Archaeological Iron

I treated seven pieces of iron from a larger collection excavated at Ham Hill hillfort in Somerset, an Iron Age site. The archaeologists wanted the iron to be cleaned for interpretation rather than display, so we were able to practice on them without too much pressure. To clean them, most of us used an air abrasive machine.

Air abrasion is like sandblasting. We use aluminium oxide, which is a finer powder than sand. It is dangerous to breathe the powder, which gets everywhere, so masks must be worn. By the end, your clothes are covered with a fine layer of powder. The objects to be treated are placed boxes with a viewing window and hand holes, which is hooked up to an air extractor. The aluminium oxide is blown through a small nozzle, which is held at a shallow angle to the metal surface. The airflow is pedal-powered, and the air pressure and amount of powder can be controlled at the machine.

Top: the air abrasive machine (top left), box (bottom middle), and extractor tube (right). Bottom: horseshoe being abraded.

The iron objects had large, bulky corrosion products. The rusty orange or brown layers were removed by air abrasion. The fine aluminium oxide powder knocks off corrosion, turning it into dust and blowing it away. Its hard to see the powder working, so it looks like the corrosion is melting off, worn down by the powder.

There is a dark grey layer that follows the original surface, called magnetite. The corrosion products have to be removed, stopping at the magnetite layer but not hit the metal core underneath. This is fairly hard in practice, as its hard to see in the boxes and with the poor lighting. The stylus is held at a shallow angle so that higher areas are removed. This can produce a relatively smooth surface if done right, but my surfaces were fairly rough. Every once in awhile, you have to pull out the object to see if you are at the right level or not. There are also often voids and blisters in the iron. These areas can pop off if you are not careful, revealing the metal beneath.

One of my objects was a scythe. The scythe had dusty orange corrosion and large blisters. I slowly wore away at the corrosion with the air abrasive machine, and removed a couple of blisters by snapping them off with dental pliers. The corrosion abraded off fairly easily. It took about 4 hours to abrade the entire piece. Although it looked like a dolphin at the beginning, the shape and edges of the blade were visible after treatment. The surface still had voids and blisters, which I had avoided, but the object could be identified and its shape studied without much problem. The end of the piece had fallen off at some point, and I adhered it back on with epoxy.

Top: Scythe before treatment. Center: X-ray of the scythe, showing dark voids, a thinner edge, and an irregular surface. Bottom: after treatment.

Another object I treated was a broken horseshoe. I knew it had holes from the X-ray, and I knew that three of the four had dense metal inside. Therefore, when I abraded the piece, I was able to look out for them. The corrosion was harder than on the scythe, and so it took longer to treat the horseshoe. In the end, it took over 5 hours to treat. The surface was rougher and more uneven than the scythe. However, the shape and characteristics were evident.

Top: horseshoe before treatment. Upper middle: X-ray, showing the holes and metal in the first three holes. Lower three images: after treatment.

I cannot say that I enjoyed this treatment. I found the air abrasive machines difficult to use well. It was slow, hard to see, and dirty. The set up caused back pain because I had to bend over the boxes for hours. I was quite glad to be done. However, it was a good skill to learn. I certainly am not great at air abrading yet, but with practice, I will improve.

Copper Alloy Fragments : Treatment

After determining that the pale blue-green corrosion was just carbonates and oxides, I decided not to remove it. I could have removed the corrosion without effecting the stability of the piece. However, if I removed it, the reddish copper oxide layer would be exposed. This is visible in the pits I excavated for sampling. Although the bright spots are not aesthetically pleasing, red spots would not blend into the smooth green patinas either. The aesthetic and stability considerations were the same whether I removed it or not. Therefore, I chose to leave the corrosion as it was and just clean up the pits from which I took samples.

I removed the dirt from the surface of the fragments. The fragments, having been buried at one point, still had dirt caught in the crevices and incisions. The pieces with a rougher patina had quite a bit of dirt remaining. Dirt can soak up moisture and pollution from the air and hold it against the metal surface. This can cause reactions and negatively effect the stability of the copper alloy pieces.

