ArthurEvans
Senior Member
I got interested in a thread by Fool on cleaning sulfur residue off copper. I know there are a lot of cases where one wouldn't want to touch the patina on a metal, and I'm not suggesting doing anything in those cases.
I mentioned in that thread a well known method I had used for silver, where you put the tarnished silver on a piece of aluminum foil, submerged in a dish of dissolved baking soda. It works, transferring the sulfur to the aluminum, and not dissolving or scrubbing off any silver.
I thought it might work on copper. I don't know whether copper patina is really sulfide, or just an oxide - maybe it can be either. But anyway, I thought it was worth a try.
But it doesn't work. Left it overnight, nothing happened.
But I thought of some other ways to take the oxide coating off without dissolving any copper. I know passing hydrogen gas over hot copper will take the oxide off in a jiffy - like instantly, like magic. But that's a little risky for home use.
So I thought of another reducing agent (technical term, just means something that reverses oxidation/corrosion) - thiosulfate ion, a.k.a. "hypo" or "fixer" in black and white photography. It's low toxicity (it's an important medicine) and not corrosive. It's easy to get, and safe to dispose of. Once made the solution should keep -- just label it and keep it away from kids, of course.
So today, I got three pennies with solid oxide/patina...
I put them in about a half cup of water with a tablespoon of sodium thiosulfate (pentahydrate form, but it doesn't matter once it's dissolved), and stirred.
In just minutes they were looking cleaner. In about 20 minutes I took them out, dried them, and polished them a bit on a soft cloth.
There was no blue tint to the solution, so no copper got dissolved. The black patina was gone. Some verdigris (green corrosion) was not removed from one penny, so I put that in weak HCl (about a 1:4 dilution of plumber's muriatic acid, which doesn't react with metallic copper) for about 5 minutes (getting to be dinner time, or I'd have left it a little longer,) and that mostly came off. I used a soft pencil eraser on it, too.
The backs:
As I say, I know this isn't appropriate for coins of historic value, would harm the value of collectibles, and a lot of us (me included) like the patina. But I thought someone might find it useful for "non-historic" applications.
I mentioned in that thread a well known method I had used for silver, where you put the tarnished silver on a piece of aluminum foil, submerged in a dish of dissolved baking soda. It works, transferring the sulfur to the aluminum, and not dissolving or scrubbing off any silver.
I thought it might work on copper. I don't know whether copper patina is really sulfide, or just an oxide - maybe it can be either. But anyway, I thought it was worth a try.
But it doesn't work. Left it overnight, nothing happened.
But I thought of some other ways to take the oxide coating off without dissolving any copper. I know passing hydrogen gas over hot copper will take the oxide off in a jiffy - like instantly, like magic. But that's a little risky for home use.
So I thought of another reducing agent (technical term, just means something that reverses oxidation/corrosion) - thiosulfate ion, a.k.a. "hypo" or "fixer" in black and white photography. It's low toxicity (it's an important medicine) and not corrosive. It's easy to get, and safe to dispose of. Once made the solution should keep -- just label it and keep it away from kids, of course.
So today, I got three pennies with solid oxide/patina...
I put them in about a half cup of water with a tablespoon of sodium thiosulfate (pentahydrate form, but it doesn't matter once it's dissolved), and stirred.
In just minutes they were looking cleaner. In about 20 minutes I took them out, dried them, and polished them a bit on a soft cloth.
There was no blue tint to the solution, so no copper got dissolved. The black patina was gone. Some verdigris (green corrosion) was not removed from one penny, so I put that in weak HCl (about a 1:4 dilution of plumber's muriatic acid, which doesn't react with metallic copper) for about 5 minutes (getting to be dinner time, or I'd have left it a little longer,) and that mostly came off. I used a soft pencil eraser on it, too.
The backs:
As I say, I know this isn't appropriate for coins of historic value, would harm the value of collectibles, and a lot of us (me included) like the patina. But I thought someone might find it useful for "non-historic" applications.