Re-Toning Project

Normally I dont engage in any cleaning or modifying of coins in any way, but I got this Maximinus Sestertius in a lot some time back. Didnt know what to do with it. I dont like to sell coins so horribly cleaned, so I decided to keep it for myself and see what can be done. I should have taken shots of it completely before, but well, things get away from us! I decided to put in on the roof of my house, in full sun. The reverse has been exposed for 30 days. It seems to be re-toning OK, but I see it might be a bit 'spotty' when done. I figure to flip it every 30 days for one year and see what happens (even through the rainy periods). The pic is from my phone, so its not great but shows the progress:

 

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Rain?, what rain in California

 

Interesting exercise! Assuming the reverse looked like how the obverse looks now prior to the exposure, it does seem to be coming along, albeit a bit patchily. Monthly updates, please!

 

Wow Ken, that coin seems to be coming along very nicely (congrats on a very cool idea)



Ummm, I may try this cool experiment as well, but I will merely flip it once per year (on Canada Day, July 1st)



... sadly, this may be a slow process

 

Definitely interesting. I think the best you can hope for by just leaving it to the elements is for the coin to turn dark brown. I'm pretty sure you're not going to end up with any of that nice green patina that a lot of nice bronzes have.

This is a wonderfully detailed coin, and I think you might have been able to get a good price for it even as is. Let's hope it turns out well.

 

It's looking good. You must live in an arid place. If I left a bronze coin on my roof I'd have a pile of green powder in few weeks.

 

I would like to pay a visit to your roof someday, Ken

Q

 

Nice coin and interesting experiment. I probably would use a different piece of metal to do reaction studies, but I am an engineer and hate to test with a limited product that took so long to make.
Some random thoughts:
I do not see any reference to speeding up the process with sun light.
Increasing temperature would increase the process.
Has anyone tried speeding up the chemical reaction by adding CO2?
This could be done in solution with DI water and baking soda (not powder).
You could get a small bottle of CO2 from a beverage store.
A baking dish in a gas oven at low temperature would have both CO2 and water.

Copper (I) oxide, Cu2O, red
Copper (II) oxide, CuO, black
Azurite, Cu3(CO3)2(OH)2, blue
Malachite, Cu(CO3)(OH)2, green
CuCl2, Copper (II) Chloride turns from brown to blue on hydration
CuCl, Copper (I) Chloride is a yellowish brown solid that turns blue green with the addition of water (di-hydrate)

 

Yea, but I am in northern California. We actually get rain! And every 10 years, snow!

Yes, I can only assume it will end up brown. I am not doing anything else to it, so there will be no greens, reds, etc (well, there are patches of original reds on the reverse, so they will remain).

Sunlight only tones silver. But with base metals it can darken what is already there. Since I dont know how it was cleaned or what was used to clean it (I assume chemical) there will be some reaction to whatever remains on the surface of the coin. Sunlight, rain, leaves that fall on it over the winter, all will have an effect. It is just an experiment, and hopefully it will have a good outcome. I thought about buying it in the ground. That would also be a good experiment. I once had a really nice intact Roman brass fibula that someone cleaned harshly (like this coin) and it was really bright. I decided to bury it and see what happens. Unfortunately I forgot where I buried it and it is now lost! Maybe in a thousand years someone will dig it up and decide that Romans colonized Northern California!

 

Well, I will have to visit your roof AND your garden then !

Q

 

interesting experiment, i didn't know what direct sunlight would do to a AE coin. here's a couple AE bayzntine turds i'm going to bury in my yard..



we'll see what they look like later. in case i can't remember where they are, NW corner of shed.

hey Q, stay out of my yard with your metal detector!

 

I'm guessing the sun isn't part of the equation , but being outside, the coin is in contact with plenty of oxygen and moisture.

Source. Gsa

Though it does not rust, exposed bronze is subject to oxidation. Copper is the major component of bronze, and it passes through several stages of oxidation in the presence of oxygen and an electrolyte such as rain water. Eventually, the copper in bronze forms a green patina on its surface that prevents further degradation of the underlying metal. Bronze also decays in contact with sulfurous compounds and chlorine, such as those present in sea spray.
Rub lightly in olive oil and put in an oven on 180 degree c to see what happens. ...

 

This is month two, both photos for comparison. The first photo was with natural light, the new one with fluorescent (which I think shows it more closely as it really appears). This may take longer than I thought. Maybe updates every few months instead of monthly!

 

i had already forgot that i buried a couple of coins in my back yard!

 

Ken,
Another atmospheric influence in the central valley is the smoke that hangs in the air when the rice fields are burned or the smoke we had last summer due to all of the wildfires. I'm bringing this up because you see a lot of green bronze on buildings in Europe and I'm told it came about via local air pollution, namely coal heating and wood cooking fires.

Please keep us updated because I've got Domitian and Vespasian bronzes that are harshly cleaned and have considered leaving them outside. However, I live in the SF Bay Area and would more than likely get different results.

Best of luck in your experiment!

 

For a nice rich Brown 'Florentine patina' use 1 tsp of Ferric Chloride, 1/2 tsp Ferric Nitrate in 1 pint of distilled water.

You can use a hot or cold solution. Ensure the metal is totally free from oil or greasy deposits then brush on the solution. Leave until the colour develops then dry using newspaper (the Carbon on the newspaper is important).
Repeat the process until the desired colour is achieved.

 

Isnt that basically an acid?

 

yup.

but that seems dilute. You get a brown solution which must react with and precipitate on the surface. I am guessing the carbon adds color or perhaps fixes the surface (i.e. stops the reaction)

 

Is your question an oxymoron?
Ferric chloride is a Lewis acid.
From an expert source, Wikipedia: "Anhydrous iron(III) chloride is a fairly strong Lewis acid".
The Engineering Toolbox lists bronze as an unacceptable metal for ferric chloride.
http://www.engineeringtoolbox.com/metal-corrosion-resistance-d_491.html
I am not sure about Ferric Nitrate.

 

Hence the use of the carbon.