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.
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?
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
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.
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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