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<p>[QUOTE="Rob Woodside, post: 6617523, member: 96187"]1) I don't think silvers oxidizes to Ag<font size="2">2</font><font size="4">O but rather to Ag</font><font size="2">2</font><font size="4">S which is black and irridescent in thin films</font></p><p><font size="4"><br /></font></p><p><font size="4">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.</font></p><p><font size="4"><br /></font></p><p><font size="4">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. </font>[/QUOTE]</p><p><br /></p>
[QUOTE="Rob Woodside, post: 6617523, member: 96187"]1) I don't think silvers oxidizes to Ag[SIZE=2]2[/SIZE][SIZE=4]O but rather to Ag[/SIZE][SIZE=2]2[/SIZE][SIZE=4]S 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. [/SIZE][/QUOTE]
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