Crystallisation - is it a guarantee of authenticity?

[John Conduitt]

Ok, there is no guarantee. But it is often said that a coin is not fake if the silver has crystallised. Then there is this coin on the Calgary Coin website. Fake but crystallised.

So. Is this very rare, and crystallisation is still a very good indicator of authenticity? Or are there several examples of crystallised fakes?

How long does it take for silver to crystallise in a coin? A decade? A century or two? Millennia? Or are crystallised coins already crystallised, and it is simply that ancient coins have lost the other metals that disguised it?

The nearest I have to a crystallised coin.

Agr Unit, 35-43

Camulodunon, Catuvellauni Tribe. Silver, 13mm, 1.17g. Wreath around legend with pellet border; AGR. Dog standing right on a snake, held in left paw and mouth; AGR in exergue (ABC 3005; S 354). The design is the same as a Cunobelin coin with CAM/CVN inscriptions.

Edited March 1 by John Conduitt

[Deinomenid]

Crystallisation is apparently very difficult but possible to do with the right mix. The mix has to be really quite pure silver. You can (if you know what you are doing!) force the process by adding tiny quantities of less noble metals to the mix (the mix is acting as a sort of battery) and the less noble will corrode giving a crystalline appearance.

I think (from bitter recent experience) that crystallization is more often used as a  tick/check mark in partially verifying a coin. In my case there were a number of  issues, one of which was a lack of crystallisation when it is common to actually have it (Neapolis, Macedonia). If it HAD been crystallized then  it would have removed one of the concerns, been very influential, but  not wholly proven the case. [I know this only answers one of the various questions, and I suppose  I should add fake crystallisation by use of acid etc...]

When I was doing my own coin postmortem, I found these  explanations quite helpful though not specifically on ease of forging.

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

(The author elsewhere said  he'd almost never seen it on a fake but put that down to probably a lack of effort. So the coin  you highlight  is very  unusual.)

[Ocatarinetabellatchitchix]

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. A very strong crystallisation could be an indication of a very fast cool down of the metal; even Medieval coins can be found crystallized, so several hundred years are enough with normal temperature.

But since we know that the speed of chemical reactions is related to temperature we can guess that you can create the same crystallisation artificially if you use very high temperature. You do of course need very pure silver about 95 % and some chemicals.

[Nerosmyfavorite68]

That's a good question.  I had also wondered how long the process takes.

Well, I guess the coin I just ordered is authentic (probably crystallized) and my poor, broken Nero is, too.

The above is 3.97g.  The weight is normal, but the porous surfaces led me to assume the coin is probably brittle.

[ominus1]

1 hour ago, Nerosmyfavorite68 said:

That's a good question.  I had also wondered how long the process takes.

Well, I guess the coin I just ordered is authentic (probably crystallized) and my poor, broken Nero is, too.

The above is 3.97g.  The weight is normal, but the porous surfaces led me to assume the coin is probably brittle.

...a denarius of Brutus Albinus?...whose portrait is it?...and welcome to the Bruti Albinus club..(put'er there paly! ^^) and the ides of March are upon us...lQQks like a fine coin! 🙂

[Nerosmyfavorite68]

The portrait is allegedly the consul Aulus Postumius Albinus.

That's scary that crystallized fakes are showing up.

 

 

[John Conduitt]

13 hours ago, Deinomenid said:

Crystallisation is apparently very difficult but possible to do with the right mix. The mix has to be really quite pure silver. You can (if you know what you are doing!) force the process by adding tiny quantities of less noble metals to the mix (the mix is acting as a sort of battery) and the less noble will corrode giving a crystalline appearance.

I think (from bitter recent experience) that crystallization is more often used as a  tick/check mark in partially verifying a coin. In my case there were a number of  issues, one of which was a lack of crystallisation when it is common to actually have it (Neapolis, Macedonia). If it HAD been crystallized then  it would have removed one of the concerns, been very influential, but  not wholly proven the case. [I know this only answers one of the various questions, and I suppose  I should add fake crystallisation by use of acid etc...]

