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.

Consolidation of a Glass Pitcher

There will be an exhibition of Islamic glass objects this fall. All the objects need to be examined before they are put on display. This includes writing or updating the object's condition report and photographs. I was given a small glass pitcher from Egypt to check and treat.

The pitcher is small and narrow, made from undecorated glass, tinted yellow-ish green. It was blown, as there is a little pontil mark visible on the underside of the base where it was attached. Bubbles are also evident in the glass, as are some manufacturing lines. The rim and spot are made from a wide disk pinched together. The handle is made from a separate, thick piece of glass, which is slightly darker and greener than the body. It twists slightly, connecting to the body further to the right than where it joins the rim. The handle is folded under and doubled up underneath the rim. There are ridges for decoration around the edge of the rim, in the middle of the top, and where the handle meets the body. The latter ring is mostly missing. The bottom is concave, with a thick central pontil mark.

Glass pitcher, before treatment.

Although originally green and transparent, much of the glass has turned brown, iridescent, and opaque. The worst of it is on the right side, where it covers most of the exterior. The left side is not as deteriorated, with some clear, transparent spots. The handle is the least weathered, with the interior and the left side of the handle clean, smooth, and transparent. Rainbow iridescence occurs when very thin deterioration layers are superimposed on one another, parallel to the original surface, that cause a distortion in the transmittance of light. Under ultraviolet light, some of the weathering crust faintly fluoresced yellow-white. The spots are seen mainly on the bottom and handle of the pitcher. It could be from microcracks in the glass, or the pitcher could have been consolidated in 1975, though no record of treatment remains.

The glass is heavily weathered from burial, with a beige enamel-like weathering crust over much of one side. In burial conditions, the alkali in the glass gets leached out and replaced with water. This process produces degradation layers with a different composition than the rest of the glass. In the best cases, this creates a weathering crust which protects the bulk of the glass. In the worst cases, the glass is completely dissolved. The discoloring weathering crust is not a layer of corrosion products on top of the glass, but rather the original glass after some of the chemicals have been replaced. Therefore, the weathering crust is never taken off. In removing the weathering, the original surface is lost and the glass is more vulnerable to chemical and liquid attacks. In this case, the crust is unstable and flaking off, particularly on the lower body. The glass is rough and iridescent where the patina layer has flaked off. In the object's old conservation file, there is a single slide from 1975 that shows that although this deterioration and delamination was happening then, it has since gotten worse. Some of the flakes have been retained in a bag, and could be analyzed later.

The worst flaking edges of the weathering crust were consolidated with a 10% solution of Paraloid B-72 in acetone. Glass is usually not consolidated or coated because it often seals water in the glass, which leads to deterioration. Paraloid B-72 is more permeable by air and water than other consolidants, and looks much like the glass. The Paraloid B-72 was applied with a small, soft brush along the laminated edge of the weathering layers on the right side and bottom of the pitcher. The adhesive was quite thin and runny. This allowed it to be brought into the weathering layers by capillary action. When the acetone dried, the consolidant tightened the layers and secured them to the rest of the glass. The excess Paraloid B-72 was removed with acetone so the consolidant is not visible on the surface. This should protect the crust and prevent further deterioration or delamination.

Top left: before treatment. Top right: applying the consolidant. Bottom: after treatment.

Parthian Vessel Fill, Part II : Microballoons

Filling a small gap in the Parthian vessel with plaster was a good experience, so I decided to try to gap fill with glass microballoons, or glass bubbles, to see the difference. I picked another small gap on the side of the large Parthian vessel and created a barrier by pressing dental wax on the inside of the vessel wall. The wax is used to hold up the fill and create a smooth interior surface, but it is not always necessary with glass bubbles, unlike plaster. When thick and set enough, the glass microballoons can be molded and pressed on without worrying about the fill material dripping or being unstable. The fill material looks much like plaster. It is a white powder which, when mixed with liquid, becomes thick and workable before drying hard. The plaster was mixed with water, but glass bubbles are mixed with adhesive. I slowly added the powder to a solution of 40% Paraloid B-72 in acetone, stirring it in until a paste formed. I had to add a lot more powder than I thought, but unlike plaster, you add and mix until it is the right consistency, rather than adding it all and then stirring, hoping it is the right ratio.

 Left: Microballoon powder, before mixing. Right: Wax barrier, from interior.

The bulked glass bubble mixture was thick, but it remained wet and workable longer than the plaster. I pushed the mixture into the gap with a spatula, taking care to get the sides filled entirely. I tried to get it as smooth and level to the surrounding ceramic as possible. Unlike plaster, microballoon fills are not carved down after drying. This fill shrinks less. As I was working, a film formed on top the fill. The top was dry and set, but still flexible enough due to the wet fill beneath it. It felt squishy and I could use my finger to press the fill into the proper shape.

