Question

Why is tamarind juice used to clean verdigris from copper and brass?

Answer 1

Undoubtedly because it works and is cheap, but as chemists we can suggest the chemical equation.

Explanation:

Copper and (sometimes) brass (a copper alloy) acquires a green patina of verdigris, which is `CuCO_3` or maybe copper acetate. The best example of this is the Statue of Liberty in New York Harbour, the which over the years has acquired an impressive chalky green coating.

Tamarind juice is quite strongly acidic, and is hence used as an acid flavouring. The acid is likely tartaric acid,

`"2,3-dihydroxybutanedioic acid"`,

which reacts with the copper carbonate in an acid base reaction.

`CuCO_3(s) + HO_2C-CH(OH)CH(OH)CO_2H(aq) rarr Cu{O(O=C)CH(OH)}_2(aq)+H_2O(l) + CO_2(g)uarr`

The copper acetate is a soluble species, and can be washed away to leave a gleaming copper surface.