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I'm considering the reaction wherein ionic silver (from the dissolved nitrate) is reduced by glucose in the presence of NaOH and forms a metalic silver layer on glass.

Reading through several topics here on stackexchange it seems that it still hasn't been clearly elucidated what exactly is happening on the glass surface for reduced silver to deposit onto / bind to it (instead of just precipitating out as a powder in solution).

I'd like to know if/how glass type might affect the process:

Can I get as perfect of a mirror on a borosilicate surface as on pure SiO2 glass?

Is the silver layer in either case uniform or will is be porous at the molecular level?

I've read that a slow reduction is key. Is it so that a silver mirror is qualitatively better produced at lower temperatures (or with highly diluted reagents) over a long time than by gently heating the solution?

Could a large temperature gradient between the solution and the glass surface be advantageous for the reaction to proceed much faster on the surface than in solution (e.g. mixing a solution at 10C into a glass container whose surface is brought to 80C)?

How can the process be optimized to produce a perfectly insulating silver layer at the molecular level?

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    $\begingroup$ My experience has been that all precipitated silver mirrors are slightly porous (or at least have numerous defects), are quickly damaged by exposure (e.g. to sulfides in air) and that adhesion is problematic. The process is of historical interest, and an interesting lab experiment, but for most purposes, a vacuum-evaporated, overcoated mirror is far more useful and cost efffective. $\endgroup$
    – DrMoishe Pippik
    Aug 16, 2020 at 2:35
  • $\begingroup$ @DrMoishe_Pippik Thanks for sharing your experience. Do you know if the thickness could easily be grown to uniformity by additional exposition, or perhaps electrolysis? What steps are involved in the vacuum evaporation: Does it imply rinsing and then vacuum drying the glass surface or vacuum evaporating the entire solution? By what mechanisms does it help? $\endgroup$
    – Hans
    Aug 16, 2020 at 12:00
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    $\begingroup$ The coating is tenuous, but theoretically, could be made thicker by electroplating - though only the glass-covered side would be usable (i.e. a second surface mirror, unsuited for most optics). Vacuum vapor deposition is a thermal, or ion-mediated, process; see en.wikipedia.org/wiki/Silvering . $\endgroup$
    – DrMoishe Pippik
    Aug 16, 2020 at 21:09
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    $\begingroup$ Consider that after the first monolayer on the glass, it is no longer silver adhering to glass, but silver to silver. Since this is precipitating from the solution, rather than being moderated by a force as in electroplating, it is a statistical process and irregularities would be expected to increase with the square root of thickness. $\endgroup$
    – DrMoishe Pippik
    Aug 17, 2020 at 16:19
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    $\begingroup$ Perhaps this link will be of interest: chemistry.stackexchange.com/q/134278/79678. $\endgroup$
    – Ed V
    Aug 17, 2020 at 22:07

1 Answer 1

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It is not so much the glass, it is the contact between the substrate and the reflecting layer.

Some of the problems with the silver/glucose approach, like their optical imperfections is related to the bad adhesion of elemental silver to glass. The «normal glass mirror» in the bathroom thus uses a tin layer between the cleansed glass, and the subsequently added silver (mostly) responsible for the reflection. Discovery channel once aired brief documentary about this; the more interesting part may be still seen on youtube here.

Modern mirrors for telescopes and optical instruments in general do not use silver, but aluminum; the reflective surface is now not behind the glass, but on top (first surface mirrors). One of the reasons to use aluminum instead of silver is that silver darkens over time (think about the formation silver sulfide, already on cutlery), while aluminum oxide forming on top of aluminum still is transparent enough (for visible light) that these mirrors are aluminized about once every two years only. Willi Koorts shows the procedure in a about 20 min video here on small scale: washing the mirror, remove of Al by KOH, washing, deposit of Al in high (real high) vacuum. (This shows the operations at a larger, $\pu{8 m}$ scale.)

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