Featured Unbending hammered medieval silver coin (Gros Tournois)

Discussion in 'Ancient Coins' started by Roerbakmix, Aug 11, 2019.

  1. Roerbakmix

    Roerbakmix Well-Known Member

    Part of the fun I get with collecting ancient and medieval coinage is attempting to clean encrusted coins, or restore damaged coins. There is a certain learning curve to this, and deciding whether or not a coin actually improves from it beforehand is difficult and, in my opinion, also differs per individual.

    This project was something I never tried before: unbending a hammered medieval silver coin. This coin has been posted at CoinTalk before (https://www.cointalk.com/threads/a-fun-lot-of-silver-roman-and-medieval-coinage.343992/). In my opinion, the coin has three issues:
    1. It's bent quite badly
    2. There is a deep scratch, probably caused by digging the coin (and thus causing problem 1)
    3. The patina is uneven and 'spotty'
    So, first a detail image of problem 1:
    upload_2019-8-11_20-5-25.png

    A high-definition image of the obv/rev (note the deep scratch and the uneven patina)
    [​IMG]

    Unbending silver hammered coins should be done with caution. I followed this approach https://www.metaldetectingworld.com/cleaning_coin_p24_bent.shtml.

    After these steps, I ended op with a (almost) perfectly flat coin.
    upload_2019-8-11_20-27-47.png
     

    Attached Files:

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  3. Jwt708

    Jwt708 Well-Known Member

    It seems like an improvement! Any chance to get another overhead obverse and reverse shot?
     
  4. Roerbakmix

    Roerbakmix Well-Known Member

    Thanks @Jwt708. I was a bit hesistant to post this restoration process, as there are some strong opinions concerning cleaning / restoration etc.

    I tried to make another overhead photograph, but it was already too dark outside. Will try again today and post the results here.
     
    Last edited: Aug 12, 2019
    Jwt708 likes this.
  5. Pellinore

    Pellinore Supporter! Supporter

  6. Roerbakmix

    Roerbakmix Well-Known Member

    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)
    upload_2019-8-12_20-12-30.png

    I will summarize the method to unbend this coin (and other silver coinage). This is not for the faint hearted (...).
    • Stuff needed:
    • Blowtorch
    • (small) anvil
    • (small) tongs
    • metal bowl of water
    • Baking soda
    • Aluminium foil
    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.
     
    chrsmat71 and Bing like this.
  7. Clavdivs

    Clavdivs Well-Known Member

    Quite an amazing result! I do not think I would be brave enough to try this ... but I have to say the coin looks great.
     
    Roerbakmix likes this.
  8. Roerbakmix

    Roerbakmix Well-Known Member

    @Clavdivs yeah, I had mixed feelings while doing it as well. Though, having reached a point of no return, I actually enjoyed the thought "the person minting this coin in the early 1300ths would enjoy this as well".
     
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  9. Justin Lee

    Justin Lee I learn by doing Supporter

    I honestly thought you were going to say "just kidding" after the list lol. Nope. Kudos to you on your bravery and results!
     
  10. Jwt708

    Jwt708 Well-Known Member

    Thanks for the new picture. It would have been interesting to see what the coin looked like when it was heated.
     
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