Silver Lysimachos Tetradrahm - white powdery composition beneath surface

[peterpil19]

Dear All,

I was a member of this forum (10 plus years ago) during my university days when I had plenty of time on my hands to completely immerse myself fully in this wonderful hobby. I haven't been an active member of this forum for many many years (in fact, my original registration of peterpil19  'lapsed' for lack of a better word) but I have maintained a continued interest in ancient coins and missed the discussion that goes with the interest.

I came across something today which I found interesting. I thought it was a good excuse to make my first post in 10 years and share it here.

I was going through my collection to discover that a Lysimachos tetradrachm which I had purchased in the mail bid sale of a well known coin dealer had chipped. A small piece had broken off. I was then surprised again (read, devastated) when picking it up, to experience another (this time much larger) piece break off in my hands and fall to the ground.

Inside I could see white powder and my first thought was the coin was moulded of plaster of paris and convincingly coated in silver. in other words, I've bought a fake!

In taking closer look at the white powder I could see silver-coloured particles mixed in between the white particles.

I was aware that silver can become brittle over time.

I've never however thought it possible however that the silver could break down first from the inside whilst maintaining a perfectly-healthy facade.

So I researched it on the internet.

I came came across (highly technical) scholarly articles on the subject of embrittlement of silver generally, as well as an article on FORVM which included a coin with a very similar powder-like composition beneath its surface.

refer: https://www.forumancientcoins.com/moonmoth/crystal_coins.html

I learned that when silver breaks down it turns into silver chloride. Silver chloride looks a lot like what is inside my coin.

I've posted the photos so you can see for yourself. I'm interested in any comments and if anyone else has experienced this, is it common? (I couldn't find other examples on the internet) or perhaps it is my wishful thinking and I've been fooled by a fake.

Either way I'm not left with much of a coin so hoping to make the most of a post...

It's been awhile so forgive me if this is not the best section to post this topic.

Regards,

Peter

[djmacdo]

I have seen this before, though seldom in such an advanced form.  I believe the coin is (sadly, was) authentic, and the insides just the result of crystallization and internal corrosion.  The outside can look fine, and the internal composition much altered and weakened. 

[areich]

I have seen this several times, with coins sometimes looking almost like silver-painted plaster casts, just as you describe it. I don't remember if they were lighter than solid silver coins would have been.

[Jay GT4]

I also had a Julius Caesar denarius that was chipped and had the same appearance.

[Din X]

Crystallisation was for a long time a sign of authenticity because experts thought it could not be achieved perfectly artificial.
Now we know that this is not true, because several fakes from modern dies with deceptive crystallisation appeared.

Here is an expample from fake reports.

https://www.forumancientcoins.com/fakes/displayimage.php?pos=-17798

Or here a bad cast fake was artificial crstallized

http://www.calgarycoin.com/reference/fakes/examples/fakeamphipolis01.htm

[peterpil19]

Thanks everyone for sharing.

I'm still surprised that chemically the crystalisation can begin from within the coin with no apparent signs on the surface. I wonder what causes that to happen?

I always thought that silver coins were less maintenance because they couldn't get bronze disease and the ones that have become brittle typically show the signs on the surface so you can judge which ones you need to be more careful with. I have plenty of silver coins which could snap in my hands and I treat them as such.
This Lysimachos more or less broke like it was a cookie.
I will glue it back together and it will probably make a nice gift for someone.

Peter

[Joe Sermarini]

This is not unusual. There seems to be greater density of the metal on the surfaces, undoubtedly due to something that happens during strike. I don't have a scientific explination. Lamination defects and this internal crystallization are the result and are fairly common.

[peterpil19]

Thanks Joe,

If they are reasonably common I'll be extra cautious around silver coins from now on. Unusual from my perspective because until now I've never broken one before.

Peter

[Joe Sermarini]

Thanks Joe,

If they are reasonably common I'll be extra cautious around silver coins from now on. Unusual from my perspective because until now I've never broken one before.

