Some of you probably know I've been working on different techniques to restore ancient coins with horn silver. I'm working on my PhD as well, and have to prioritize things, so it will take some time. However, this fun time lapse of a Sev-Alex denarius with black horn silver is too fun (and cool, imho) not to share: (nb: the GIF takes 5.2 second, and then restarts) 50 ml of distilled water with two tablespoons of sodium thiosulphate at warm temperature (usually I will warm until boiling, but as the camera lens became condensed, I had to use a lower temperature). The time lapse (5 seconds) took like 10 minutes in reality. Horn silver, or silver chloride (or bromide, or a combination) reacts with sodium thiosulphate as following: AgCl + 2Na2S2O3 → Na3[Ag(S2O3)2] + NaCl Usually, the coin will turn black (which was eventually also the case here, but not present in the video, as the reaction was considerable slower due to the lower temperature). This can easily be removed by either rubbing the coin between finger and thumb under flowing water, or using a mild abrasive such as baking soda. Sodium thiosulphate does not react with silver or copper, and will subsequently not cause pitting. However, the horn silver is (not in all cases) essentially a form of corrosion of the coin, so beneath the layer of horn silver, pitting may be present. There are various ways to determine this beforehand, which I've posted on other threads. My intend however is to write a comprehensive overview and guide on the restoration of horn silver.
NICE! Nice to see a write up on the reaction. Not enough talented and CREATIVE chemists in America. Keep at it brother!!!!!! Funny that Ive noticed in this "coin hobby" that there are alot of backyard type chemists that actually know a bit and are dangerous. I dont think I would have predicted that. It makes sense though given that basic chemistry and a little understanding of metals goes a long way here.
Very cool!! Thanks so much for posting this and for your ongoing efforts in determining the best way to manage horn silver .
Wonderful post and great cleaning results. Thanks for sharing this information. Do you think sodium thiosulphate is preferable to using EDTA or a similar chelating agent for dissolving horn silver? If so, would you like to tell us more? (I have posted some mixed EDTA cleaning results here.) Last but not least, and from one PhD candidate to the other: good luck with writing your thesis!
Thanks all for the comments. I rather enjoy doing this, and sharing it is part of the fun, given these reactions. Sure, here it is. I don't have the photo of the horn silver all gone, unfortunately, and the coin has been sold already (it was bought as a proof of principle and sold as a nice denarius of Sev Alex ) The changes in colour are due to the lighting. As you can see, at the chin is a somewhat rough layer. With this type of horn silver, it's difficult to predict what's under it, and this came as a suprise. It's more visible on photo than 'in hand', but still ... Yes, I do think that. EDTA is an acid (Ethylenediaminetetraacetic acid), and has no specific action towards the chlorides (or bromides) that constitutes to horn silver: in mainly just disolves the silver in the AgCl. Furthermore, it dissolves copper which may cause pitting. Sodium thiosulphate only 'targets' the chloride / bromide. Thanks ... will it ever be finished
