Silver nitrate for mirror silvering

Silver nitrate dissolved in water and treated with ammonia forms the diamminesilver(I) ion, [Ag(NH₃)₂]⁺. In alkaline solution a mild reducing agent, invert sugar, glucose, formaldehyde or tartrate, reduces it to metallic silver. If the reduction is slow and the solution is in contact with a surface that already carries silver nuclei, the silver deposits as a coherent, mirror-bright film rather than as a black precipitate. The sensitising step with tin(II) chloride puts those nuclei on the glass, and the method you choose sets the rate of reduction.

Two reducers are in common use. The Brashear process uses a sugar reducer and deposits a bright film in a few minutes at room temperature; it is the standard for telescope mirrors and demonstration work. The Rochelle salt (potassium sodium tartrate) process is slower, often 20–60 minutes with gentle warming, and gives a harder, finer-grained film that polishes well. Commercial production uses sprayed two-component solutions on the same chemistry.

A good mirror film is about 100 nm thick. At the density of silver, 10.49 g/cm³, that is about 1.05 g of silver per square metre, or 1.65 g of silver nitrate (63.5% silver). In practice a third to a half of the silver in a Brashear bath ends up on the glass and the rest in the sludge and rinse water, so budget 4–5 g of silver nitrate per square metre, or roughly 0.2 g for a 20 cm telescope mirror and 2.5 g for a 60 × 80 cm sheet, plus whatever volume of solution is needed to cover the surface to a depth of 5–10 mm.

Nothing else matters if the glass is not clean. A film deposited on a greasy surface peels, pinholes or deposits unevenly.

1. Wash with warm water and a few drops of detergent, rubbing with a cotton pad; rinse.
2. Polish with a slurry of precipitated chalk, rouge or cerium oxide on a cotton pad, rinse, then swab with dilute nitric acid (about 10%) and rinse again. Wear gloves from this point so skin oils never touch the surface.
3. Rinse with distilled or deionised water. The test is simple: water must sheet off evenly with no beading or dry patches.
4. Sensitise by flooding the surface for one to two minutes with a fresh 0.1% tin(II) chloride solution (1 g of SnCl₂·2H₂O in 1 L of water with 1 mL of concentrated hydrochloric acid to keep it clear).
5. Rinse thoroughly with distilled water; any tin left in solution would precipitate silver in the bath. Keep the glass wet until the silvering solution goes on.

Quantities for about 350 mL of silvering solution, enough for a 20–25 cm diameter mirror in a shallow tray:

• Solution A (silver): dissolve 5 g of silver nitrate in 300 mL of distilled water.
• Solution B (alkali): dissolve 2.5 g of potassium hydroxide in 50 mL of distilled water.
• Reducer: dissolve 90 g of cane sugar in 800 mL of water, add 4 mL of concentrated nitric acid, boil for 10 minutes to invert the sugar, cool, add 175 mL of ethanol and make up to 1 L. It keeps indefinitely and improves with age; you need about 10 mL per batch.

1. Set aside about 10 mL of solution A. To the rest add concentrated ammonia dropwise with stirring: a brown precipitate of silver oxide forms and then just redissolves to a clear solution. Do not overshoot.
2. Add solution B. A dark precipitate forms again; add ammonia dropwise until it just redissolves.
3. Now add the reserved solution A dropwise until a faint permanent turbidity remains. This back-titration ensures silver, not ammonia, is in slight excess, which makes a brighter film and, more importantly, reduces the risk of fulminating silver forming later. Filter through cotton wool if the turbidity is heavy.
4. With the sensitised glass face-up in the tray, add 10 mL of reducer to the silvering solution, swirl once and pour it over the glass immediately. Rock the tray gently and continuously.
5. The solution darkens and the film appears within a minute; after 3–5 minutes, when the solution has turned murky and the mirror looks opaque against a lamp, pour the spent liquid into the waste container.
6. Rinse at once with distilled water, stand the mirror on edge to drain and dry it in a dust-free place with gentle warm air. Work at 18–25 °C; colder solutions deposit slowly and dull, warmer ones deposit too fast.

• Silver solution: dissolve 2 g of silver nitrate in 100 mL of distilled water and add ammonia dropwise until the precipitate just redissolves, then add 0.5 mL more of a 2% silver nitrate solution to restore a faint turbidity.
• Reducer: dissolve 1.6 g of potassium sodium tartrate (Rochelle salt) and 0.2 g of silver nitrate in 100 mL of water, boil for a few minutes until a grey precipitate forms, cool and filter.

Mix equal volumes immediately before use, pour over the sensitised glass and keep at 35–45 °C with gentle agitation. The film forms over 20–60 minutes. The result is harder than a Brashear film and can be burnished lightly with a chamois and a trace of rouge once dry, which is why this method was preferred for front-surface mirrors before vacuum aluminising.

A back-silvered mirror (glass in front, silver behind) is protected by painting the silver with a mirror-backing paint, traditionally after a thin electroless copper layer. A front-surface mirror has no protection and tarnishes in weeks to months in sulfurous air; a dip in dilute potassium dichromate or a modern thiol-based tarnish inhibitor slows this, as does storing the mirror in a closed box. Faults are easy to read: pinholes mean dust or poor cleaning, a milky film means the solution was too cold or too dilute, and a loose film that wipes off means the sensitising step failed or the glass was greasy.

Ammoniacal silver solutions that are left to stand, and especially allowed to dry, deposit fulminating silver (silver nitride, Ag₃N, with silver amide), a black solid that detonates on the slightest friction or warming. Every year this injures someone who kept a bottle of Tollens-type solution or let residues dry in a sink.

• Never store a mixed silvering solution, not even for an hour. Make it immediately before use.
• As soon as the job is done, acidify all spent solutions and rinses with dilute hydrochloric acid until acid to litmus; silver precipitates as harmless silver chloride and any nitride is destroyed.
• Collect the silver chloride sludge for recovery (it is worth money) and dispose of the acidified liquid as silver-containing waste, not down the drain.
• Rinse trays, flasks and the sink with dilute acid before anything can dry on them.