When I was doing my own coin postmortem, I found these  explanations quite helpful though not specifically on ease of forging.

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

(The author elsewhere said  he'd almost never seen it on a fake but put that down to probably a lack of effort. So the coin  you highlight  is very  unusual.)

I suppose then the question is how likely it would be for a forger to create a crystallised coin by accident. If it can only be done with a lot of knowledge, skill and equipment, it's not going to be widespread and only seen on the types of coins produced by certain forgers. If they can accidentally create the right conditions, even 1% of the time, there could be a lot of them about.

As you say, it's like looking for casting bubbles or smooth edges - not conclusive, but an indicator amongst others.

[John Conduitt]

12 hours ago, Ocatarinetabellatchitchix said:

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. A very strong crystallisation could be an indication of a very fast cool down of the metal; even Medieval coins can be found crystallized, so several hundred years are enough with normal temperature.

But since we know that the speed of chemical reactions is related to temperature we can guess that you can create the same crystallisation artificially if you use very high temperature. You do of course need very pure silver about 95 % and some chemicals.

Do you think it can be achieved simply by using pure silver, making it very hot and then cooling it quickly? Are the chemicals needed specific to this process (so they would need to know what they are and to add them correctly) or might they be present anyway?

[Ocatarinetabellatchitchix]

1 hour ago, John Conduitt said:

Do you think it can be achieved simply by using pure silver, making it very hot and then cooling it quickly?

Very good question. Over time metal alloys do not always remain stable, they can slowly separate into their more stable natural components. An example of this happening with ancient silver coins becoming fragile is the crystallization of silver. Stable silver only contains about 1% copper at room temperature. This proportion increases when the silver is heated, so the heat is used to combine silver and copper into an alloy which slowly degrades at room temperature.

Monetary silver is almost always a combination of silver with roughly 1.5% to 15% copper. Adding a little copper to silver normally makes the silver softer and more resistant to wear. Silver and copper do not mix well, and over time (300-500 years or more), at normal temperatures, the copper will begin to separate from the silver.

Silver crystallization is the selective precipitation of copper from the silver alloy within crystalline boundaries, it is only the copper crystals that become visible when they separate from the alloy and they begin to corrode, weakening so does the money. The crystallization of silver is a natural process which, over time (300 years or more), separates the copper from the silver because this alloy is not stable. In this way they take empty spaces among the silver particles making the currency porous and fragile. This phenomenon mainly occurs on silver alloys with a copper content of 1.5% to 10%. Silver alloys with more copper content are resistant to this phenomenon. So theoretically it could be artificially produced by high heating the silver, but in reality it would be very difficult because the heating required would be towards the melting point of silver. Maybe we need the expertise of a metallurgist, a chemist or even better a skilled forger….

[DLTcoins]

Since this idea of "crystallized counterfeits" is based on subjective observation of the surface only, I wonder if something is possibly being misinterpreted. For example, might a surface craquelure resembling crystallization be an unintended artifact of the casting process and not evidence of actual internal crystallization and embrittlement? Only in the case of a broken coin revealing the diagnostic chalky interior would I say that crystallization demonstrates authenticity.

[Nerosmyfavorite68]

Crystallized!  And sadly, my only Hadrian denarius, unless there's some hanging out in the old section of my collection.

Are there any photographs of fakes said to be crystallized?  I'm hoping my Hadrian is flawed, but real. It was a 2022 Zurqieh purchase.  It would have been a nice coin if undamaged.  It came that way, and wasn't a result of shipping mishaps.

Were I a better photographer, I could offer up a picture of my broken Nero, which would show the insides poking out of the crack.

[Nerosmyfavorite68]

Another question: Are all crytallized coins lighter than normal?

Is porosity necessarily a symptom of it being in that state?

[Deinomenid]

25 minutes ago, Nerosmyfavorite68 said:

Are there any photographs of fakes said to be crystallized?

