Ridge Runner said:
If they are green when you find them wrap them up until you get home and then put them in a tub of olive oil for a day or two and then clean them with a soft tooth brush, "NEVER" put them in water, I dropped 2 extremely rare coils in the bath once and it destroyed them in seconds, They flew out of my pocket as I was getting undressed and landed right in the Tub, Both were worth in the Mid 5 figures and looked perfect but dark green in colour, By the time I got them out of the very hot water they were stuffed, So always soak them in olive oil and once you have the loose dirt off wipe them carefully with a soft cloth.
The only purple coppers I have seen is when my car was faster than theirs, :Y: :lol: :lol: :lol: :lol:
I suspect that they were no longer copper when they dissolved in the bath, but that copper carbonate had completely replaced them (ie it can preserve the appearance such as date etc despite the copper being gone). Hot water will not dissolve copper metal. Or alternatively, your bath water is a bit acid (enough to rot your goolies off).
Every time you clean a coin you remove some of its surface and degrade the sharpness of the motifs on it. There are ways to clean them, e.g. for verdigris: add a few, tiny grains of baking soda to about 2 tablespoons of water and dissolve it. Soak the coin in the solution for about an hour. Remove the coin, rinse with fresh water and pat dry (do not rub). You can probably get the green off this way that you put there yourself, if you think it is only a dusting of green.
I don't clean coins myself when it is only a mild tarnish.
Basically, acid dissolves copper. It is like polishing your silverware when it goes blackish - you are removing silver so should not do it regularly. In that case the silver reacts with hydrogen sulphide in air to form black silver sulphide (the mineral acanthite Ag2S - the other silver sulphide argentite does not form at low temperatures). The coating then protects it a bit, so only clean it when it has to be displayed (the same reason most people don't clean copper and silver coins unnecessarily). I suspect that storing coins in an airtight container with a dessicant (packets of silica gel warmed in the oven first to drive off their moisture) might reduce the tarnish they get with time.
For those with some chemistry and mineralogy:
2Cu(copper metal) + H+ (in the acid) is giving you Cu2+ ion on the coin surface, then Cu2+ + CO3 2- ion (atmospheric carbon dioxide dissolved in the water as H2O + CO2 = 2H+ + CO3 2- ion) + 2H20 (the water itself) are reacting together to give a coating of CuCO3.2H20 (the mineral malachite - you don't need those other fancy names like verdegris). So acid is dissolving your coin slightly and forming a coating of malachite.
This is probably also what happens when copper coins sit around exposed to air. H2O + CO2 from the air react together on the coin surface to give 2H+ + CO3 2- ion, forming a very thin coating of CuCO3.2H20 and/or possibly a dark copper oxide coating (Cu + 2O2 = Cu2O, the mineral cuprite) .
This is how minerals like azurite (CuCO3.H2O), malachite and cuprite (and acanthite) form near surface in the upper parts of copper-silver ore bodies, so that we collectors can display the pretties in glass cabinets. Chalcanthite crystals (hydrous copper sulphate - take care it is poisonous and dissolves in water - if left in the sun it dehydrates and destroys the mineral) form in much the same way. However rather than getting the sulphur from hydrogen sulphide in air, it is usually derived from sulphate ions in mine water, formed by oxidation of sulphide minerals such as chalcopyrite and pyrite.
Thus endeth Copper Oxidation 101.
