Copper, corrosion and curbing the damaging effects of Bronze Disease

Editor’s Note: Alexis North is a third-year graduate student in Conservation of Archaeological and Ethnographic Materials at UCLA. She specializes in the conservation of archaeological objects and is working at the Michael C. Carlos Museum at Emory University this summer, preparing a group of objects for display here at HMNS. Read the first blog from her series here.

You may think of metal as a strong, impervious material. It’s used in bridge and building construction, and many of the tools we use today are made of metal (like silverware, hammers and screwdrivers, medical scalpels, etc.). Despite its strength, however, metal can be one of the more fragile materials found in archaeological sites. This is because different types of metal can very easily corrode in the presence of moisture and salts, both of which are found in the burial soils of archaeological sites. If you’ve ever seen red rust on an iron fence, or an old penny turn green, then you’ve seen what corrosion can look like.

Five of the objects I am working on this summer are made of copper alloy. An alloy is a mixture of metals. Copper is most often alloyed with silver, tin, arsenic or lead (or any combination of those) and the resulting mixture will have different strengths and working properties depending on the components and the proportions of those components. Here at the conservation laboratory at the Carlos Museum, one way we can determine which metals are present in an alloy is by using X-ray fluorescence spectroscopy (XRF).

XRF analysis uses X-rays to excite the electrons within a material. These electrons jump to a higher energy level when they come into contact with the X-rays. The electrons of each element give off a characteristic amount of energy when they return to their unexcited state.

By measuring the amounts of energy emitted, we can determine which elements comprise a certain object. Here, the XRF spectrum of the cat figurine seen in my first blog post shows that the metal is an alloy of copper (Cu) and lead (Pb), with a possible trace amount of silver (Ag). The iron (Fe) most likely comes from the burial environment.

Copper, corrosion and curbing the damaging effects of Bronze DiseaseXRF spectrum of 1999.001.043, revealing copper and lead as major components.

Copper and its alloys are susceptible to several different types of corrosion, some of which are good or protective corrosion, and some of which can be very damaging to the objects. After a copper alloy object is buried, it forms a protective layer of copper oxide (cuprite) on its surface. Cuprite can be bright to deep red in color, and will preserve the original surface of the object, even when additional corrosion layers form on top. That upper layer of corrosion is usually made of copper carbonates, called malachite and azurite. These compounds are bright green and blue in color, respectively, and have historically been used as pigments, in Egypt and elsewhere.

The real bad boys of copper corrosion are the copper chlorides. These appear as a pale turquoise green compound, usually in spots on the metal’s surface. When copper metal comes into contact with chloride anions, it forms deep pits full of copper chlorides. These pits disrupt the metal’s surface, damaging the original appearance of the object and obscuring surface details. These pits are also autocatalytic, meaning that once one appears, it will continue to grow and form additional pits until the copper chlorides are removed. This cycle of corrosion is commonly called “Bronze Disease,” like a kind of copper Chicken Pox!

Copper, corrosion and curbing the damaging effects of Bronze DiseaseSchematic diagram of copper alloy object with various types of corrosion products.

All five copper alloy objects that I am working on show evidence of Bronze Disease, as well as malachite and cuprite formations. The cat figurine has very little corrosion, and will not require much treatment at all before it will be ready to pack up and ship to the HMNS. This mirror, on the other hand, has significant corrosion all over its surface. In the detail image on the right, you can see where I’ve found an area of Bronze Disease, and the powdery light green copper chlorides are erupting onto the surface.

Copper, corrosion and curbing the damaging effects of Bronze DiseaseBefore treatment image of copper alloy mirror (left) and close-up image of Bronze Disease pit with copper chloride corrosion products (right).

Treating Bronze Disease is a two-step process. First, the copper chlorides must be mechanically removed. I do this using a variety of tools, including scalpels and dental tools (if they work for cleaning your teeth, then they should work for cleaning copper!). The copper chlorides are gently scraped away, while making sure that I don’t damage the rest of the mirror’s surface. The pits made by the copper chlorides are carefully cleaned out, so they can then be chemically treated to help prevent the formation of new copper chlorides. Once the corrosion products have been removed, the objects are treated with Benzotriazole (BTA), a corrosion inhibitor that forms a stable coating with the superficial copper ions, so they cannot react with any chloride ions which may come around.

Corrosion cannot be stopped completely, but these treatments help to significantly slow down the deterioration process, allowing the objects to continue to be displayed and studied. While the corrosion may not be vanquished entirely, with careful consideration the right conservation treatment can be undertaken, allowing these objects to be enjoyed both by scholars and museum visitors like you for many years to come!

References:
“Benzotriazole,” Conservation and Art Material Encyclopedia Online (CAMEO), Museum of Fine Arts, Boston, http://cameo.mfa.org/wiki/Benzotriazole, accessed 7/16/2013
Scott, David A. Copper and Bronze in Art: Corrosion, Colorants, and Conservation. Los Angeles: Getty Publications, 2002.

