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.