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FORVM`s Classical Numismatics Discussion Board  |  Numismatic and History Discussions  |  Coin of the Day (Moderator: LordBest)  |  Topic: Denarius on fire 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Denarius on fire  (Read 2002 times)
Rich Beale
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« on: April 12, 2012, 07:35:02 am »

It's been a while since I saw a denarius with such a vivid tone so thought I would share.

Both Roma and the Dioscuri here appear to be aflame - a fitting aura for the divine.

Click the image to enlarge. Hope you all enjoy!
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« Reply #1 on: April 12, 2012, 08:32:16 am »

Wow, that's really nice.  What causes that on silver coins?
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Rich Beale
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« Reply #2 on: April 12, 2012, 09:06:53 am »

In brief, it is a reaction between the surface layer of silver and hydrogen sulphide in the atmosphere and/or other sulphur compounds in close proximity to the coin.

A thin layer of silver sulphide is formed as a result; while silver sulphide itself is black, the colours are produced as a result of thin film interference - the same principle that causes the iridescent sheens on bubbles or oil on water, whereby varying thicknesses of sulphide cause light to be refracted and reflected to greater or lesser degrees (and speeds).

A beam of light striking the coin will be partially reflected off the layer of silver sulphide and will be observed by the viewer; the remainder of that light however will pass through the film and strike the surface of the coin, from where it is reflected and again, observed by the viewer. However, because light travels slower through a denser medium, this part of the light is now out of phase with the light that did not pass through the silver sulphide, and this phase shift causes us to perceive a colour.
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Andrew McCabe
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« Reply #3 on: April 12, 2012, 09:32:13 am »

I wonder might it be a specific metal feature of the coins of this era (150s BC). Below are four denarii from my collection of this date, not as much on fire, but at least smoldering despite their more worn condition. I really like the blue colours.



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benito
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« Reply #4 on: April 12, 2012, 09:42:46 am »

I do own  some RR with that type of toning . Pic 1
But you can also find it in much later coins. Pic 2.
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Andrew McCabe
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« Reply #5 on: April 12, 2012, 10:35:24 am »

I do own  some RR with that type of toning . Pic 1
But you can also find it in much later coins. Pic 2.

The common thread is that it seems to be a feature of lovely coins! Do the good surfaces and sharp edges of lovely coins invite nice toning? Or does the nice toning on otherwise-average coins make them lovely? Chicken or egg?
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David Atherton
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« Reply #6 on: April 12, 2012, 04:20:42 pm »

Rich, that is a beautiful denarius, with or without the toning!

The toning is icing on the cake.
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« Reply #7 on: April 12, 2012, 05:47:15 pm »

very neat!
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mix_val
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« Reply #8 on: April 12, 2012, 06:32:10 pm »

It's been a while since I saw a denarius with such a vivid tone so thought I would share.

Both Roma and the Dioscuri here appear to be aflame - a fitting aura for the divine.

Click the image to enlarge. Hope you all enjoy!

double wow!
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Bob Crutchley
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Rich Beale
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« Reply #9 on: April 13, 2012, 03:17:30 am »

Thanks to all for your comments, I'm pleased you like the coin. It is not easy to capture iridescence with a camera, mostly because a single-lens apparatus only 'sees' the coin from the one perspective, whereas of course we ourselves have the benefit of stereoscopic vision which enhances the appearance of iridescent toning.

I wonder might it be a specific metal feature of the coins of this era (150s BC).

The appearance of colourful toning requires only that a coin be mostly silver. True iridescence, i.e. colours that shift depending upon the angle at which a coin is viewed, occurs only on coins that 1) have a high silver purity, and 2) are well struck and preserved, such that the surface retains the lustrous (and therefore reflective) qualities it possessed when originally minted. 

Hence, corroded or heavily crystallized coins rarely display this quality. Roman Republican denarii are ideal candidates for iridescent toning because they were almost always minted from high purity silver, and most come to us from large hoard deposits that have well preserved the majority of the coins contained therein from their surrounding environment.
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« Reply #10 on: April 13, 2012, 04:16:58 am »

While I agree that toners are found in RR and late siliquae of similar purities, it would seem we should have a similar showing from Greek coins of good silver.  Are the best Greeks just not fine enough to make it happen? 