The dirt was removed mechanically. Under the microscope, I gently scraped off the dirt with a scalpel. A rounded blade was used because it was smaller, easier to control, and would be less likely to scratch the surface than a pointed one. I softened the worst of the dirt with a cotton swab dampened with a solvent, IMS (industrial methylated spirits). The solvent was not used to remove the dirt because it could have just as easily pushed the dissolved dirt into the porous corrosion products. However, by dampening the dirt slightly with the solvent, it was easier to take off the dirt. I felt like I had more control, and needed to apply much less force.

A half completed fragment: cleaned on the left, with the soil remaining on the right.

There was a question regarding two of my pieces, broken fragments from the same bracelet. I did not know whether the incisions were filled with dirt or if it was deteriorated enamel or inlay. I analyzed them and another piece with SEM again to evaluate which was more likely. Enamel is mainly made of silicon and oxygen, much like dirt. In fact, it is fairly hard to accurately distinguish degraded enamel from dirt. However, most inlays would have had a colorant added, such as copper or manganese. Most colorants are either not found in soil or only found in small quantities. I compared the material in the decorative incisions on the bracelet with dirt found on another piece. I looked at them using SEM-EDS, backscatter imaging, and secondary electron imaging.

The material on the bracelet did not have any large, distinct peaks indicating a colorant. Unfortunately, I could not be sure from this test as it does contain elements that could have potentially been colorants, though in small enough quantities that they probably came from the soil. In fact, its composition was fairly similar to the dirt found on the other pieces. I decided that it was far more likely to be dirt, and removed it from the bracelet.

Compared spectra: the yellow is the spectrum of the dirt found on another piece, and the red line is the spectrum from the material in the bracelet's decoration.

The tested bracelet: the top bracelet fragment has not been cleaned, and the dirt has been removed from the incisions on the bottom bracelet fragment.

Using the SEM, I was able to use high magnification to see the surface. A spot that looked completely smooth and undamaged, even under an optical microscope, revealed to be cracked at x 700 magnification. This agrees with my theory that the smooth tin oxide patina had dehydrated and cracked, allowing for continued reaction with the copper metal beneath.

Once the fragments were clean, I treated them with benzotriazole (BTA). BTA is commonly used in the conservation of copper alloys as an inhibitor, though usually as a barrier to chlorides. The BTA complexes with the copper so that the metal cannot react with chlorides. In this case, I wanted the copper to be protected from further oxidation. I immersed the fragments in jars of 3% w/v BTA in IMS for 24 hours in the fume hood.

The BTA darkened the surface of the patina slightly. This is one of the downsides of BTA. The other downside is that the fragments are now toxic. BTA is thought to be a carcinogen. Gloves must be worn at all times when handling objects that have been treated with BTA.

I negated the toxicity slightly by coating the fragments. I applied two coats of 3% w/v Paraloid B-72 in toluene with fumed silica. Paraloid B-72 is a clear adhesive. Dissolved in toluene, and at such a low percentage, the Paraloid mixture was more like water and did not dry out too quickly when thin coats were applied to the metal surfaces. The fumed silica was added to the Paraloid B-72 so that there was a matte finish, rather than a smooth and shiny appearance. This, too, slightly changed the appearance of the fragments. However, it is fairly small and now they can be handled.

Top: after treatment. Bottom: before treatment.

In the end, the copper alloy pieces really did not need much treatment. They would have been fine without me cleaning, inhibiting, and coating them. It was a good experience for using analytical techniques, but I hope to do more treatment next time.

Copper Alloy Fragments : Corrosion Products

The copper alloy fragments were sent in for conservation because of the appearance of pale blue-green pits. The museum was worried that it could be bronze disease. Bronze disease is the active corrosion of the copper by chlorides, which can cause complete deterioration. I tested these spots of corrosion for chlorides using a silver nitrate test. If they were copper chloride corrosion products, then I would have to remove them.

