Why is tamarind juice used to clean verdigris from copper and brass?
Undoubtedly because it works and is cheap, but as chemists we can suggest the chemical equation.
Copper and (sometimes) brass (a copper alloy) acquires a green patina of verdigris, which is
Tamarind juice is quite strongly acidic, and is hence used as an acid flavouring. The acid is likely tartaric acid,
which reacts with the copper carbonate in an acid base reaction.
The copper acetate is a soluble species, and can be washed away to leave a gleaming copper surface.