Currently the fad among collectors of US coins are toned Morgan silver dollars which show rainbow hues at only  90% silver so I wonder if there might be a range of alloys prone to this characteristic with it also being possible to be too pure to tone well.  Does such toning require a polished die surface quality missing earlier and lost when a coin weathers so toning might be more common on minty fresh coins? 

There also is the question if these toned coins might have been cleaned a century ago and this is what happens on their journey to solid black.  I do not recall seeing the toned Morgans 50 yeras ago when they were younger and ask if 'natural toning' might just take a while.  The other option is that we previously considered it a flaw and cleaned such coins.  Coins are subject to fads and toning is currently 'in'.
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Rich Beale
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« Reply #11 on: April 13, 2012, 08:09:04 am »

While I agree that toners are found in RR and late siliquae of similar purities, it would seem we should have a similar showing from Greek coins of good silver.  Are the best Greeks just not fine enough to make it happen?  

Hi Doug,

On the contrary. Greek coins, having been mostly struck in high purity silver, also come to tone beautifully when kept in the right environment. For example, I had the pleasure to (albeit briefly) own this coin, which had the most spectacular tone I have seen on any piece, ever.

...toned Morgan silver dollars...only  90% silver so I wonder if there might be a range of alloys prone to this characteristic with it also being possible to be too pure to tone well.  Does such toning require a polished die surface quality missing earlier and lost when a coin weathers so toning might be more common on minty fresh coins?  

Referring back to my last post, it is sufficient that a coin be well preserved such that the original surface, lustrous from the striking, is preserved. A polished die or flan is not necessary. 90% silver is more than sufficient.

Natural toning can be a slow or fast process - it depends primarily on the availability of sulphurous compounds, but the process is accelerated by heat. The colourful tones are indeed only a stage between a 'white' coin and a black one.
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« Reply #12 on: April 13, 2012, 09:35:15 am »

Perfect illustration of a recent discussion on choice between rarity and quality. There was one post really right in arguing that some kind of quality makes real rarity irrespectively of the number of coins. That's the case of this splendid denarius.
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SPQR Matt
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« Reply #13 on: April 14, 2012, 12:13:16 am »

A truly beautiful coin!
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gallienus1
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« Reply #14 on: April 14, 2012, 07:29:41 am »


A truly fabulous coin Rich. Andrew’s, benito’s and Doug’s examples are also highly beautiful.

Now the consensus seems to be that iridescent toning is desirable, but after contact with gases in the environment we know it is a transitory phase on the way to grey (cabinet toning) then black (typical silver hoard patina). Would it then a good idea to artificially lengthen a coins time in an iridescent state by preventing further oxygen and sulfur in the atmosphere from reaching the coins surface?  Oiling or waxing the surface would probably do the job but leave an unpleasant “shiny” appearance.  What about using silicone spray? Gun owners often use it to protect metal surfaces. Silicone is not as reflective as oil or wax and can easily be removed with soap and water. Does anyone think it is a good idea?

Best Regards,
Steve
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4to2CentBCphilia
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« Reply #15 on: April 14, 2012, 10:07:07 am »

Keep in mind what has been mentioned here at least once, that heat will accelerate this rainbowing process. This means that there is a possibility to create this effect "artifically" making rainbowed coins more commonplace....although pretty none the less.

In doing some reading on-line (and in past converstions with collectors) there are ideas on the best methods for producing rainbowing on silver coins and suggestons on how to accelerate this process.

One method is rather inocuous in that is changes the atmosphere that the coin resides in, rather than actually touching the coin in any way and requires no heating. Placing a coin in a manilla coin sleeve (higher sulfur content)and leaving it on a windowsill to warm is said to work, but it could require months of longer. A method to accelerate this is to place the coin in tuperware along with a crumbled boiled egg yolk and let the closed container sit in the sun. This creates a higher concentration of sulfur in the air and should accelerate toning. Supposedly rainbowing will occur sometime in this process and the coin must be removed and rinsed to stop the process.