Yes, the coin from the first post.

The Calgary one.

[John Conduitt]

29 minutes ago, Nerosmyfavorite68 said:

Crystallized!

A common spelling error 😉

 

13 minutes ago, Nerosmyfavorite68 said:

Another question: Are all crytallized coins lighter than normal?

Is porosity necessarily a symptom of it being in that state?

This comes down to how they become crystallised. If it is only in a process where copper comes out of the coin, then it would have to be lighter on average and more porous.

[Nerosmyfavorite68]

Is that a U.S. vs. UK thing or is it really crytallised in all forms?  I was unsure.

[Brennos]

The coin from calgarycoin isn't very dangerous and the zoomed-in area doesn't look like crystalisation at all . 

early stage exemple :

later stage :

even later stage :

 

[Nerosmyfavorite68]

Uh-oh, that kind of looks like the surface of my recent Albinus Bruti (above).  I bought it from a relatively reputable dealer on vcoins. Should I be worried?  So far, the only problem from them's been crappy packing.  I've bought many lovely and real coins from them.

Crystallisation is an interesting phenomenon.  I forget which is crystallised, my Sextus Pompey denarius with the Pompey portrait or the pseudo Brutus (the Rufus issue).  Perhaps both.  I bought them almost thirty years ago, and the crystallised one was cheap because of that, although it's in pretty decent shape.

Does crystallisation ever involve actual crystals?  One of my earliest coins, an Allen Berman purchase, I think it was the Menander drachm, had some itty bitty crystals, and they glinted.  I've noticed that these Indo-Bactrian coins are prone to crystallisation.

Edited March 2 by Nerosmyfavorite68

[Deinomenid]

48 minutes ago, Nerosmyfavorite68 said:

Is that a U.S. vs. UK thing

Yes.

32 minutes ago, Nerosmyfavorite68 said:

Crystallisation is an interesting phenomenon. 

The Empire welcomes you (back).

32 minutes ago, Nerosmyfavorite68 said:

Does crystallisation ever involve actual crystals? 

Numiswiki - Although crystallization is the popular term used to describe this fragile condition, the term is a misnomer.  Granularization or embrittlement are perhaps better terms (but not customary).  Embrittlement of silver has been studied for a long time. It is the cause of some concern in museums holding archaeological silver and in archaeology itself. Embrittlement seems to be linked to inter-crystalline corrosion (see Ravitch, Lehmann, Organ, and Werner). Inter-crystalline corrosion can be exacerbated by the alloying elements present in the silver. Copper and lead are commonly encountered in brittle silver (Lehmann, Bhowmik, Toda, Thompson) but bismuth has also been detected (Rematullah). Discontinuous preservation of copper at the edges of the silver grains can also lead to embrittlement. Lead can make silver brittle even without corrosion (Toda).

 

[DLTcoins]

2 hours ago, Nerosmyfavorite68 said:

Is that a U.S. vs. UK thing or is it really crytallised in all forms?  I was unsure.

Two great nations divided by a common language. 😉

[DLTcoins]

3 hours ago, Nerosmyfavorite68 said:

Does crystallisation ever involve actual crystals?  One of my earliest coins, an Allen Berman purchase, I think it was the Menander drachm, had some itty bitty crystals, and they glinted.  I've noticed that these Indo-Bactrian coins are prone to crystallisation.

Many (most?) of the early Indo-something silver coins in the marketplace over the last 30 years have come from the Mir Zakah hoard, hundreds of thousands of coins found in a well in Afghanistan, often porous from leaching over the centuries.

