Making dark copper and bronze coins lighter in color

Having been collecting coins for several decades I am well aware of the pitfalls of "cleaning" coins but I have a problem that may be amenable to some king of process that would lighten in color and tone very dark copper and bronze as in the Aes and Sesterces of Roman coins that are so dark that is difficult to make out much on them. I display my coins for student use. I have noticed that they will linger over silver, gold or brass coinage as the lighter color allows details to be seen, but simply pass over the darker coins. I am not talking about high grade or rare pieces, simply common type ancients, perhaps in VG or fine condition in terms of wear. So, what I am asking is for any suggestions that would allow very dark brown, almost black Roman Aes to be treated in such a way that they become lighter in color, say a milk chocolate appearance, but not to the point where they assume a hideous pinkish orange tinge in color. Is it possible to do this? Thanks for any assistance.

 

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I know dealers who soak in pepsi, if you dont care about the 'grade' you could try hot sauce. We did that and tooth paste to our wheat collection as a kid wait for others, cause that is crazy, but....

 

A better idea would be to read up on photography. I find natural sunlight works well with darker patinas.

 

The problem with desert patina is that the chances of buying a coin with genuine sand deposits is slim nowadays as rogue sellers know that an applied patina will make a sows ear look like a silk purse, so they spray a mist of glue on the coin and drop it in very fine sand. Later they whiz it with a dremel and nylon attachment to remove sand until the detail is visible. I'll bet you wouldn't spot the difference between genuine and applied.
In answer to the OP question,
Only by removing some of the dark patina will you lighten the coin. Acid or alkaline, or use something mechanical and abrasive.
Dark is a natural patina for many find spots...
In pasture here in the UK, I detect them and they are black in colour ..
Light them differently for photography or display using a cool lamp maybe?

 

Perhaps try a water based paint that can be removed at a later date.

 

If you use anything acidic to lighten the patina, you risk eating away some of the metal along with it. If you want to remove the patina without removing metal, electrolysis is probably the best way to go.

Another option is scrubbing it hard with one of those thin green scouring pads from 3M and some dish-washing liquid and water. It will remove the patina down to bare metal, but remove it from all the highest points first, leaving all the raised areas bright and shiny and the lower areas dark. The result is similar to desert patina but much uglier. It's a good way to reveal detail at the expense of winding up with a very unnatural-looking coin. I recommend using this technique only on beat-up LRB junkers that you don't care about, but it does work.

 

If you can swing it, I'd just find some different coins for the students to use, rather than try to doctor your coins to suit the use.

 

Sell the ones that don't present well to students and use the money to buy more suitable coins. Anything you do will damage the coins you have and most likely not make them look better.

I have cleaned 100s of coins and I don't know a way to achieve what you want.

 

I have this same coin in you’re picture any idea about it mine has different letters on the back tho

 

I'm in the midst of a bronze disease treatment of a coin I DO value very much. The patina may or may not be sacrificed in the process..
I've read that sodium sesquicarbonate will strip patina safely but so far (after 1 week) the patina seems perfectly intact with a 5% solution.

Fwiw I quite like the look of bronze coins whose high points are lighter in color than the fields, however that is achieved through cleaning or handling.
I find they grab attention much better in a tray of coins when there is some contrast present.

 

I’d avoid the sodium sesquicarbonate approach. I’ve treated multiple coins for bronze disease and the effect on patina and “patina” is uneven and unpredictable.

You could strip the patina with the baking soda and aluminum foil approach, and then tone them back with one of the liver of sulphur methods. But that’s extreme and artificial.

If the coins aren’t too dear, you might consider permitting the students to handle the dark bronzes under supervision. Their skin oils and sweat will naturally alter the patina. It won’t happen quickly, but it will happen. Stop when the coins get to the desired shade of brown. Plus the students get to hold a piece of history.

 

First, I'll bet you could put a large AU bronze coin (many with not attractive overall designs) in a case with an artistic Greek silver piece in VF and any gold ancient and the students will still linger over the precious metal coins.

I see a solution. ALTER THE SURFACE of the inexpensive, large bronze piece to bring out its design either with paint, dip, wire brush, or the method above to add sand. Then I'll guarantee they will stop to examine the large bronze coin too!

 

I find gold Rustoleum particularly attractive.