I scraped off some of the corrosion products with a scalpel into a test tube and added deionized water. I dissolved a small amount of silver nitrate in deionized water and added a small amount to the test tube with the sample. I added a few drops of dilute nitric acid. The acid should have encouraged the dissolution of the copper chloride, which should react with the aqueous silver to form silver chloride, which precipitates out of the solution. Silver nitrate and copper chloride are both soluble in water, but silver chloride is insoluble. I ran this test on two pits on two different pieces, but a solid precipitate did not form.

There were a few possible reasons for this negative result. The sample could be too cohesive and thus it did not dissolve and react with the silver nitrate. This could possibly be remedied by soaking a entire fragment in water, then using that water in the reaction, but I do not want to immerse the copper fragments in water. Water is damaging to copper, and is often the root cause of active corrosion. The second option is that this is not a copper chloride. Without chlorides, the precipitate wont form.

I then decided to analyze the fragments using a scanning electron microscope (SEM). The SEM can reveal the elements in a sample through energy dispersion spectroscopy (EDS). This is a non-destructive analytical technique. An electron beam hits the fragment. The excited electrons that bounce back off are collected and create a back-scatter image, where light elements are dark and heavier elements are brighter. The fragment also generates x-ray emissions, which are gathered to create a spectrum. The more 'counts' of these emissions, the greater the amount of the element, and the higher the peak.

Top: The SEM with its three dedicated computers. Bottom left: the SEM stage, with the door open. Bottom right: looking through the view hole at the fragment on the stage in the SEM.

 I placed each fragment in the chamber and picked a place or two to test with the electron beam. I tested both the patina and the pit on two different fragments to compare. The fragments only have trace amounts of chlorine and sulphur, eliminating chlorides and sulphates as possible sources of corrosion. In fact, the SEM spectra and back-scatter images showed that the pit has a composition close to that of the patina, with no major differences in elements.

One of the tested fragments, showing the sampled sites. The top images are the EDS spectra, and the bottom images are the back-scatter images, with pink boxes around the specific sample sites.

Although the SEM-EDS did not tell me what the corrosion product was, I was able to determine the alloy composition of all the fragments. Seven of the fourteen are brass (copper and zinc), most with both zinc and tin. A couple had arsenic, and all of them contained lead. The other seven were bronze (copper and tin), with five of them as high-tin bronzes. These too had lead inclusions, and one had arsenic.

Knowing the composition, I was able to see a pattern. All the of pitting corrosion occurred on the bronzes, and the most severe corrosion was on the high-tin ones. The brass fragments had a rough patina, and a couple had a light green patch, but none seemed to have active corrosion. This made me think that it could have something to do with the tin content rather than trace elements.

I scraped out all the corrosion product from a pit. I analyzed this sample using Fourier transform infrared spectroscopy (FTIR). The easier method is to place the sample on a stage with a small crystal. A beam is bounced through the crystal, and the interaction with the sample causes adsorption peaks on a spectrum. The peaks can be attributed to various bonds in the material. The spectrum I got from this test was noisy and unhelpful.

The unassuming FTIR machine, which looks much like a printer.

To get better resolution on the spectrum, I made a KBr pellet to test. I ground the sample with potassium bromide for half an hour until it was a very fine powder. In theory, the potassium bromide is invisible on the spectrum. I then pressed the powder into a disk using 10 tons of force. It looked like a thin white plastic wafer, with a hint of green. The disk was placed in a slide mount, then the FTIR beam passed through it. The resulting spectrum was much more resolved and not very noisy, but it was nothing like anything in the database.

Left: grinding up the KBr and sample with a tiny mortar and pestle. Right: placing the slide mount with the inserted KBr pellet into the machine.

I went through and tried to assign all the peaks to possible bonds. I created a list of all the copper corrosion products that can be green, blue, or blue-green. I eliminated the ones that had elements that did not show up on the SEM-EDS spectrum. I looked at the peaks of the remaining corrosion products and compared them to mine. Most had too many peaks missing, so I eliminated them as possibilities. The closest product was copper carbonate, or malachite, which shared many of the peaks with my spectrum. I looked at other corrosion products of tin and lead to see if I could find anything to explain the other peaks. Hydrated tin oxide matched well, taking out the peaks assigned to malachite. Copper oxide and lead carbonate, or cerussite, were also possible. Therefore, my mixture seems to be oxides and carbonates, the similar to the patina.