Edited March 2 by DLTcoins

[rasiel]

There are a lot of myths and misconceptions in these answers. While I'll be the first to admit I don't have a solid background in chemistry I do think I have a solid understanding of the principles at play here. The gist of it is this: any metal alloy whose ingredients have a differing level of susceptibility to corrosion can become "crystallized" as it's understood here. In reality there's no crystallization happening. That happened entirely on the day of mintage. Let's take a typical silver coin, a 2nd century denarius. Let's say it has 88% Ag, 11% Cu, 1% P and the remainder a hodgepodge of other metals like Sn, Au, Zn, etc. When you make an alloy of this or similar makeup what happens at a microscopic level is that, as the molten mass cools, the metals begin to separate into solidifying blobs.

Think of a cooling soup. The fats, the tomato sauce, cream, etc. all start to congeal imperfectly as the temperature drops. In the coin, those now solid "islands" are surrounded by a mixture of metals with a higher melting point. As a new analogy now think of a glass of lemonade with ice cubes. Freeze the whole glass and the ice cubes are surrounded by frozen juice all around. Back to the coin. Once everything is a solid you're left with a mix of high-purity blobs bordered with what appear to be corridors of more well-mixed metals. In the case of this hypothetical scenario is that the "corridors", end up having a disproportionately high amount of the base metals, and so tend to be easily corroded away leaving little support for the big blobs of silver. Think again of the frozen lemonade how if the juice were selectively removed the cubes of ice would not hold very well. In the coin likewise this is a structural weakening that results in the coins cracking easily.

At the same time you can see why it's wrong to suspect that this would happen to high purity silver: there are no "corridors" of crappy base metals to begin with, just a giant crystal of silver that is uniformly resistant to corrosion. To create fake "crystalliized silver" therefore all one needs to do is purposefully mix base metals (iron would be perfect) into the melt, strike or cast the coin, then subject the coin to an acid that will dissolve that iron or whatever away to leave behind a fragile core of high purity silver that can break from a small amount of pressure. 

Another method would be to use sintering. This is a process whereby powdered metal is fused under heat and pressure to result in what looks like a solid but under a microscope appears to be swiss cheese due to incomplete welding of the starting particles.

Rasiel

[John Conduitt]

1 hour ago, rasiel said:

There are a lot of myths and misconceptions in these answers. While I'll be the first to admit I don't have a solid background in chemistry I do think I have a solid understanding of the principles at play here. The gist of it is this: any metal alloy whose ingredients have a differing level of susceptibility to corrosion can become "crystallized" as it's understood here. In reality there's no crystallization happening. That happened entirely on the day of mintage. Let's take a typical silver coin, a 2nd century denarius. Let's say it has 88% Ag, 11% Cu, 1% P and the remainder a hodgepodge of other metals like Sn, Au, Zn, etc. When you make an alloy of this or similar makeup what happens at a microscopic level is that, as the molten mass cools, the metals begin to separate into solidifying blobs.

Think of a cooling soup. The fats, the tomato sauce, cream, etc. all start to congeal imperfectly as the temperature drops. In the coin, those now solid "islands" are surrounded by a mixture of metals with a higher melting point. As a new analogy now think of a glass of lemonade with ice cubes. Freeze the whole glass and the ice cubes are surrounded by frozen juice all around. Back to the coin. Once everything is a solid you're left with a mix of high-purity blobs bordered with what appear to be corridors of more well-mixed metals. In the case of this hypothetical scenario is that the "corridors", end up having a disproportionately high amount of the base metals, and so tend to be easily corroded away leaving little support for the big blobs of silver. Think again of the frozen lemonade how if the juice were selectively removed the cubes of ice would not hold very well. In the coin likewise this is a structural weakening that results in the coins cracking easily.

At the same time you can see why it's wrong to suspect that this would happen to high purity silver: there are no "corridors" of crappy base metals to begin with, just a giant crystal of silver that is uniformly resistant to corrosion. To create fake "crystalliized silver" therefore all one needs to do is purposefully mix base metals (iron would be perfect) into the melt, strike or cast the coin, then subject the coin to an acid that will dissolve that iron or whatever away to leave behind a fragile core of high purity silver that can break from a small amount of pressure. 