From Hotlanta to Houston: A Carlos Museum conservation specialist shares her work prepping artifacts for travel

Hi everyone! My name is Alexis North, and I am working at the Michael C. Carlos Museum at Emory University this summer preparing a group of objects from their collection to come to Houston to be installed in the new Hall of Ancient Egypt. I am currently a third-year graduate student at the UCLA/Getty Program in the Conservation of Archaeological and Ethnographic Materials, specializing in the conservation of archaeological objects. I have worked on archaeological sites in Kenya, Greece, and Turkey, and have experience conserving a variety of objects, from antiquity through the 20th century.

I arrived in Atlanta a few weeks ago, and have been getting situated in my new surroundings, learning all about the Carlos Museum and its amazing collection of archaeological and ethnographic objects.The objects from the Carlos Museum that will be traveling to Houston include some fantastic examples from all areas of Egyptian life, including tools, clothing, and ritual and funerary objects. A few of my favorite objects are: a colorfully painted coffin in the form of a falcon, with red, yellow, blue, white, and black pigments; a child’s pair of leather sandals with detailed stamped decoration; and a wonderful copper alloy cat figurine.

From the Carlos to the Coast

Before treatment photos of the falcon coffin (left), leather sandals (center) and cat figurine (right).

I have been spending the last couple of weeks pulling the objects from storage, examining them and documenting their condition. Some of them had just been taken off display here at the Carlos, and some have long been kept in storage. All of them are due for a thorough check-up, and I am so excited to have the opportunity to work so closely with such a great collection of Egyptian objects.

Archaeological objects have many different condition issues, which can affect their appearance and stability. The painted coffin, for example, has many areas of paint loss on its surface. This is usually caused by the wood structure underneath, which swells and contracts with changes in humidity, making cracks in the paint layer that can cause lifting and flaking. The areas of loss on the falcon coffin will have to be carefully stabilized before the coffin can be packed for travel to Houston.

I’ll share more of my work as I start to dive into the treatment process. I look forward to working on these amazing objects, and sharing all my findings with you!

Kid Curators lit up the mic at Wednesday night’s Intellectual Insights Q&A

On Wednesday evening, we hosted the inaugural Intellectual Insights, an innovative lecture/question-and-answer session helmed by Hall of Ancient Egypt curators Dr. Dirk Van Tuerenhout and Tom Hardwick, with the Carlos Museum’s Dr. Peter Lacovara joining remotely via a video call.

We took questions from a members-only live audience and from our HMNS Twitter feed, so our online friends could join in the fun. It was all pretty nifty, but the highlight had to be our Kid Curators: Jacob Blackman and Abby Myers.

KidCurators

Dr. Dirk Van Tuerenhout, Abby Myers, Jacob Blackman, and Tom Hardwick.

We revived our Kid Curator program earlier this week by asking young Egyptology fans to submit a one-minute video explaining why they’d be best suited to help us open up our new Hall of Ancient Egypt alongside our bona fide big-guy curators.

We were so inspired by our young fans that even with our Museum President weighing in, we had a two-way tie between four fabulous finalists, which led to the selection of two Kid Curators. Both Abby and Jacob were on-hand at Intellectual Insights to grill our curators with their questions, and had the chance to show off a few objects to our live studio audience that’ll end up in the new Hall of Ancient Egypt. In the coming weeks, they’ll also get to bask in flashing lights at a press opp, get a personal guided tour of the new hall and earn their very own Museum memberships.

In appreciation of all our entrants (more than 40!), each hopeful Kid Curator received tickets to Intellectual Insights, with the finalists also receiving family tickets to view the Hall of Ancient Egypt when it opens later this month.

Check out our super-talented finalists, below:

Kid Curator: Abby

Kid Curator: Jacob

Runner-Up: Marcel

Runner-Up: Lorena

Field Notes: The vividness of Ramesses II at Abydos

Editor’s Note: Peter Lacovara, Senior Curator at Emory University’s Carlos Museum, has worked on numerous expeditions in Egypt and published several books on his work and experience, including The Pyramids and Sphinx, Tombs and Temples of Giza, and Excavating Egypt: Great Discoveries from the Petrie Museum of Egyptology. Other blogs in this series can be found here.

While not nearly as well known as his father’s temple at Abydos, Ramesses II also built a temple there which is open for tourists to see. While it is smaller, less well-preserved and not as finely carved as Seti’s temple, Ramesses II’s monument is notable for the vivid color that is preserved on many of the remaining walls.

MCCM R II reliefMany of the kings of the New Kingdom built temples at Abydos to honor Osiris at the edge of the desert, but few are well preserved. Ramesses’ temple used both limestone and sandstone along with black granite for the doorways and alabaster for the central shrine.  This temple was built during his early reign, when he ruled alongside his father, and the quality of some of the carving approaches that of Seti.

Ramesses II Tempe Abydos offering procession

While Osiris was the chief god worshiped here, many other deities where honored in the temple. A head of the god Amun with ram horns and filled in with blue paint, now in the Michael C. Carlos Museum, may have come from this very destroyed structure.