Cleaning a Gothic Florin

The first object that everyone was given was a coin. They had been glued to a textile-covered board in a museum exhibit. The coins had adhesive residue and fibers stuck to one side, and tarnish on the other. Mine had a fair amount of adhesive and fibers on the reverse and a tarnished obverse.

Mine was a silver florin from 1852. A florin is one-tenth of a pound and was one of the first attempts to decimalize the British currency system in 1849. The obverse has a profile view of Queen Victoria. An inscription in Gothic lettering around the rim is the reason this type of florin is called "Gothic". The reverse has the coat of arms of England (three lions), Scotland (framed lion), and Ireland (harp). In between, there are roses, a thistle, and a shamrock.

Obverse, before treatment: Queen Victoria, "Victoria d : G [Dei Gratia (“by the grace of God”)] : brit / reg : f : d [Fidei Defensor (“defender of faith”)] : mdccclii"

Reverse, before treatment: Coat of Arms of England (top and bottom), Ireland (left), and Scotland (right), “One / Florin // one tenth / of a pound”

The condition of the coin was not too bad. There are scratches over most of the surface. The obverse was covered in tarnish, especially the areas around the edges and inscription. These areas approached black, whereas the figure and empty spaces were browned. The silver had tarnished in between the threads of the reeded edge as well.

The reverse was dulled, but did not have the tarnish or corrosion that the obverse had. This indicates that the tarnish on the obverse was formed in the museum and is not original. The adhesive on the reverse was only in certain areas rather than coating the entire side. It was clear, or slightly yellowed. There was trapped dirt in the adhesive, as there were some brown and green particles.

Small spots of rust color are on the edge of the obverse. This is because the coin is made from a silver alloy with copper in it. Copper corrodes before silver, so it has deteriorated to copper oxide before the silver is affected. This corrosion product does not need to be treated or removed at this point, though if it gets worse something may have to be done.

Obverse, copper oxide corrosion above date and dark tarnish, before treatment

Reverse, red fibers and clear adhesive, before treatment

The adhesive seemed to be modern as it was smooth, clear, and plastic-like, like a Paraloid. It could have been an acylic or a synthetic thermoplastic. For most of the acrylics and synthetics, acetone, toluene, 1,1,1-trichloroethane, ethanol, and cyclohexane are the best solvents. Out of these, acetone and toluene seem to be the most useful, with a middling polarities. Ethanol is more polar, and cyclohexane is fairly non-polar. I removed the adhesive from the reverse of the coin with acetone, using a small cotton swab. This dissolved the adhesive well and removed the fibers. I also cleaned the obverse with acetone.

The second objective was to remove the tarnish from the obverse. Tarnish is not harmful. In fact, it is a protective layer and often desirable. Because of the way it is formed, the tarnish layer is actually the original surface of the coin which means removing it removes original silver. The tarnish was not bad, so I did not want to remove it all, just the worst of it. Mechanical cleaning with a fine, soft abrasive paste is preferred if the silver is strong enough, which mine was. Other methods of cleaning, such as chemical dips to remove corrosion and electrochemical reduction to turn the tarnish back to metal, can damage the silver and cause it to tarnish more quickly in the future. They are harder to control, so mechanical cleaning was the better option in this case. 

The best abrasive system appears to be precipitated calcium carbonate in water. This removes the most tarnish and least silver with little scratching. I made the powdery white calcium carbonate into a dilute paste with deionized water, then applied locally with a small cotton swab. I barely applied any pressure to the swab. The polishing action came from moving the small particles of calcium carbonate around the surface rather than rubbing them in. The worst sections of tarnish were removed, particularly around the edge, but some was left untouched. I did this process twice. The first time, I just went over the areas that had particularly dark tarnish. Unfortunately, it was very obvious where I polished and where I had not. Therefore, I did it a second time barely polishing but gently going over the entire surface. The layer of tarnish was reduced and evened out, but not completely removed.

After treatment, the remaining polish particles were removed. The coin was washed well and rinsed in deionized water, then dried with a cloth.

Obverse, after treatment

Reverse, after treatment

I think this treatment was quite successful. I retained a thin layer of protective tarnish and removed the foreign material. I liked the calcium carbonate polish, and I felt like I had a lot of control with it. If it is put back on display, the museum has to be careful about not letting it tarnish again. The spots of copper oxide also have to be monitored. Otherwise, the coin is clean and stable.