Re: [ATM] silvering vs mercury: Another Question: Silver removal

["Bill Kelley" <flex@commspeed.net>]

Another word.
We silver a lot of mirrors in our telescope making classes using "Peacocks"
kits.  And of course many don't care for them properly afterward and we
have to re-silver them. Cost about $1.00 per mirror.The old silver strips
quickly in clorox.
Bill Kelley


----- Original Message ----- 
From: "Mark Holm" <mdholm@telerama.com>
To: <atm@atmlist.net>
Sent: Wednesday, November 09, 2005 9:34 PM
Subject: Re: [ATM] silvering vs mercury: Another Question: Silver removal


> Pete wrote:
>> Hi all,
>> Silver won't stick well to glass without an adhesive bonding layer.
>> So, with chemical silvered mirrors, get some good sticky tape (clear
>> packaging tape works well) and apply it to the mirror in small sections
>> at a time and the silver will be gone very quickly.
>> Then use acetone to remove any sticky glue residue from the tape.
>> Finally just take a cotton ball with some polishing compound and water
>> and lightly rub all over the surface to remove the last traces of silver.
>> Hope this is helpful,
>> Peter Nance, Jr.
>> lenses@adelphia.net
>
> I think the first sentence is incorrect.  Silver will stick to glass
> with no adhesive bonding layer if the glass is sufficiently clean and is
> kept wet with distilled water between the cleaning and silvering
> operations.  I have never heard of anybody using an adhesive in the
> usual sense to stick silver to glass.
>
> As far as getting the silver off, I can't work up much enthusiasm for
> using polishing compound.  You are certain to do some damage to the nice
> smooth surface you worked so hard to get on the glass.  Silver is very
> easy to dissolve off with either weak or strong nitric acid.  As for the
> idea of using circuit board etchant, ferric chloride will convert the
> silver to silver chloride.  Whether the silver choride will wash easily
> from the glass, I simply don't know.  Silver chloride is not very water
> soluble, but it may not adhere well to the glass, so it may rinse off
> despite not being soluble.  Silver nitrate is quite soluble (as nitrates
> often are), so the nitric acid treatment converts silver metal to a
> solution quite reliably.  The nitric won't need to be very concentrated
> for the job of dissolving silver.  Very strong (fully concentrated)
> nitric acid also is a strong oxidizing agent for organic materials.  It
> will burn off traces of oils, pitch residue, etc. that even acetone
> won't fully remove.  I say, give the mirror a warm soapy water wash to
> get the worst of any crud off.  Rinse it well several times, then blot
> it dry and give it a couple of acetone washes to get rid of most of the
> organic residues.  Then pour on a little concentrated nitric and swab
> that around gently with a wad of cotton wool.  Then rinse several times
> with distilled water (don't pour water into acid.  drain off the bulk of
> the acid first), taking care not to touch the front surface with
> anything.  This should result in a surface that is quite clean, except
> for an adsorbed water layer, that you will have anyhow, and is a normal
> part of the glass to silver interface.  Keep the glass under distilled
> water until you pour on the silvering solution.  You don't want it to
> dry out.  Dispose of the nitric soaked cotton wool in a large container
> of water to dilute the nitric acid in it.  You don't want to leave it
> undiluted, because it can catch fire, as well as being hazardous from
> the nitric acid.
>
> The only hang up in this procedure is working with concentrated nitric
> acid.  It is nasty stuff, not to be mucked about with carelessly.  OTOH,
> chemists who know what they are doing and have the appropriate safety
> gear, wouldn't hesitate to do it.  They play with a lot worse stuff than
> nitric acid on a pretty regular basis.
>
> Appropriate safety gear for concentrated nitric acid:
> Long cuffed rubber gloves.
> Safety goggles & a full face shield.
> Rubber apron.
>
> The full face shield plus safety goggles double protection for your
> eyes is partly in compensation for the fact that, in a home workshop,
> you are unlikely to have an eyewash fountain or safety shower.  You
> really, really don't want concentrated nitric acid in your eyes, and if
> by accident it should get there, you really, really want to have a very
> good eyewash fountain immediately available.  The other good safety rule
> is to have a responsible person standing by who knows the correct drill
> for the hazards involved.  The correct drill for nitric acid exposure is
> flooding the skin or eye with lots of fresh water as quickly as possible
> and lasting for at least 15 minutes, followed by a rapid trip to the
> emergency room.  A person with a strong irritant in an eye will not be
> able to open their eyelids voluntarily.  You have to pull them open by
> hand to get the water in.
>
> There may well be other methods for getting the glass clean enough that
> don't involve concentrated nitric acid.  Of course, if they involve
> chromic acid or nitric-sulfuric mixture, then they are even worse than
> nitric from a safety point of view.  Even chemists think twice before
> mixing up nitric-sulfuric.  (Appropriate thoughts: Do I really need to
> do this?  Do I really know how to do it correctly?  What the hell am I
> going to do with the stuff afterward?)
>
> Mark Holm
> mdholm@telerama.com
>
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> 


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