Another method would be to use sintering. This is a process whereby powdered metal is fused under heat and pressure to result in what looks like a solid but under a microscope appears to be swiss cheese due to incomplete welding of the starting particles.

Rasiel

So does this mean all coins will 'crystallise' that are made of metals that don't mix well? It doesn't matter how old they are, but you have to wait for the base metals to come out. And to fake crystallisation, all you need to do is speed up the leaching of the base metals, such as with acid.

On the other hand, I found this post, which says much the same thing but suggests you need quite high silver purity for crystallisation to happen (under 5% base metal):

"Nearly all metals and alloys are crystalline in their natural state, but objects made from silver are not one big crystal. They are composed of millions of tiny crystals that are metallurgically bonded to each other at their boundaries.

These tiny crystals are called "grains" and alloying elements, such as copper in debased silver, tend to be concentrated at their surfaces. When the proportion of copper exceeds a certain amount - more than, say, 5 percent - it effectively seals the grain with a coating of copper rich silver. That is why Severan denarii (for example) never "crystallize."

When the proportion of copper is small - say, between one and five percent - it does not reach a concentration at the grain surface that seals the grain. The copper rich boundary, in the presence of electrolytes, forms a "battery" with the interior of the grain leading to corrosion of the boundary layer that was enriched in copper. When this happens the boundary layer swells and breaks the intergranular bonds. The coin is then likely to fracture easily where the grains meet."

Presumably, that only applies to natural crystallisation, as with a fake you'd be forcing it regardless of the purity.

Edited March 3 by John Conduitt

[rasiel]

2 hours ago, John Conduitt said:

So does this mean all coins will 'crystallise' that are made of metals that don't mix well? ...

I don't want to commit the sin of talking out of my ass on a topic I readily admit I have only a superficial knowledge of so, again, I'd prefer someone with deeper knowledge in this area chime in. However, I'm pretty confident that while I might have some of the details wrong I have the big big picture more or less right.

The term "crystallization" is problematic as noted in the first email. When a molten metal cools below its melting point its atoms will (typically) begin to arrange themselves into an orderly pattern. That's the crystal. It weas born on the day the blank which will become the coin was made and there will be no more crystal growth after that. What the numismatist means by crystallization is really just embrittlement of the alloy. The process of becoming increasingly more brittle naturally happens as the coin, usually due to the exposure of corrosive chemicals in dirt, chemically interact with the base metals in that coin. Silver, being less reactive than copper and the other metals normally encountered in ancient silver coins, can often superficially appear to be in good shape but is at a microscopic level riddled with fault lines formed through the chemical weathering of the interstitial areas.

"When the proportion of copper exceeds a certain amount - more than, say, 5 percent - it effectively seals the grain with a coating of copper rich silver. That is why Severan denarii (for example) never "crystallize."

As far as I can tell this is completely false. Highly purified silver is very ductile - the opposite of "crystallized". While it is true that silver objects will be made up of many small crystals (polycrystalline) rather than a single crystal, the boundaries in this case will remain almost as strong thanks to a force known as Van der Waals. All that serves as a boundary here is the atomic orientation of the facet wall. There is no, for lack of a better word to visualize it, "corridors" in which environmental corrosives can seep in and create the type of division that would allow the structure to become brittle. You can, by the way, prove this to yourself by taking a silver bullion bar that is 99.9% or higher and cake it in salt or even dunk it in battery acid for a year. Take it out and whack it with a hammer. It will not shatter.

The last paragraph I think is alluding to the process of galvanic corrosion. While this is another process that I don't pretend to have a full grasp of the long-story-short here is that when two dissimilar metals are in close contact an electric field is created which, when in combination with an electrolyte (say, salt water) forms an acidic layer which will begin dissolving the lesser noble metal beneath, in this case copper. I don't know to what extent this affects ancient silver coins in particular but it probably can be credited with destroying an untold amount of copper-based coinage. Orichalcum sestertii probably being particularly vulnerable due to their high zinc content... I don't know... just musing here on this count.

