FORVM`s Classical Numismatics Discussion Board

Numismatic and History Discussions => Coin Photography, Conservation and Storage => Topic started by: Smak on May 11, 2019, 02:50:46 pm

Title: Questions about horn silver
Post by: Smak on May 11, 2019, 02:50:46 pm
I have two coins that are clean, good quality, and reasonably valuable, but they have some horn silver.

I read that horn silver can be treated with sodium thiosulfate baths. However, before taking this step I was wondering about the nature of horn silver. It's hard to find information about it.

Does horn silver spread if the coin is in a capsule in a dry environment? How fast does horn silver spread?
If I decide to proceed with the treatment, are there any good guides on it?

Title: Re: Questions about horn silver
Post by: Callimachus on May 11, 2019, 07:48:40 pm
There are numerous posts here about horn silver.
Go to the top of the page and you sill see a link that says Search Discussion.
Click on that link, and then type "horn silver"  (with the quotation marks) in the search box.

Title: Re: Questions about horn silver
Post by: Smak on May 12, 2019, 01:09:12 am
Thank you for the suggestion.

I tried searching for it, but horn silver is mostly mentioned in passing. Even those discussing it more left out the mechanics of it spreading on the coin as well as many details of how they cleaned it (which is why a proper guide if one exists would be appreciated).

Title: Re: Questions about horn silver
Post by: Smak on May 24, 2019, 11:07:38 am
I decided to go ahead with the treatment. I had two coins with a thin layers of horn silver covering large areas and one coin with two patches that I thought might be horn silver.  Unfortunately I did not take any pictures.

I prepared a bath of 10% sodium thiosulfate and 90% distilled water, and another bath of 100% distilled water. Sodium thiosulfate dissolved very easily even without heating up the water. I started with one coin but eventually had all three in the bath at the same time. I did not let the coins touch each other.

The effect of the bath was almost instant. You could see areas turning dark when the horn silver reacted with sodium thiosulfate. The speed of the reaction surprised me and I took the first coin out after just 30 seconds for the first distilled bath and brushing. Brushing removed the dark crust that had formed as a result of the reaction. Most of the horn silver didn't turn dark, only somewhat dimmer.

I had read that extended periods of time in a sodium thiosulfate bath could cause toning so I soaked the coins in 100% distilled water whenever I brushed them. I used a soft toothbrush with trimmed bristles. This worked perfectly and left no marks on the coins. I kept cycling between bathing and brushing with few minutes in the bath at a time and eventually a final 10 minute bath followed by the final brushing. Overall the coins spent probably around 25 minutes in the solution though I think in this case only Coin 2 showed signs of improvement after the 5 minute mark. Most of the horn silver was removed during the first 2-3 soakings.

I do not have pictures so a verbal description based on my initial impressions will have to suffice. This is not ideal and I apologize for that:

Coin 1
Obverse before: Half covered by a thin layer of horn silver, other half bright with no encrustations. Sharp details.
Reverse before: Bright with no encrustations. Sharp details.

Obverse after: No visual changes to the areas without horn silver, areas with horn silver brightened and cleared up. There is slight roughness in the areas that had horn silver and the surface is still dimmer compared to the unaffected areas with one 1x2 mm darker spot. Even sharper details were revealed from underneath the horn silver.
Reverse after: No visual changes

Coin 2
Obverse before: Most covered by a very thin layer of horn silver with some thicker areas on one side. Details pretty sharp but hazy.
Reverse before: Two patches of horn silver, covering about 1/4 of the coin. Details sharp but also hazy in the areas with horn silver

Obverse after: Slight roughness especially in the areas that had thicker layers of horn silver. The surface is overall brighter and clearer than before, though still dim. The surfaces seemed to improve further once the coin had a chance to dry. Much of the haziness in the details is gone.
Reverse after:  Brighter and clearer but the rough surface revealed underneath doesn't reflect the light as well as the horn silver did so the patches are more visible. One small darker spot also about 1x2 mm revealed underneath. The details sharpened up noticeably. No visual changes to the areas without horn silver.

Coin 3
Obverse before: Dark toning with no encrustations. Sharp details.
Reverse before: Dark toning, sharp details. Two patches of 3x5 mm and 2x3 mm that are either encrustations, horn silver, or some combination of these with maybe some corrosion thrown in.

Obverse after: No visual changes
Reverse after: Very slightly sharper details revealed from underneath one of the patches. The surface is now clearer but rough similarly to what was revealed with the other two coins, suggesting that the patch was horn silver. Toning in the area lightened up just a bit. No radical changes to the other patch, possibly a very slight improvement in the sharpness of the details. No visual changes elsewhere.

Overall the treatment was a success. I anticipated much worse, though admittedly the horn silver layers were fairly thin.