Peter

This is why the drop test for detecting cast fakes is not recommended.  It is also why padding is especially necessary when shipping silver coins. Sadly, I have had silver coins arrive as crumbled tiny pieces. It is only a small percentage of coins that are affected. Usually they will be underweight, however, there is no certain way to know.

[n.igma]

My former mineralized Eukratides I Tetradrachm (Bactria)....  it started to crumble to dust sitting in the tray, without any provocation.

I then gently wiped the crumbling edge across a piece of paper with the resultant pile of chips and scrapings shown in the image.

I then sold it at auction for a fraction of the original purchase price.

The auction house was the same one I purchased it from and kindly waved the commission which helped ease the pain.

[n.igma]

Forgot to add that I have reason to believe that this coin (above post) originated from the infamous Mir Zakah II Hoard, in which case is sat submerged in a well-spring for around 2,000 years. And yes it was about 1.5 grams underweight.  I think that sitting below the water table in a water saturated environment for a thousand years or so, may be a key factor in the development of such internal mineralization in silver coins. Most hoards are not buried so deeply as to be below the ground water table and this may explain the relative rarity of such destructive mineralization.

[Joe Sermarini]

Interesting. It does seem very reasonable that a water saturated environment is a possible factor. That might explain how some portion of the alloy could migrate out of the less compressed center.

[djmacdo]

Many of the Mir Zakah II coins were badly altered by the highly mineralized water in which they sat for so long.  Some looked good from the outside but had not unaltered silver left in the interior at all.  I was cleaning some that I can only call silver chloride lumps and the just melted away.  In frustration I tried to cleave one and in the middle was the most beautiful Menander tetradrachm I have ever seen--great style, full pinwheel lust, but the coin was as back and shine as obsidian.  Half an hour later, it began to dull, then craze, then crack, then crumble.  At the end of four hours, it looked like a pile of black ash, completely gone.

Sorry for all the typos--was using a friend's laptop and I found it awkward.

[n.igma]

That explains why so few coins from this hoard have reached the market in the last decade despite Bopearachchi reporting in 2011 that around 3 metric tons of hoard coins had found their way to Basel awaiting sale.  

Notorious in more ways than one, for some reason, even on this discussion board, mention of the Mir Zakah hoard appears to provoke a  hostile reaction from some https://www.forumancientcoins.com/board/index.php?topic=83147.0  Yet I find it a fascinating discovery that unfortunately has gone largely undocumented and ignored in mainstream numismatic study.

[n.igma]

I wonder whether this thread is not more appropriately placed under the category of Greek Coins as it really has nothing to to with Fake Coin Reports where it was originally posted.

[Mycenae]

Ouch! How does one avoid buying a coin that will succomb to this disease?

[n.igma]

It is a relatively rare phenomenon. I experienced one case in over 400 coins purchased. 

Avoiding it in the absence of detailed physical inspection is difficult unless the coin exhibits an obviously crumbled edge and is underweight. Too light a weight relative to the standard for the type is clue to the potential problem. All these mineralized coins are underweight due to silver loss over centuries of immersion. But not all underweight coins are mineralized. I generally avoid substantially underweight coins for this and other reasons.

[Viriathus]

Sorry for replying to an older post, I don't know if I should've started a new topic. I was looking for information about silver crystallization/embrittlement and I found this post interesting.

Forgot to add that I have reason to believe that this coin (above post) originated from the infamous Mir Zakah II Hoard, in which case is sat submerged in a well-spring for around 2,000 years. And yes it was about 1.5 grams underweight.  I think that sitting below the water table in a water saturated environment for a thousand years or so, may be a key factor in the development of such internal mineralization in silver coins. Most hoards are not buried so deeply as to be below the ground water table and this may explain the relative rarity of such destructive mineralization.

It's such a shame this happened... But it's great that you could trace back the coin's history and give a likely explanation for its embrittlement, it's very interesting. I wonder how often embrittlement causes silver coins to be underweight.  

I've never however thought it possible however that the silver could break down first from the inside whilst maintaining a perfectly-healthy facade.

