Treating Bronze

I began working on a bronze figurine, accession number 0000.02.1596. Unlike the Parthian jug, this one has been conserved before, though it was last treated in 1972. The figurine is Greco-Roman and excavated at Fayoum, Egypt, putting it's creation date around 100 BCE - 300 CE. The form is simplistic, and with the corrosion some of the details are hard to see. The active corrosion can been seen in the picture below. The green patina has spots of light green corrosion, though the worst spots are not shown here. The incised details around the paws of the animal and the arms and legs of the man are almost completely covered in yellow-green corrosion. The old conservator thought it was a prostrate man with a bear on his chest, perhaps a wrestling scene. With its pointed snout and ears and narrow body, I personally think it is more likely to be a wolf, which would make more sense if the figurine Roman.

Bronze Figurine: Bear or wolf on top of a man. Spots of active corrosion are lighter than the surrounding patina.

Before I started treatment, I did not have to wear gloves. With the patina and corrosion in place, I was not actually touching any metal. Once I began to scrape off the corrosion, then I put on gloves to protect the metal from the oils on my fingers and to protect my skin from the chemicals on the metal from old treatments.

First, I tested the active corrosion to see if it was a chloride. Copper chloride is one of the chemical reactions that can cause damage to bronzes. Copper oxide turns the metal green but is more protective than harmful. Copper chloride, on the other hand, is unstable and damages the metal irreversibly  Extreme cases of this is called the bronze disease, which corrodes until no metal remains. The only way to combat this is to remove all the copper chlorides from the object, with either chemical or mechanical cleaning. Chemical cleaning is harsher than mechanical cleaning and can further damage the metal, so the conservators at the Kelsey mostly mechanically clean the metalworks. To test the figurine, I scraped off some of the active corrosion, placed the flakes in a test tube, and dissolved them in deionized water. I then added nitric acid and silver nitrate. The silver reacted with the chlorides present in the corrosion forming silver chloride, a white precipitate.

Chemistry in action: The figurine tested positive for chlorides.

Knowing what the corrosion was, I was able to begin treatment of the bronze figurine. The corrosion has to be scraped off by hand. At the Kelsey, metals are not entirely cleaned. Instead, it is preferable to get very close to the metal without actually exposing it, as long as all active corrosion is removed. Sometimes, pockets of corrosion form, erupting from the surface of the metal through the patina, which have to be excavated leaving an uneven or pockmarked surface. In that case, it can be filled with a silver compound or left to show the damage. Using scalpels, paint brushes, picks, glass brushes, and cotton swabs in various sizes, I began to remove the corrosion from the surface of the figurine. I used a microscope to see better what I was doing. I also worked on a double-headed Greco-Roman coin found at Karanis, Egypt to get a feel for working under a microscope. Working under magnification, I felt like I was taking off a lot more corrosion than I actually was.

 Microscope: The magnification allowed me to see the flakes of active corrosion that I was removing from the coin (left) and bronze figurine.

Desalination of the Parthian Jug

 Today, I worked a bit more on the Parthian jug. Last time, I had noticed salt crystals on the fabric, so I wanted to see if they would come out. To do this, I set up a desalination bath. I placed the potsherds in plastic baskets and soaked them with deionized water. Just rinsing them got off a lot of the dirt. I then submerged them in tubs of deionized water. The initial reading of the conductivity of the water was about 15 μS/cm. A couple hours later, I took out the potsherds, stirred the water, and again took a reading of the water's conductivity. This time, it measured about 300 μS/cm indicating a fairly high concentration of salts in the solution. This means that salts were, in fact, coming out of the potsherds. Although it is still probable that some of the salts that I saw are insoluble, the soluble salts cause more damage than insoluble salts because they change state when the humidity fluctuates. When the relative humidity is high, the clay seems to 'weep' from the moisture in the salts, and when the humidity is low, the salts crystallize within the fabric. This change in state can cause cracks and swelling. The lower the conductivity reading, the better, but to stabilize ceramics you want it below 30 μS/cm (deionized water is around 5, whereas tap water is around 50). The potsherds will remain in the desalination bath for at least a few days until the water tests low enough or the concentration of salts plateaus. Because I will not be there, Claudia will test the conductivity and change the water each day for me.

Bath time: The potsherds are placed in deionized water to draw out any soluble salts.

Day One: Dec 5, 2012

Today was my first day, both at the Kelsey and in conservation. I was excited and nervous to move from archives and images to actual objects. The conservation lab is on the third floor of the old section of the museum. The lab consists of two rooms, both with a central work bench set up. The first room is the newer section of the lab, which used to be a research and study space. There is a large window which lets in a lot of natural light. This room is where the computer stations are set up, and where I worked today. The other room is the old conservation lab space, which now seems to be mainly storage. The drawers of objects being worked on is in this room, and the photography studio is set up in a niche to one side. 