I am going to test this on a coin over the next few weeks and see what sort of results are achieved in 21 days. I don't know what sort of acceleration is possible and I don't know if it will go past rainbowing and into graying too quickly. Steve mentions this in his post. His question about preserving the patina is valid. Keeping a rainbowed coin away from heat and sulfurous paper sleeves should minimize the coin over toning to a gray. But simple chemistry implies rainbowing won't last forever.

However if this method works...............  then I will begin to wonder how many of the coins I now see with rainbowing, have been artifically induced. People seem to pay a premium for a nicely rainbowed coin, so there is a financial incentive here for folks to do so before selling.


Mark


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« Reply #16 on: April 14, 2012, 12:01:19 pm »

A method to accelerate this is to place the coin in tuperware along with a crumbled boiled egg yolk and let the closed container sit in the sun. This creates a higher concentration of sulfur in the air and should accelerate toning. Supposedly rainbowing will occur sometime in this process and the coin must be removed and rinsed to stop the process.

I am going to test this on a coin over the next few weeks and see what sort of results are achieved in 21 days. I don't know what sort of acceleration is possible and I don't know if it will go past rainbowing and into graying too quickly.

Mark, rest assured you will not need 21 days! I experimented with this on a denarius of mine and noticed a change of colour almost instantly, certainly after 30 seconds. I had to remove the coin from the tuperware after 5 mins as the process was going past rainbowing into blackness. The results were too dazzling if anything and the spread of the toning was not realistic - nevertheless I could imagine the process being refined somewhat so as to produce a more realistic outcome. A fun science experiment if nothing else - the results can be removed with a little LJ if not to your liking.
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« Reply #17 on: April 14, 2012, 01:33:29 pm »

Opinion:  Toning should be a very few molecules thick and would seem to me to be changed significantly by adding any surface coating.  I have not tried this but would expect it to be a way of reducing a $500 coin into a $50 coin with one squirt.  Anything done to a coin that starts with the word 'artificial' is likely to have the same effect. 

I once met a coin show dealer who showed me a tetradrachm of Uranius Antoninus that he was carrying in his pocket with his change.   At the time the coin was only worth about $1000 but needed a bit of natural tone and smoothing after a bit too harsh cleaning.   I always wondered how it came out or if he spent it by accident. Roll Eyes 
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Rich Beale
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« Reply #18 on: April 15, 2012, 07:34:16 am »

I experimented with this on a denarius of mine and noticed a change of colour almost instantly, certainly after 30 seconds. I had to remove the coin from the tuperware after 5 mins as the process was going past rainbowing into blackness. I could imagine the process being refined somewhat so as to produce a more realistic outcome.

The thing is, whatever you do to a coin to produce an artificial tone, it is and will remain just that - an artificial tone. Yes, there are ways and means by which one can tone a silver coin, but I do not advocate any of them. Most of these methods produce a horrible effect that I call 'clown toning', and frankly they decrease the appeal of a coin.

There is no substitute for natural toning.
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4to2CentBCphilia
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« Reply #19 on: April 22, 2012, 03:45:43 pm »

A method to accelerate this is to place the coin in tuperware along with a crumbled boiled egg yolk and let the closed container sit in the sun. This creates a higher concentration of sulfur in the air and should accelerate toning. Supposedly rainbowing will occur sometime in this process and the coin must be removed and rinsed to stop the process.

I am going to test this on a coin over the next few weeks and see what sort of results are achieved in 21 days. I don't know what sort of acceleration is possible and I don't know if it will go past rainbowing and into graying too quickly.

Mark, rest assured you will not need 21 days! I experimented with this on a denarius of mine and noticed a change of colour almost instantly, certainly after 30 seconds. I had to remove the coin from the tuperware after 5 mins as the process was going past rainbowing into blackness. The results were too dazzling if anything and the spread of the toning was not realistic - nevertheless I could imagine the process being refined somewhat so as to produce a more realistic outcome. A fun science experiment if nothing else - the results can be removed with a little LJ if not to your liking.

