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I had taken as a given that silver tarnish is a layer of silver sulfide on the surface of a utensil, and that cleaning it using a thiourea based agent like Tarn-X dissolves the sulfide layer.

However, while recently cleaning a variety of tarnished sterling and silver plated utensils, I noticed (1) the sulfureous odors emitted during the cleaning process varied considerably from one piece to the next; and (2) the tarnish on a few pieces could not be removed completely via the non-abrasive chemical process.

Knowing that many foods such as members if the cabbage and onion families have various sulfur compounds in them as flavor and aroma components, and that the sulfur containing amino acids cysteine and methionine are almost universally present in living organisms (including those we eat), I wondered: is silver tarnish a complex mixture of silver complexes that might reveal a history of the foods a person has eaten. Sherlock Holmes would know for sure.

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  • $\begingroup$ The affinity of silver for sulfur is so high that silver may finally react with the S atom from thiourea. That is why I would not use thiourea for cleaning silver surfaces. Sodium bicarbonate works much better. . $\endgroup$
    – Maurice
    May 23, 2020 at 13:00
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    $\begingroup$ Note that thiourea is a carcinogen. This is not the best cleaning approach, especially if you are smelling sulfurous compounds. You should search X-ray photoelectron spectroscopy of silver tarnish, such as researchgate.net/figure/… or chemistryworld.com/news/… $\endgroup$
    – AChem
    May 23, 2020 at 17:01
  • $\begingroup$ Upvote, Silver chemistry is always interesting! $\endgroup$
    – AJKOER
    May 24, 2020 at 1:18
  • $\begingroup$ Please see What happens during cleaning silverware?. $\endgroup$
    – Ed V
    May 24, 2020 at 12:32

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The composition of Silver tarnish is Silver sulfide, to quote from Wikipedia:

Silver sulfide is an inorganic compound with the formula $\ce{Ag2S}$. A dense black solid, it is the only sulfide of silver. It is useful as a photosensitizer in photography. It constitutes the tarnish that forms over time on silverware and other silver objects. Silver sulfide is insoluble in most solvents, but is degraded by strong acids.

The last sentence is important as Silver cleaning is not accomplished with harsh chemicals, but via electrochemistry which leads to the hydrogen atom radical formation, which then reduces the $\ce{Ag2S}$ liberating $\ce{Ag}$ and $\ce{H2S}$. Here is a short summary of likely mechanics:

$\ce{Al -> Al3+ + 3 e-}$

$\ce{H2O ⇌ H+ + OH-}$

$\ce{HCO3- ⇌ H+ + CO3(2-)}$

$\ce{H+ + e- ⇌ •H }$

Effectively, $\ce{•H}$ can operate as a $\ce{(e-, H+)}$ pair:

$\ce{Ag2S + 2 •H -> 2 Ag + H2S (g)}$

Here is a confirming source, which states to quote:

The nascent hydrogen may also act as a reducing agent as it evolves from the metal surface.

And also, per the same source, the formation reaction of Ag2S can be reversed:

In that case silver suffers attack by dissolved oxygen and present sulphide species according to Reaction (5):

$\ce{4Ag (s) + 2HS- (aq) + O2(g) ⇌ 2Ag2S (s) + 2 OH- }$ (5)

So working in air, it is advised to employ a base to drive the above equilibrium to the left. Also, in alkaline conditions expect the reaction:

$\ce{H2S + 2 OH- -> 2 H2O + S(s)}$

Which is also noted in the referenced source, to quote:

However, after the cleaning of heavily tarnished samples (immersed for 30 and 60 min) some small quantities of sulfur (~0.3wt.%) are detected on the surface by EDS analysis, and the colour is changed to light yellowish.

[EDIT] With particular reference to the thiourea system, it is described in this hydrometallurgy source. A cited reaction equation is:

$\ce{ Ag_2S + (SC(NH)(NH)2)2 + 2 H^+ + 4 SC(NH2)_2 -> 2 Ag(SC(NH2){_2}){_3}^{+} + S_o }$

Silver metal can then be recovered with reductive electrolysis.

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    $\begingroup$ I don't dispute the chemistry in your answer, but it addresses how tarnish--specifically, silver sulfide-- may be removed electrochemically or via sodium bicarbonate in the presence of Al metal, from a silver surface. My question referred to the use of thiourea - admittedly a controversial method since it strips off the tarnish rather than restores it to silver metal, and because of its adverse health effects. I am certain that other sulfur species besides hydrogen sulfide were released during the process, based on their unusual odors. Hence there was more than silver sulfide in the tarnish. $\endgroup$
    – iad22agp
    May 24, 2020 at 0:19
  • $\begingroup$ I have added a hydrometallurgy reference on a thiourea system. $\endgroup$
    – AJKOER
    May 24, 2020 at 1:59

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