From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.chem
Subject: Re: need to clean tarnish off of silver
Summary: Reduction of sulfide by aluminum
Keywords: silver tarnish, sulfides, reduction, aluminum
Date: 9 Apr 91 04:13:11 GMT

In article <1991Apr4.011625.1904@minyos.xx.rmit.oz.au>
tylmnb@minyos.xx.rmit.oz.au (Michael Barnett) writes:

>>The process involved placing the tarnished silverware in a boiling solution
>>of sodium carbonate, along with a wadded-up piece of Al foil.  Naturally,
>>if you use an Al pot do do the boiling, you can omit the foil, but this
>>will do unkind things to the Al pot!
>
>While we are on this topic, does anyone know what reactions are occuring?
>I asked several chemistry lecturers, but they weren't too sure themselves.
>One suggested that hydrogen gas was being formed (in a form that he called
>'nascent' - or something like that), which was reacting with the AgS.

	I don't see any mystery here.  The hydrogen gas is merely evolved
through the reaction of aluminum and sodium carbonate.  The hydrogen plays
no part in the removal of the silver tarnish (i.e., silver(I) sulfide).

	Aluminum is a reducing agent, and thereby reduces silver sulfide
to elemental silver while forming aluminum sulfide (which is yellow in
color).  The aluminum sulfide thus formed readily hydrolyzes to form
aluminum hydroxide and hydrogen sulfide.

	So, we really have *two* gasses evolved: hydrogen and hydrogen
sulfide, with the quantity of the former being greater than the latter.

	Aluminum in pure water would not work since the surface film of
aluminum hydroxide readily stops further activity.  Since hydroxyl ions
from the sodium carbonate readily dissolve any aluminum hydroxide thus
formed, a ready supply of aluminum ions for reduction purposes is assured.

>I have used this process, and found that cold water will work, but
>lukewarm is better. Boiling water will probably work even better, but the
>aluminium is used up too fast, and the smell is quite bad (probably from
>H2S or similar).

	See, I told you there was hydrogen sulfide! :-)  Now you know why.

>I also find that a scum forms on the surface of the water after some
>time. Can anyone suggest what this might be and how it is formed?

	Probably aluminum hydroxide, perhaps with some loose silver sulfide
formed due to the reverse reaction with evolved hydrogen sulfide.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231       {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635   [note: ub=acsu.buffalo.edu] uunet!/      \aerion!larry

From: "Barry Ornitz" <ornitz@dpnet.net>
Newsgroups: rec.radio.amateur.homebrew
Subject: Cleaning tarnish from silver
Keywords: silver, tarnish, cleaning, oxidation-reduction, electroplating
Date: Tue, 2 Jan 2001 00:48:24 -0500

Boy, some of the answers on cleaning tarnish from silver have been quite
humorous but completely wrong.  To correct a few things: all "thio"
compounds are sulfur containing (not mercury), silver oxide is not more
conductive than metallic silver but it is more conductive than silver
sulfide (black tarnish),

The simplest process for cleaning silver of tarnish is described below.  It
is likely what Henry Kolesnik was trying to remember.  It does not remove
any silver, unlike polishes.  It reduces the silver sulfide to metallic
silver.  The only problem is that the silver or silver plated part must be
immersed in an aqueous solution.

To begin, find an aluminum pan that is deep enough to hold the part.  It is
best not to steal the wife's Mirro cookware.  A disposable pan like grocery
stores sell for holiday baking is excellent.  Fill the pan with water and
add a little table salt (sodium chloride) or baking soda (sodium
bicarbonate).  No more than a half teaspoon is necessary.  Now dunk the
silver part in the pan and allow it to rest on the aluminum.  It must make
electrical contact to the aluminum.  To speed things up, heat the water.
You will quickly find the black discoloration on the silver dissolves and
the aluminum pan will darken.
Rinse the part well after removing it from the solution.  The finished
silver part may not come out very shiny (but it will be bright).  This is
because the process only removes the sulfur.  If you mechanically polish
the silver, you WILL remove some silver.

What is happening is an electrolytic oxidation-reduction chemical reaction.
The silver sulfide is reduced to metallic silver with the sulfur ions going
into solution.  They react with the aluminum making it black.  The
electrical contact is needed for the electrolytic reaction to take place.
The aluminum reaction is the oxidation.

If you did accidentally use the wife's fancy pot and you are too lazy to
scrub it with a scouring pad, you can cook some tomatoes in the pot.  Their
acidity will dissolve the aluminum sulfides.  The added sulfur in the
tomatoes is generally not harmful but it can give some awful flatulence!

The fancy product sold on television that has a liquid and a "magic" metal
plate to put in the bottom of a bowl is essentially just this - at a far
higher price!  That metal is just aluminum.

Cool-Amp powder is sold in large electrical supply stores.  You add a
little water to it to make a thick paste and rub it on copper to produce a
thin silver coating.  It works quite well.  The main use is to coat
electrical buss bars and breaker contacts with a thin silver plating to
reduce contact resistance.  The stuff is quite expensive and they only sell
it in one pound jars.  A jar will last you and your friends a lifetime
however.

By the way, while I am the "resident chemist" on the Old Tube Radios
mailing list (Boatanchors), I am really a chemical/electrical engineer.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@usa.net