Suzanne had me start today by reading sections from a couple of publications about ceramics. The first was a handbook designed to quickly explain how to handle objects for non-conservators. I also read two chapters in a book about ceramics, one about deterioration and the other about preservation. This book went through the ways and reasons ceramics and glass deteriorates, such as low porosity makes the object more prone to breakage and high tension pottery (which is fired in high temperatures) can be 'sprung' where the tension inside the ceramic is released when it breaks and thus it cannot be put back together.

The third book was a short summary about the excavation at Seleucia. The ceramic that I was given was from that dig, so it was interesting to read. The excavation was led by the University of Michigan from 1927 to 1937. In Iraq, Seleucia was the Parthian capital on the Seleucid empire. By the Tigris and Euphrates, much of the ceramics found at Seleucia have salt deposits.

After reading about ceramics, I was able to start working on an object. I was given a zip lock bag with potsherds from Selecia. The piece I was given was 0000.03.4030. Working on the lab bench in the main room, I took out each piece and examined it. I did not use gloves, as I needed to feel and grip the sherds better. The oils on my hands should not effect the pot, but being unable to properly feel or hold the ceramic could result in damage.

I wrote a condition report and created a record in the Kelsey conservation database, describing the ceramic and its condition. It was a small jug with one handle, probably less than a foot high. The wall thickness varied, particularly at the base where it was very thin in the middle but very thick along the rim. The bottom rim was a distinctive shape with incised circles and flattened sides. There was a second rim that mimicked the thick bottom rim between the neck and body of the jug. The clay was a dun brown color and quite flaky. The exterior had a light glaze with small sections of blue-green and iridescent colors. The interior of the jug was also glazed, with a dark brown color that lighten to grey in some areas near the bottom. Both the interior and exterior glazes had chipped off in places and showed cracks throughout the piece. The interior also has salt deposits on perhaps half the the sherds, and the entire piece has dirt and grime on it. There are 33 pieces, the largest of which was about 9cm by 7cm.

After the condition report, I started to piece the potsherds together. I was able to fit 30 sherds together, but I have yet to figure out where the final three small pieces go. This will probably be easier when I actually start to adhere the pieces so that I can see spots where things are missing. Right now, I was just putting pieces together to see if they fit, then trying to carefully space it out on the tray in order. It is surprisingly hard to find a good way to lay potsherds so that the 3D object is laid out on a 2D surface but still recognizable. After fitting most of the sherds together, I noticed that the upper rim of the jug as well as some spots around the base are missing. The pieces along the base of the jug might need to be filled in for structural purposes, but the missing top will probably not be replaced. At the Kelsey, the primary aim is the preserve the archaeological artifacts, not reconstruct or restore them.

The next step was to photograph the potsherds for documentation. Using the photography studio set up in the next room, I arranged the potsherds and took pictures. The camera and tripod system was quite nice, allowing me to photograph the sherds from directly overhead. The sherds were photographed on each side. These images were then edited in photoshop, where I added labels, cropped the images, and corrected the color. I created records in the Kelsey photograph database and added the edited images.

Next week, I will begin the treatment. Claudia thinks the salt deposits are insoluble, but out of curiosity, I will probably try desalinization by soaking the potsherds in deionized water and checking the conductivity to see if salts come out. When that is done, the potsherds need to be cleaned, and then I can join the pieces together with an adhesive. Because of the missing pieces, and the fact that most ceramics cannot be joined perfectly, I will work from top to bottom so that any gaps or issues occur at the base where there is more flexibility and space for work.

Work space: The potsherds on the work bench. The bag in which the pot sherds had been stored can be seen on the left, and my condition report is on the right. At this point, most but not all of the sherds are in the correct place.

A work in progress: The potsherds are all laid out, mostly in order. The sherds on the left are one side, showing the interior from neck to base, and the sherds on the right are the other side of the jug, showing the exterior. The different glazes can be seen.

A Beginning

I am often asked what conservation is. My advisor at the University of Michigan had only heard of environmental conservation. Five years ago, even I did not fully know what conservation was when I started to explore it as a career path. I double majored in Classics and History of Art, and studied abroad in Greece. I wanted to work in a museum or archive, preferably behind the scenes working with objects. As the care and treatment of objects, conservation seemed perfect. I have since been to half a dozen conservation labs and studios. In order to pursue my dream of becoming a conservator, I applied for a conservation internship at the Kelsey Museum of Archaeology at the University of Michigan. A year later, I have finally started. This blog will be a way both for me to keep track of what I do during the internship and to show people what I want to do.