I'm truly sorry this happened. It's curious how n.igma's Eukratides tetradrachm and your Lysimachos tetradrachm look so excellent on the outside and were so brittle on the inside. Do you know by any chance how much your coin weighs?

[djmacdo]

Many of the Mir Zakah II coins from the bottom of the hoard were converted to nothing but corrosion projects.  I had one very lumpy specimen with no signs of type visible.  I had tried to clean several similar pieces chemically with no good results.  One crumbled completely; another deteriorated to a thin sliver of silver.  So I decided to try to split the corrosion off the coin.  I placed a knife on the edge and gave it a sharp wack.  The lumpy coin split into three pieces.  The middle to my delight was the most perfect, beautiful tetradrachm of Menander I had ever seen.  Perfect condition, glinting mint luster, though the coin was jet black.  I admired it greatly and set it aside.  When I looked at it a couple of hours later, the minty luster had dulled.  A few hours later, the surface was becoming grainy.  By the next morning, it had crumbled to a black cinder.  Damn!

[Kevin D]

I've read that most ancient silver coins have some amount of 'crystallization', though the vast majority are stable and strong enough that they will not break. I've bought a number of ancient silver coins over the years that were in NGC holders where the coin was obviously too large for the holder insert, and under so much pressure that the insert prongs were markedly bent and pressing against the holder window pane. I remove all ancient coins I acquire in 'slabs' from the holders, but I realized that coins under such pressure from the holder are likely to be 'healthy' coins as far as 'crystallization' goes, or they would have been chipped or broken by the holder. (not a recommendation for buying slabbed coins, just an observation)

I like the 'ring test', or drop test (done before I bid on or buy the coin), and I also like the 'weight test'. Below are my notes from an interesting article that pertains to this subject.

E. T. Hall and D. M. Metcalf
‘Methods of Chemical and Metallurgical Investigation of Ancient Coinage’
Pages 49-66, ‘Changes Suffered by Coins in the Course of Time and the Influence of these on the Results of Different Methods of Analysis’. There is much worthwhile information in this article, presented in text and photographs, only a small portion of which is quoted below.
Page 50, “The difference between the present total composition [of a coin] and the original composition results mainly from two groups of phenomena, namely (A) the oxidation of the alloy; (B) the formation of holes inside the alloy…As a rule the volume of an oxide is higher than the volume of the metal it comes from…As the volume of the coin does not vary, the increase of volume due to the formation of oxide is necessarily accompanied by the elimination of a part of the alloy towards the surface of the coin. This part, which has been eliminated, is to be found in the crusts of oxides and carbonates that are frequently seen on the surfaces of ancient coins before they are cleaned…The above considerations also apply to various compounds other than oxides which, like the latter, have been formed in the course of time under the action of the surroundings. Usually they are chlorides, and sometimes even carbonates and sulphates in the levels closer to the surface.”
Page 51, The Formation of Holes Inside the Alloy. “Oxidation is generally accompanied by the formation of holes.” I believe the authors describe what is often referred to by numismatists as ‘crystallization’.
Page 52, “As far as we can judge from our own experience, there is no connection between the presence of oxides and holes inside the coin, and a bad condition of the surface. It would appear rather to the contrary, as most of the coins whose inner part was very oxidized had a very good-looking surface. This is apparently due to the redeposit of silver, which we shall be dealing with below.” See page 57 notes.
Page 53, Alloys Containing Lead. “Oxidation and corrosion are made easy by the fact that we are dealing with polyphase alloys in which lead is found in isolated grains. The variation of the total composition in the course of time generally comes from the fact that the grains of lead are more easily attacked.”
Page 55, “Furthermore, when dealing with identical compositions, the coins for which the blanks were cast are more easily oxidized in their interiors than those of which the blanks have been hammered and tempered, because the dendritic structure resulting from the casting makes the process of oxidation easier.”
Page 57, “Another cause of the superficial enrichment in silver is the redeposit of silver which replaces more or less completely the grains of copper oxide in the vicinity of the surface. This phenomena, still incompletely understood, affects the composition only of specimens that are strongly oxidized.”