Discussion in 'Ancient Coins' started by hotwheelsearl, Mar 5, 2021.
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Honestly, I'm not sure. I just know that some silver coins break easily because the silver "crystallizes." I feel like @desertgem may be the chem guy in the house.
The crystalized part looks unlike regular silver, it has a crystalline structure although why that is is beyond me.
The coin in question is L Memmius Galeria:
Most materials can be crystalline or amorphous. Metals tend to form crystals. The material is 100% (or whatever) silver, it's just in its crystalline form rather than its amorphous form.
Sorry for the worse-than-usual photo - got a bit of sun glare on this one:
Thrace, Kallatis Hemidrachm
(c. 300-101 B.C.)
Head of Herakles right wearing lion skin headdress /ΚΑΛΛΑ, club and ear of barley below, bow in bow case (gorytos) above.
AMNG I-I, 201.
(2.13 grams / 14 mm)
These often have a letter in the field (Φ, A, O, Σ etc.), which might be off flan on this specimen.
Attribution from Numista, with no letters noted.
The silver looks as if it totally altered to a silver oxide (Ag2O).
except that Ag2O is black.
That's true, the embrittlement is a combination of oxidation and the leaching of copper alloy, leaving a dark, brittle crystalline surface that over time becomes increasingly thicker, until the entire coin is altered.
Now I have a tetradrachm that had very heavy oxidation that did penetrate fairly deeply, much to my dismay when I attempted to remove some of, only to see a lot of the detail melt away.
Severus Alexander Denarius
(c. 222 A.D.)
IMP C M AVR SEV ALEXAND AVG, laureate, draped bust right / SALVS PVBLICA, Salus seated left, with patera, feeding serpent rising from altar.
RIC 178; Sear 7952.
(2.38 grams / 19 mm)
2O but rather to Ag2S which is black and irridescent in thin films
2) you are looking at age hardening that can embrittle the coin severely. If the alloy is severely debased it may be unstable and the impurities diffuse to the grain boundaries embrttling the coin. On the fracture you see a typical brittle fracture that looks like the pictures. Basically the alloy is recrystallizing to stable compounds.
With the advent of electron microscopes in the '60s it became possible to identify metals in a micron sized patch. When they did this to lab crystallized Pt alloys they couldn't find the same compounds occuring in ancient Pt nuggets. This was explained by saying the lab grown stuff wasn't in equlibrium as it hadn't the millions of years for the alloys to diffuse to their most stable state. I'm amused that classical coins have been around long enough to do so.
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