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Unbending hammered medieval silver coin (Gros Tournois)
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<p>[QUOTE="Roerbakmix, post: 3647963, member: 100731"]So here is a combined picture of the before (above) and after (below). 'In hand', the colours show more contrast (i.e. a darker patina on the surfaces with less patina on the lettering)</p><p>[ATTACH=full]979709[/ATTACH]</p><p><br /></p><p>I will summarize the method to unbend this coin (and other silver coinage). This is not for the faint hearted (...).</p><ul> <li>Stuff needed:</li> <li>Blowtorch</li> <li>(small) anvil</li> <li>(small) tongs</li> <li>metal bowl of water</li> <li>Baking soda</li> <li>Aluminium foil</li> </ul><p>This method only works with silver coinage of a higher silver purity (according to the metal detecting website >0.200/0.1000). This specific coin has a silver purity of well above 0.900/1.000.</p><p><br /></p><p>First, as most older silver coins contain a layer of silver sulphide (Ag2S) or silver chloride (AgCl), heating the coin may cause the sulphides and chlorines to further react with the silver this causing pitting. So, the patina needs to be removed.</p><p><br /></p><p>Silver sulphide can easily be removed using a redox reaction with aluminium foil in a solution of baking soda. In my understanding, silver chloride will not react in the same manner.</p><p><br /></p><p>In this specific case, the patina did not dissolve, strengthening my hypothesis that the uneven, spotted patina of this coin is silver chloride - which is more dangerous to silver than silver sulphide. I removed the patina by gently scrubbing the coin with undissolved baking soda.</p><p><br /></p><p>Second, the coin needs to be heated until it becomes dull-pink in colour. I noted that some small spots of patina that were left evaporated.</p><p><br /></p><p>Third, when the coin reaches this point, it needs to be dropped in a metal bowl of water. This process is called annealing, and essentially a) relieves the metal of stress, e.g. caused by bending, and b) improves ductility and reduce hardness.</p><p>After dropping the coin in a bowl of water, lay it on a flat and smooth surface and gently apply some pressure. Straighten no more than 1-2mm (depending on the silver purity). Repeat steps 2 and 3 until the coin is sufficiently flattened. Avoid short high-pressure (e.g. hammering), which may damage the coin (duh!), but placing the coin in a bank screw between two pieces of smooth, flat wood may be helpful. </p><p><br /></p><p>Lastly, heat the coin until dull-pink, and let it cool without dropping it in water.</p><p><br /></p><p>I noted a white dust on the coin, which may have been silver oxide.</p><p><br /></p><p>After this, you end up with a flat coin. I decided to apply some elemental sulphur to the coin, in order to create a silver sulphide patina and hopefully reduce the chance of a silver chloride patina. An alternative may be to place the coin somewhere safe, and wait for a few months.</p><p><br /></p><p>The whole process lasted about half an hour. After this, the coin reduced 0.01 g (13.14 g to 13.13 g) which may well be a measurement error.[/QUOTE]</p><p><br /></p>
[QUOTE="Roerbakmix, post: 3647963, member: 100731"]So here is a combined picture of the before (above) and after (below). 'In hand', the colours show more contrast (i.e. a darker patina on the surfaces with less patina on the lettering) [ATTACH=full]979709[/ATTACH] I will summarize the method to unbend this coin (and other silver coinage). This is not for the faint hearted (...). [LIST] [*]Stuff needed: [*]Blowtorch [*](small) anvil [*](small) tongs [*]metal bowl of water [*]Baking soda [*]Aluminium foil [/LIST] This method only works with silver coinage of a higher silver purity (according to the metal detecting website >0.200/0.1000). This specific coin has a silver purity of well above 0.900/1.000. First, as most older silver coins contain a layer of silver sulphide (Ag2S) or silver chloride (AgCl), heating the coin may cause the sulphides and chlorines to further react with the silver this causing pitting. So, the patina needs to be removed. Silver sulphide can easily be removed using a redox reaction with aluminium foil in a solution of baking soda. In my understanding, silver chloride will not react in the same manner. In this specific case, the patina did not dissolve, strengthening my hypothesis that the uneven, spotted patina of this coin is silver chloride - which is more dangerous to silver than silver sulphide. I removed the patina by gently scrubbing the coin with undissolved baking soda. Second, the coin needs to be heated until it becomes dull-pink in colour. I noted that some small spots of patina that were left evaporated. Third, when the coin reaches this point, it needs to be dropped in a metal bowl of water. This process is called annealing, and essentially a) relieves the metal of stress, e.g. caused by bending, and b) improves ductility and reduce hardness. After dropping the coin in a bowl of water, lay it on a flat and smooth surface and gently apply some pressure. Straighten no more than 1-2mm (depending on the silver purity). Repeat steps 2 and 3 until the coin is sufficiently flattened. Avoid short high-pressure (e.g. hammering), which may damage the coin (duh!), but placing the coin in a bank screw between two pieces of smooth, flat wood may be helpful. Lastly, heat the coin until dull-pink, and let it cool without dropping it in water. I noted a white dust on the coin, which may have been silver oxide. After this, you end up with a flat coin. I decided to apply some elemental sulphur to the coin, in order to create a silver sulphide patina and hopefully reduce the chance of a silver chloride patina. An alternative may be to place the coin somewhere safe, and wait for a few months. The whole process lasted about half an hour. After this, the coin reduced 0.01 g (13.14 g to 13.13 g) which may well be a measurement error.[/QUOTE]
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Unbending hammered medieval silver coin (Gros Tournois)
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