Glass bubble gapfill, before setting.

After drying, the glass microballoon fill was hard and rough. The surface felt like soft sandpaper. Microballoon fills are often used because they match the feel and look of ceramic so well. I could see a tinted fill blending in perfectly with the texture of many ceramics. In my case, it was decided that the fill should be polished down. Paraloid B-72 is soluble in acetone, and, as the base of the fill mixture, so is the glass bubble fill. To smooth down the rough parts, I dipped a spatula in acetone. The acetone softened the fill, and I was able to slowly push the fill around slightly. Using this method of steady strokes over the surface with a spatula dipped in acetone, I was able to smooth the fill, flatten the imperfections, and create lines along the edges to simulate material loss like the other cracks. At the end, the fill felt much like the plaster one.

After polishing. The dark smudges are trapped dirt particles from the ceramic.

After polishing, I painted the fill. Using the same water-based acrylic paints as the plaster fill, I tried to match the buff ceramic color. Once again, I was not exact. It is much harder than it looks to match, especially since the paint looks different once it dries. This time, I tried to add a bit of a subtle spattered effect, like the ceramic, instead of a single color field. In theory, this break up of the color makes it look more natural and blend better.

Left: Close up of fill in process, with yellow and brown speckles. Right: The finished product.

Overall, I quite liked the glass microballoon fill. I thought that it was fairly easy to work with and looks good with the ceramic. For rougher pieces, leaving it unpolished would make the fill look quite natural. The glass bubbles fill was good over a small area, but could be more difficult to mold into the proper shape if the gap is too large, complex, or curved. I found mixing and working with plaster harder, but it would work better for a large gap.

Islamic bowl reconstruction

The Islamic bowl was difficult to reconstruct. It had many issues with it: large gaps, worn or broken edges, and laminated pieces. 

Top: Front of bowl, with gray areas indicating plaster; Bottom left: Back of bowl, with grey areas indicating plaster; Bottom right: pieces after disassembly.

I started with the easy part, the rim, and worked my way inward. The rim was mainly intact, but even so, there were weak joins and unstable sections. I glued each section together, placing the sections in a box of glass beads, which acted like a sandbox, to prop the pieces up. While the Paraloid B-72 dries, you want the join to be horizontal so that gravity helps keep the pieces in place. I then mounded up tissue for a support when I put the rim sections together.

Left: sections of the rim standing in glass beads; Right: the completed rim.

I put on as many pieces as possible, but then it became hard to place the pieces exactly. I then switched to gluing the small slivers together to create larger pieces. The center is mainly intact, but there are only a few places where a sherd touched both rim and center, and none of those had good join edges. 

Small sherds grouped into larger pieces, drying against a weight.

Claudia called Stephen Koob, a ceramics conservator at the Freer who has written books and articles on ceramic reconstruction methods. He said that he has seen a Seljuk bowl with laminated pieces, though not as bad as mine. It could be as issue with a particular clay or firing. He suggested using a synthetic material called Flügger to gap fill. Flügger is like plaster, but it is more flexible and stays workable for longer. It is used more in Europe than in the States, though it started to be used fairly recently.

To cast the bowl properly, all the joins had to be perfect and the angle of the sherds correct. I decided the best way to do this was to break up the rim into six sections and fix each before putting them back together. I used acetone to dissolve the worst rim joins, then I used a hairdryer to heat up the thermoplastic adhesive on each join to manipulate the pieces back into shape. I also heated and fixed the joins on the smaller interior pieces.

I made a new support for the bowl out of carved foam, cotton, and tissue. I used a razor and scalpels to carve out the general shape of the bowl from the pieces of thick foam. I cut a large depression in the middle so that the top of the base was level with the sides of the bowl. This was padded with cotton, then a sheet of tissue was placed over top.

Carving the foam.

I nestled the pieces into the support and pinned them in place. The central section was quite stable, but it took some work to get the rest of the ceramic to fit. I left off the top pieces of laminated sherds. When they lay over a back section, I cannot put the pins in place. The first time I tried to put all the rim and back pieces in, I was left with a centimeter-wide gap which I physically could not close. I took the pieces all off, padded the support again to change the angle of the sides slightly, then tried again. This time, I was able to close the gap and get all the pieces to fit. Although it is still not exact, the pinned pieces looked good.

Top: First attempt, with a large gap in the rim on the right; Bottom: Pieces pinned into the support.