coating with very precisely controlled thickness.




= Metals

=



Silver

The reflective layer on a second surface mirror such as a household mirror is often actual silver. A modern "wet" process for silver coating treats the glass with tin(II) chloride to improve the bonding between silver and glass. An activator is applied after the silver has been deposited to harden the tin and silver coatings. A layer of copper may be added for long-term durability.
Silver would be ideal for telescope mirrors and other demanding optical applications, since it has the best initial front-surface reflectivity in the visible spectrum. However, it quickly oxidizes and absorbs atmospheric sulfur to create a dark, low-reflectivity tarnish.



Aluminum

The "silvering" on precision optical instruments such as telescopes is usually aluminum. Although aluminum also oxidizes quickly, the thin aluminum oxide (sapphire) layer is transparent, and so the high-reflectivity underlying aluminum stays visible.
In modern aluminum silvering, a sheet of glass is placed in a vacuum chamber with electrically heated nichrome coils that can evaporate 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 or beryllia 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.



Tin

The first tin-coated glass mirrors were produced by applying a tin-mercury amalgam to the glass and heating the piece to evaporate the mercury.



Gold

The "silvering" on infrared instruments is usually gold. It has the best reflectivity in the infrared spectrum, and has high resistance to oxidation and corrosion. Conversely, a thin gold coating is used to create optical filters which block infrared (by mirroring it back towards the source) while passing visible light.




See also


Dielectric mirror
List of telescope parts and construction
Optical coating
Mercury glass
Mercury silvering
Metallizing




References






External links


Tions.net, Diy mirror / mirroring / silvering

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Daftar Isi

• Dial silvering - NAWCC Forums
• Silvering Powder? - NAWCC Forums
• What is Silvering on a Clock Face and How to Identify it?
• clock Dial resilvering, | NAWCC Forums
• Restoring silvered dials - NAWCC Forums
• Dial Silvering Question - NAWCC Forums
• dial silvering not working!!! - NAWCC Forums
• Re-silvering engraved clock face - NAWCC Forums
• Re-silvering a 1913 Junghans bracket clock face. - NAWCC Forums
• Resilvering for newbies - NAWCC Forums

Dial silvering - NAWCC Forums

Dec 10, 2017 · As I understand it, it relies on using silver powder during the silvering process along with silver chloride, which helps disrupt the optical properties of the silver. If you have benefited from the discussion here on the NAWCC forum, please consider joining the NAWCC, or donating to the message board

Silvering Powder? - NAWCC Forums

Jun 7, 2023 · Looking for recommendations as to which silvering powder I should use to re-silver the chapter ring on my clock. It was marred, stained and really in need of attention so I polished it down to the brass tonight as per a couple of videos I was linked to here on the forum. The silvering process looks simple enough and pretty straight forward.

What is Silvering on a Clock Face and How to Identify it?

Jan 29, 2019 · After silvering cream of tartar is applied in the same way. In cases where the silvering is still present but tarnished it is possible to recover the finish with cream of tartar alone. Preparation for silvering involves removing the lacquer and rubbing down with wet & dry. I …

clock Dial resilvering, | NAWCC Forums

Jan 24, 2017 · Silvering is a chemical reaction where the silver ions replace the copper ions in the surface of the brass. On an enamel dial it would just be a really expensive abrasive powder, like millionaire's scouring powder.

Restoring silvered dials - NAWCC Forums

Dec 28, 2009 · The dial silvering was badly tarnished and scratched by the hands, and there were a few dings in it. First thing to do was to photograph it, of course, and measure the diameter of the chapter ring. Restoring the dial: Obviously all the printed parts had to be removed, and the dial needed a bit of gentle panel-beating to flatten the dings.

Dial Silvering Question - NAWCC Forums

Oct 28, 2020 · yeah, the process is the same. Clean off the brass with wet-dry paper worked in the direction you want the grain on the dial. Soak a piece of cotton T-shirt or the like in water, wring it out to only slightly wet, apply the silvering compound to the T-shirt material, and rub it on the dial surface. It may take a couple of passes.

dial silvering not working!!! - NAWCC Forums

Oct 3, 2011 · As to silvering generally: I had a long conversation with a electroplater in the 70s, when a client wanted a teapot re-plated. He said it would cost too much, when he saw the piece, and went on to explain the method of "scouring" the metal, to make it mat. The piece in question had too many crevices etc.

Re-silvering engraved clock face - NAWCC Forums

Aug 21, 2023 · Yes, give it some thought. I started out with several simple, plain dials before I ever attempted one with engraving. The wax (shellac) for filling the numbers and minute ring takes some hands on experience to get the right amount of fill without making a mess. It makes the silvering process easy compared to the wax.

Re-silvering a 1913 Junghans bracket clock face. - NAWCC Forums

Mar 23, 2022 · Antique Clock Dial Silvering Powder & Finishing Powder - Priory Polishes However, I misunderstood the original poster's question. If the silvering compound is commercially produced, there is no difference in preparation of the dial for the silvering process, whether they list silver chloride or nitrate.

Resilvering for newbies - NAWCC Forums

Dec 5, 2005 · One contains the silvering powder and the other is the polishing/shining powder, which I assume is nothing more than cream of tarter. I've had good luck with cleaning the dial off with Comet cleanser - just rub and rub with it until the dial is totally clean - make sure that if it was lacquered, that the lacquer is off and the dial then cleaned.