crystallization of silver in ancient coins

I know that any metal will allow the formation of crystals while cooling.
Is there any mechanism that would account for the crystallization of silver
coins at ambient temperature minted over 1800 ago? (The silver content is
rather high, about 95% pure in many cases.)
Andrew Thall
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Andrew Thall
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You need to be a little more specific.
In general, your silver containing coin almost certainly was a polycrystalline mass when made 1800 years ago - during cooling from whatever melt stage it underwent. It did not transform _into_ crystalls slowly at room temperature, almost certainly.
What are you calling "crystallization"?
How can you tell it with the tools or eyeballs that you have?
The claim that it has undergone crystallization over the last 1800 years is maybe pretty tough to prove.
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Most ancient coins display what is commonly (at least within the numismatic community) described as crystallization. The coin can become extremely brittle and the insides of the coins that break show strong crystals under a microscope (20 power).
I am guessing that rather than continued crystallization, the surrounding matrix of copper (I'm assuming that it is about a 5% alloy) leaches out over time, exposing the crystals.
Is that a plausable scenario for the observed crystals inside ancient coins?
Andrew Thall
Reply to
Andrew Thall
Have you heard of the "Crystallization of Old Brass Doorknobs"?
When new, the brass doorknobs were bright and shiny. After many years of outdoor exposure and contact with sweaty human hands, you can see "Crystalization" appearing on the brass doorknobs.
What you are seeing then is the grain structure of the original cast brass. Gradually the brass has been etched by sweat and the enviornment and any "polished layer" has been removed.
The very mild salt and environmental etching reveals the grain structure by preferential etching of the grain boundaries.
Not having seen the actual structure you are describing, I cannot tell you what caused it.
There is also a long history of newspapers calling structural failures from classical fatigue loading, "Metal Crystallization", because the fracture surface shows some grain boundaries and possibly some grain cleavages. (The fracture is revealing what was already there, but the newspaper people generally never knew that almost all useful structural metals were polycrystalline aggregates in the first place, and that such structure was vital to their useful function.)
Some newspaper reporters falsely used to conclude that therefore the metal "Crystallized" and this caused the failure. Some newspaper writers might still do this.
We should all take science and technology from the lips and pens of the describer, and ignore the lack of training of the describer.
Your explanation of leaching to reveal the pre-existing grain structure is quite like the etching of old brass doorknobs, but more severe.
The phase diagram of Ag-Cu is at
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With debased currency, say more than 10% Cu, you could get a semi-continuous phase of Cu that could preferentially leach out leaving a weak crumbly alloy.
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Silver in its lowest energy state is a face centered cubic and the solid from the melt will be a collection of crystalline metallic grains. Amounts of copper below the solid solubility limit will occupy some of the face centered cubic lattice sites and the copper will be in a true solid solution. But it is well established that atoms at the boundaries of crystalline grains as well as the boundary zone will be in locally higher energy states. This is why metallic crystal grains can be revealed by etching and also explains why sweaty hands or the action of corrosive gases can render the grains visible. It is also known that sulfur reacts readily with silver (look inside grandma's sterling cabinet years after the last silverware polishing onslaught - usually around Christmas time or Easter or VIP birthdays, etc) so that the sulfur will preferentially react with the grain boundary zone silver atoms.
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Jack Ferman

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