Quick update. I have attempted this twice with two coins. I suspect the purity of the silver at the surface is a major differentiating factor in the durataion and results of the process. The sulfur content and possibly the humidity of the atmosphere have an effect, but the if two coins are in an identical environment, they each tone at a far different rate.

First attempt was with a fairly bright coin. After three days in a tupperware container unopened, there was a light grey/rainbowed tone, however there were also small, black sludgy deposits that accumulated in the crevices of the engraving. The coin was moist from humidity. Once rinsed, the black deposits disappeared and the coin had a pleasant tone...........but it was not really rainbowed since it had already started to turn grey.

Second attempt is still ongoing. This time, two coins in the same tupperware container (but a new boiled egg yolk) with the crumbled yolk on the oposite side of the container (as in the first test.) This time I opened the container a couple times a day to check progress and eliminate humidity as the moisture from the egg was released. In this case, after 5 days one coin is gaining an even rainbow tone across it's surface that is noticeable and pleasant..........I will let this go another day or two. The other coin has only gained a faint amount of rainbowing in a few small recesses, but you have to look hard. This one will need to sit another several days at a minumum.

The rainbowing effect looks nothing like the effect using the chemicals sold on-line. So far it looks completely natural, because in efffect.......it is. If someone wants to argue this isn't natural, then neither is putting a coin in a manila coin sleeve for 30 years.

Which leads me to a question I posed earlier.............would/should a person still pay a premium for a nicely toned rainbowed coin, when it no longer takes 30 years to get that patina?

After I finish with these tests, I will see If I can get a decent photo of the coins, although I know it can be tricky to capture it in a photo.

BR

Mark

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« Reply #20 on: April 22, 2012, 03:59:16 pm »

Thanks for the update, Mark. Looks like you are getting some interesting results. In my experience the longer you overcook the egg the quicker the effect on the coin. An egg boiled for 15+ minutes will discolour coins in a sealed container almost instantly.
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SPQR Matt
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« Reply #21 on: April 22, 2012, 04:55:15 pm »

This is a very interesting topic.  I do have a couple questions, forgive me if they are stupid, but how are the coins being placed in the tuperware containers?  I assume they are laid flat on the bottom.  If that is so is the toning effect the same on both sides of the coin?
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« Reply #22 on: April 22, 2012, 05:05:57 pm »

@Kained
Interesting about how overboiling increases the sulfur content. This is true.............and in fact, for my first test, I used the yolk of an easter egg which was certainly over boiled. The second test used a perfectly hard boiled egg.....thus less sulfur......and a subtler and slower increase in irridesence.

@SPQR
Yes, the coin is flat and only toning on the upper surface. Flip the coin over and the other side is untouched. The coin would need to be flipped and the process repeated to tone the other side.


Doing some more reading, I found that this is a huge topic on Modern coin collector sites...........especially since rainbow/irridescent toning catches huge premiums................and it seems that if done well, nobody can tell the difference between artificial and natural irridescence..........thus the controversy.

Finally, I did find much discussion that heat, humidity, and the silver content of the coin all effect the process.

Anyway, as with everything else in the collecting arena, if something will increase the value of a coin..............someone will find a way to duplicate it.

BR

Mark
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Andrew McCabe
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« Reply #23 on: April 22, 2012, 05:22:29 pm »

The four coins I illustrated as lovely toned examples all, from recollection, started life bright and shiny when I purchased them, together, in 1989 (though perhaps they were lightly toned and my memory is fuzzy; they are certainly darker and more colourful today).

http://www.forumancientcoins.com/board/index.php?topic=79439.msg496939#msg496939

Just a short time, 23 years, was enough to change their colour. Would this count as accelerated toning? It's easy after all to sit and wait for 23 years. No special action required and it saves on eggs.

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« Reply #24 on: April 24, 2012, 02:55:11 pm »

I've bought coins which were completely white, and in one case left one in that state myself after cleaning. All of them toned in a couple of years, and there can't be much sulphur dioxide in the air here, as several species of lichen grow quite happily on the trees outside. I don't know whether chemical cleaning leaves silver in a state where it's more susceptible to toning, or whether that would be normal.
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