If I am correct, than this is not active corrosion. Although damaging, no treatment can fix it but it will not continue or spread. The fragments had a hydrated tin oxide patina, a tin-rich area above the copper oxide layer. This dried out and cracked at some point, perhaps when placed in the museum environment where the relative humidity was lower than during burial. The cracks allowed the copper below to continue to corrode, and the lead inclusions in the alloy reacted with the atmosphere or carbonic acid. Lead carbonate formed, which increased in volume. Copper corrosion is is similar size and volume to the original material, but lead products expand significantly when lead corrodes. The lead carbonate pushed the patina up, forming a pustule. At some point, this pustule could have burst, or been brushed off, or somehow failed mechanically. This caused the top layer of patina to fall off and the lead carbonate could have flaked off as well. A pit with malachite and oxides was left, a damaged spot with the same corrosion products as the patina.

My interpretation of what is happening: cerussite is lead carbonate, malachite is copper carbonate, and cuprite is copper oxide.

These products are stable. The cracking has already occurred, the damage has been done. Removing the pale, damaged products in the pits will not effect the fragments. Therefore, I will not be cleaning off the pale blue-green corrosion surfaces.

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.

Moving On and Up

This September, I finished my internship at the conservation lab at the Kelsey Museum of Archaeology. I will really miss the Kelsey and all the opportunities I was given through the University of Michigan. I was lucky to have had this wonderful internship, and I am grateful for everything Claudia and Suzanne has done for me.

I am now in Cardiff, Wales for my masters degree program, MSc in Conservation Practice at Cardiff University. Cardiff has one of the few archaeological conservation programs in the English-speaking world. They have an undergraduate conservation program as well, which shares some classes with the masters program. The people here are very friendly and helpful, and the professors are wonderful so far. Cardiff is quite a nice city, with all the perks of being a capital but more compact and friendly than most.

The MSc program is two full years, with a placement over the first summer and a dissertation over the second. This year, the classes focus on organic material, and next year it will be inorganic. My classes are Practical Projects (lab), Essentials of Conservation (which includes an introductory course in conservation practice, a chemistry course in polymers, and investigative cleaning), Conservation of Wet Wood, Structure and Decay of Organic Objects, Museums Collections Management, and Analysis of Artifacts. There are also conservation seminars and research seminars weekly.

There are 9 students in my year, and 7 in the year ahead. There are far more Americans than British in the program. In my year, 7 people are from the US, 1 is from Finland, and 1 is from England. It is also dominated by women, with only one man. I do not know what this says about the American programs, but it is nice that most of the people are adjusting to the new city and new schooling system like I am.

In lab, everyone gets a coin and a wooden peg. I was assigned a Victorian silver coin and a wet wooden trenail to stabilize. We are also assigned other individual objects to work on. They asked us what type of object we wanted, then came up with some things they thought we might like from local museums and clients. I was given copper alloy fragments from Roman bracelets, buckles, and wires with active corrosion; archaeological iron from a hoard; and a large, monochrome painted wood statue with cracks, flaking paint, rusting nails, insect damage, and missing pieces. I will be working with another girl on the statue, who has experience with paint conservation. I think they will all be good experiences, but the statue seems like a huge, daunting task.

The benches in the main lab. The room beyond has a reference library. Down the hall and on the next floor, there is a microscope room, X-ray and SEM rooms, a waterlogged wood room, an object reference and archive room, and a photography studio.

The large object lab room. The covered object on a cart on the far left is my statue. Off of this room, there is a fume extraction room and an air abrasive room.

                          Left: My new toolkit. Right: The storage room, where the objects are kept on assigned shelves.

I look forward to the next two years. I will learn a lot about conservation, both in theory and in practice, and hopefully get to work on many different objects.