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<p>[QUOTE="Dafydd, post: 26263387, member: 86815"]From my non-ferrous foundry days we were taught that crystallization is a structural rearrangement not a loss of mass.</p><p><br /></p><p>To explain this simply without a thesis.</p><p><br /></p><p>Imagine the silver owl. Over two thousand years, it won't actually <b>lose weight</b> because of something called "crystallization." That's just the atoms rearranging inside, not disappearing.</p><p><br /></p><p>However, if it has copper mixed in like all owls, a serious problem called <b>embrittlement</b> can occur in coins alloyed with copper. The copper content, over centuries, can slowly separate and gather at the edges of the tiny silver grains. This creates weak spots. Then, environmental issues like pollution can enter into these weak spots, causing tiny internal "rusting." The silver doesn't get lighter, but it becomes incredibly fragile and can easily crack or crumble, like old, dry wood. This is a common issue with ancient silver. These can mineralise and literally fall off like dust.</p><p>I don't see evidence of crystallization on the original post owl although it looks like [USER=128351]@GinoLR[/USER] 's third example looks somewhat crystalline.</p><p><br /></p><p>If you take a look at my "best" example of embrittlement , this is a Lucius Titurius L.f. Sabinus denarius, Crawford 344/1a minted in 89 bc over 300 years after the Owl tetradrachm. This is a fourree and you can see some fantastic chemical reaction going on as the core is made of copper. We can see Verdigris and also Cuprite or Copper Oxide. My image isn't great but in the hand you can also see pronounced red spots as well as the green spots.</p><p><br /></p><p>Some assume that iron was used in the core of many fourree's but this was seldom the case because Copper is denser than iron at 8.96 g / cm3 compared to iron at only 7.87 g/cm3. Silver tops both at 10.49 g/cm3. A iron core denarius would be lighter and of course magnetic. I have yet to encounter a magnetic fourree.</p><p>[ATTACH=full]1677705[/ATTACH] [ATTACH=full]1677706[/ATTACH] </p><p><br /></p><p>This coin approximates the diameter of a "normal" Sabine denarius I have but is only 2.3 g compared to my other silver coin at 3.8 g. Clearly, when first made, it would have had to have approximated the real coins weight but over two thousand years the copper has mineralised and there is a really significant weight loss.</p><p>I bought this many years ago purely for the interesting metallurgy it exhibits. </p><p>When I first picked it up I thought it was an Antonius Pius denarius until I looked at it closer with a magnifying glass.</p><p>There is much interesting data on coin metallurgy but if you want to really dive down a deep rabbit hole you can start by reading Hall E. T. and Metcalf D. M. Methods of Chemical and Metallurgical Investigation of Ancient Coinage A symposium held by the Royal Numismatic Society at Burlington House London 9-11 December 1970. The Royal Numismatic Society, London, 1972, Special Publication 8. This digs deep into silvered coins and the effect of time on ancient coins. </p><p>[ATTACH=full]1677707[/ATTACH] </p><p><br /></p><p>It's amazing how one coin can trigger such a debate and that's what is so great about this forum. You never know where the next rabbit hole is.....[/QUOTE]</p><p><br /></p>
[QUOTE="Dafydd, post: 26263387, member: 86815"]From my non-ferrous foundry days we were taught that crystallization is a structural rearrangement not a loss of mass. To explain this simply without a thesis. Imagine the silver owl. Over two thousand years, it won't actually [B]lose weight[/B] because of something called "crystallization." That's just the atoms rearranging inside, not disappearing. However, if it has copper mixed in like all owls, a serious problem called [B]embrittlement[/B] can occur in coins alloyed with copper. The copper content, over centuries, can slowly separate and gather at the edges of the tiny silver grains. This creates weak spots. Then, environmental issues like pollution can enter into these weak spots, causing tiny internal "rusting." The silver doesn't get lighter, but it becomes incredibly fragile and can easily crack or crumble, like old, dry wood. This is a common issue with ancient silver. These can mineralise and literally fall off like dust. I don't see evidence of crystallization on the original post owl although it looks like [USER=128351]@GinoLR[/USER] 's third example looks somewhat crystalline. If you take a look at my "best" example of embrittlement , this is a Lucius Titurius L.f. Sabinus denarius, Crawford 344/1a minted in 89 bc over 300 years after the Owl tetradrachm. This is a fourree and you can see some fantastic chemical reaction going on as the core is made of copper. We can see Verdigris and also Cuprite or Copper Oxide. My image isn't great but in the hand you can also see pronounced red spots as well as the green spots. Some assume that iron was used in the core of many fourree's but this was seldom the case because Copper is denser than iron at 8.96 g / cm3 compared to iron at only 7.87 g/cm3. Silver tops both at 10.49 g/cm3. A iron core denarius would be lighter and of course magnetic. I have yet to encounter a magnetic fourree. [ATTACH=full]1677705[/ATTACH] [ATTACH=full]1677706[/ATTACH] This coin approximates the diameter of a "normal" Sabine denarius I have but is only 2.3 g compared to my other silver coin at 3.8 g. Clearly, when first made, it would have had to have approximated the real coins weight but over two thousand years the copper has mineralised and there is a really significant weight loss. I bought this many years ago purely for the interesting metallurgy it exhibits. When I first picked it up I thought it was an Antonius Pius denarius until I looked at it closer with a magnifying glass. There is much interesting data on coin metallurgy but if you want to really dive down a deep rabbit hole you can start by reading Hall E. T. and Metcalf D. M. Methods of Chemical and Metallurgical Investigation of Ancient Coinage A symposium held by the Royal Numismatic Society at Burlington House London 9-11 December 1970. The Royal Numismatic Society, London, 1972, Special Publication 8. This digs deep into silvered coins and the effect of time on ancient coins. [ATTACH=full]1677707[/ATTACH] It's amazing how one coin can trigger such a debate and that's what is so great about this forum. You never know where the next rabbit hole is.....[/QUOTE]
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