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I have a small metal, most likely silver, cross. It is an old object and has turned dark. In order to clean it, my mother told me to boil it for a few minutes in water containing sodium bicarbonate.

Could someone explain what kind of reaction(s) occur(s)?

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If originally made of silver, the dark layer on your jewlery is silver sulfide, $\ce{Ag2S}$. The removal suggested by your mother is an electrochemical one, that will reduce the silver back into metallic, shiny $\ce{Ag}$, if in addition of hot water and bicarbonate, salt and an other metal acting as reducing component is in electrical contact with the silver piece are present. Here, some foil of aluminum will serve well as sacrifical anode.

To quote UMass Amherst, presenting it as a lecture demonstration, the chemistry may be summed up to this equation

$$\ce{3 Ag2S(s) + 2 Al(s) + 3 H2O (l) –> 6 Ag(s) + 3 H2S (aq) + Al2O3(s) }$$

The addition of salt (which is absent in the equation) serves to increase the electric conductivity of the solution, hence increasing the rate of this process. Immerse the jewlery in the hot solution (it needn't be boiling), for example wrapped in the creased foil for some minutes; then rinse and wipe it with a cloth.

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  • $\begingroup$ Thank you. Any idea how the silver sulfide was produced in the first place? $\endgroup$ – Αντώνιος Κελεσίδης Apr 22 '17 at 1:45
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    $\begingroup$ @ΚελεσίδηςΑντώνιος Silver sulfide is the product of the reaction of silver and hydrogen sulfide. Hydrogen sulfide is a colourless gas, with the smell of "rotten eggs". It is, for example, result of the digestion of organic matter (especially in absence of oxygen, as in landfills, wastewaters) already containing sulfur -- cysteine and methionine, for example, are two amino acids containing it. In addition, small amounts of hydrogen sulfide are produced by humans as signaling molecule, too. It is found in small quantities in the air; sufficient -- over some time -- to cause the reaction obser $\endgroup$ – Buttonwood Apr 22 '17 at 8:00

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