Copper Alloy Fragments : Fiber investigation

I have been working on a collection of fourteen Roman copper alloy fragments. A few of them are labeled as 'bronze', but without analyzing the composition of metal, its better just to call them copper alloy. Some of the fourteen pieces are bracelets, but there are also bits of metal strips, a piece of a belt buckle, a nail cleaner, and a wire. The museum had recently noticed the formation of pale green corrosion, and wanted them to be assessed and treated as necessary.

Most of the pieces have beautiful, smooth patinas. A patina is smooth and protective, a form of desirable corrosion. The patinas ranged from dark green to brown. These, though corrosion, will not be touched. Active corrosion, on the other hand, is damaging and usually needs to be removed. Ten of the fourteen had pale green spots of corrosion on them. On half of those, the spots were microscopic. However, five of the fragments had more corrosion. The surfaces were pitted, and a few pieces had pustules, or bumps. Because it is a recent and active corrosion that is detracting from the appearance of the fragments, I will probably end up removing it.

While looking at the pieces under a microscope for corrosion, I found a mass of fibers on one of the fragments. This fragment has a dark green patina with light brown corrosion products inside the incisions that formed the design. On one end on the interior, there is a red-brown area with fibers. The fibers were adhered well to the surface and appeared to be the same color as the corrosion around them.

I took a sample from the mass and put it on a microscope slide. I first tried to use tweezers to grab a single fiber, but it did not come away. I then used a porcupine quill to gently poke and flick at part of the mass. Unfortunately, the cohesive properties within the mass were greater than the adhesive properties of the mass attached to the metal. This means that the mass wanted to stay together more than it wanted to stay on the metal surface. A large chunk came off. I put a cover slip over the slide and placed it under the microscope with transmitted light.

Most of the fibres were long with distinct twists. One of these had a knot on on end. Others were flat with a lot of ridges and bumps along their length. A few were almost completely smooth with just a few bumps along their length and no twisting visible.  

The second and third images are composite images to show the fiber and its structure in focus.

The mass itself is not orderly, so I do not think this is a textile. Therefore, this could be a mixture of fibers. All of them look relatively new and do not seem to be mineralized. Mineralization is when the corrosion from the metal coats or completely replaces the organic fiber as it decays. When completely mineralized, none of the materials from the fiber remains; it has been replaced with metal. The fibers were fairly clean of corrosion material, once they had been extracted from the mass. They did not seem to be coated with corrosion products, nor were they colored with corrosion products. 

The structures were still visible and did not seem damaged, with the exception of the flattened, ridged fibers. Perhaps they were accidentally attached during some process in the museum fairly recently, and just loosely embedded in the corrosion products forming on the fragment. For instance, some cotton wool could have snagged on the rough corrosion when it was being cleaned, and the conservator did not notice. Or it could come from a cloth or cotton when it was packaged. Nesting in packing material, it could have had a piece adhere that was not noticed when it was removed and rehoused. This would also help explain why they came off so easily when force was applied between the metal and the fibers.

The long, twisted fibres seem to be cotton. One of them has a bit of a knot in it, and they all appear to be S-spun (twisted clockwise). This spin is very evident in longer pieces, such as the first image above. I compared my fibers to reference slides in the lab, and decided it was probably cotton. The flat, rough fibers, such as the fourth image, may be flattened or degraded cotton fibers. Cotton once flattened will not have the same twisted structure as undamaged cotton. 

On the other hand, the smooth, straight fiber seems to be an entirely different vegetal fiber. Unfortunately, many of the vegetal reference slides are missing from the microscope room. Bast fibers, such as flax, jute, or hemp, usually are fairly straight and fairly smooth with straight or crossed (X) ridges at joints. Twists are not readily seen. Although I could not identify which one the fibers are, they are probably flax, jute, or some other variety of bast. The mass, therefore, is a disorderly mix of cotton and bast fibers.

                                    Cotton                       Flax 

(Diagrams: Canadian Conservation Institute)

This is not a textile, and they are not mineralized. I think the most logical explanation is that this is from a rag or cotton swab used to clean the fragment in the museum, or if it was packaged with cotton. Part of it caught on the rough corroded surface and was not noticed or removed. If this is indeed the case, then I probably should remove these fibers. They are not original, nor do they add insight to the fragment's history. They were just accidentally attached due to cleaning or packaging in the museum.

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.