Rasiel

[Brennos]

Thank you @rasiel for these very interesting explanations. I don't manage to understand all the technical explanations, but I conclude that the forger who succeeds in artificially reproducing the phenomenon would deserve the Nobel Prize in Chemistry.

The most deceptive forgery in terms of the quality of the metal that I have ever been able to examine in hand is this Punic coin from Sicily. The surface must have been treated with some acid :

[John Conduitt]

13 hours ago, rasiel said:

I don't want to commit the sin of talking out of my ass on a topic I readily admit I have only a superficial knowledge of so, again, I'd prefer someone with deeper knowledge in this area chime in. However, I'm pretty confident that while I might have some of the details wrong I have the big big picture more or less right.

The term "crystallization" is problematic as noted in the first email. When a molten metal cools below its melting point its atoms will (typically) begin to arrange themselves into an orderly pattern. That's the crystal. It weas born on the day the blank which will become the coin was made and there will be no more crystal growth after that. What the numismatist means by crystallization is really just embrittlement of the alloy. The process of becoming increasingly more brittle naturally happens as the coin, usually due to the exposure of corrosive chemicals in dirt, chemically interact with the base metals in that coin. Silver, being less reactive than copper and the other metals normally encountered in ancient silver coins, can often superficially appear to be in good shape but is at a microscopic level riddled with fault lines formed through the chemical weathering of the interstitial areas.

"When the proportion of copper exceeds a certain amount - more than, say, 5 percent - it effectively seals the grain with a coating of copper rich silver. That is why Severan denarii (for example) never "crystallize."

As far as I can tell this is completely false. Highly purified silver is very ductile - the opposite of "crystallized". While it is true that silver objects will be made up of many small crystals (polycrystalline) rather than a single crystal, the boundaries in this case will remain almost as strong thanks to a force known as Van der Waals. All that serves as a boundary here is the atomic orientation of the facet wall. There is no, for lack of a better word to visualize it, "corridors" in which environmental corrosives can seep in and create the type of division that would allow the structure to become brittle. You can, by the way, prove this to yourself by taking a silver bullion bar that is 99.9% or higher and cake it in salt or even dunk it in battery acid for a year. Take it out and whack it with a hammer. It will not shatter.

The last paragraph I think is alluding to the process of galvanic corrosion. While this is another process that I don't pretend to have a full grasp of the long-story-short here is that when two dissimilar metals are in close contact an electric field is created which, when in combination with an electrolyte (say, salt water) forms an acidic layer which will begin dissolving the lesser noble metal beneath, in this case copper. I don't know to what extent this affects ancient silver coins in particular but it probably can be credited with destroying an untold amount of copper-based coinage. Orichalcum sestertii probably being particularly vulnerable due to their high zinc content... I don't know... just musing here on this count.

Rasiel

To be fair, there don't seem to be any studies that would give anyone much deeper knowledge.

Here's some pseudo-science of my own. This is by sampling ACSearch to see if purer silver produces more crystallised coins. The silver %s are obviously generic and the numbers are full of duplicates and misidentified coins.



This gives a correlation coefficient of 0.63, which given the flaws in the method, doesn't seem too bad. On the other hand, you'd expect coins at auction with 50% or less silver not to have crystallised, since there would be no coin left if they had. You'd also expect auction houses to mention crystallisation more often when a coin is higher value. This might, then, just show that if coins have crystallised, the ones with a higher silver purity are more saleable, because there's more of the coin left. That might lead to the conclusion that coins with higher silver purity are more likely to crystallise.

It was noticeable, though, that certain coins do seem to crystallise more often than others for no obvious reason. This search is for Constantine silver coins with Victory advancing left and gives 60 coins of the anepigraphic obverse type. If you add the word 'crystallized' you get 5. So 8.3% of them have crystallised, which is way higher than the average of 0.28%. Julius Caesar and Hadrian seem to have more than their fair share too. So perhaps there is something in the method of production causing it in addition to the silver content.