Interesting, with this controlled process, how differently the coins tone. Which is what I expected, though I'm still not clear on why these 90% silver coins of different eras react so differently. I can always seem to pick out a Trade dollar in a pile of Morgans at a distance.
OK... chemistry is a bit more complicated than that, as far as I remember.
Many metals do react with chemicals found in the air. Copper reacts with oxygen, iron reacts with water vapor (and your hands - that musty "iron" smell is actually a certain oil present on your fingers reacting with the metal to produce a volatile compound), silver reacts with sulfur, and gold doesn't react with much unless you try really hard.
However, the characteristics of the compound produced on the coin's surface are all completely different!!
Quick, dig into your world coin grab-bag and pull out an aluminum coin. It's toned! It is coated with a layer of aluminum oxide. But unlike iron oxide, aluminum oxide is strong, transparent, and does not flake off. It still reacts with oxygen enough so that native aluminum is a geological ultra-rarity (flakes of the stuff have been found in extremely oxygen-hungry soil), but this will not evolve into pitting.
Meanwhile, iron and zinc are only used for very low-value coins or in military emergencies for good reason - they react with almost anything, and turn black and pitted.
As for silver... I am pretty sure that silver sulfide will not become corrosion. Most dug silver coins come out of the ground fairly nice... more so for gold. So... don't clean silver coins out of a misguided desire to preserve them indefinitely - even sealing them off in a small container halts the process of toning (sulfur isn't too common in our atmosphere).
Copper will corrode when water and oxygen team up, but without water, I am also fairly sure that copper oxide alone is visible but benign.
So, on an atomic level... a layer of compound is building up on the pure-metal surface. At some point, if the compound is stable, the process will stop, as the reactive metal is completely covered by protective oxide. If the compound is unstable, it will flake off, revealing more metal, causing more reaction. It all comes down to how stable the compound is - rust is not stable, aluminum oxide is very stable. Why do these tiny, thin layers change the colour so much? It's like oil on water - the light is bent by the oil, then reflected off the water, then bent differently by the oil again so it comes back to your eyes differently. Like an oil slick, toning can come in all colours, but in quantity it just turns dark.
How silver coins look that come out of the ground depends entirely upon the ph of the soil. I live in coal country. The soil here tends to have high amounts of sulphur, and more elemental iron in the clay layer ,which some areas don't even have, and I've recovered some silver coins that were so heavily sulphated that the entire surface is pitted. The soil in this neck of the woods tends to be quite acidic.
Here are the last 4 silver coins I've excavated. The 1857-O quarter was horribly corroded. It was only 3 inches deep, but was near the road, soI expect it's seen it's share of exposure to road salt over the past 75 winters.
I recall reading that the toning of silver is indeed a progressive process, albeit slow. The estimate for a silver coin to be reduced to a pile of black dust was something over one hundred thousand years.
This thread reminded me of a video I was watching the other day that was showing how to tone a silver object a little quicker using electrochemistry. I'm sure there are better ones out there but this is the one I know of:
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Makes you wonder if some of the toners I see out there were created using this process.
A few of the results shown above are fairly attractive, much moreso than the pre-processed specimens. This thread may indeed become a tutorial for the AT monkeys living in the trees above your coin house . . . "Don't try this at home" can become an irresistible challenge to some folks. This + an ebay account = more trouble for the inexperienced coin buyer.
mysilveryears: ".... This + an ebay account = more trouble for the inexperienced coin buyer. "
Don't fret too much about that. There are many buyers like myself who think buying a toned coin is like buying a new car with a dented fender. Why settle for impaired when you can get new?
My opinion ... for what that's worth ... is that "original surfaces" only becomes an issue on high grade coins. Coins in the grades shown here ... it just doesn't matter to me. The coins are more market acceptable. The natural process has been kick started, and my guess is that in a couple years, especially if stored in an old Waite Raymond album ... they'll be indistinguishable from naturally toned coins. The toning forms exactly where it was originally, because the surfaces are compromised from the previous toning. That is to say ... the corrosion had already begun....but was "OK" because it was still at an attractive stage. The resellers that dipped the coins out to begin with have done as much a disservice to the novice collector as anything I've done here. All I've done is to propose a method of restoring bleached out silver coins to make them more palatable to me.
Funny thing about the cleaning and artificial toning "boogie men" is that when they sell coins that have been monkeyed with ... they may disclose the fact that they've been cleaned etc ... but it doesn't seem to impact pricing. They still price them as any "natural" coin would be priced.
The knowledge imparted here can be used either way ... by those who wish to deceive, or by those who wish to learn to spot AT. Knowledge is power. How the power is wielded is up to the individual possessing it.
I agree with dsfreeworld about the 1825 CBH. That coin was horrible and this process has it looking pretty darn good. I am not an expert by any means, but I think it would be difficult for most to pick this out as AT.
The difference that I can see ... and I'm not going to say that it's "the" definitive indicator or "smoking gun"... is that most of the coins I have that are absolutely naturally toned, contain hues of brown as well as gray. This is primarily gun metal gray, though a bit of blue will appear from time to time.
I'm way happier with them now than I was. It will be interesting to see how they age from this point.
The 1919 still shows a wee bit of polishing ... mainly evident in the skirt lines. I wish I had a before picture of that one, but I didn't even take a picture of it back when I bought it.
Here's a naturally toned 1834 bustie ... shown with the artificially toned 1824. Its easy to see the difference in color, the original having more browns .... but overall, the 1834 has a wider range of color and an overall softer transition from dark to light.
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Posted 11/10/2013 12:48 am
