Guest blogger: Danielle Montanari, U of U chemistry Ph.D. candidate/UMFA conservation volunteer
If you’ve been following this blog, you might remember the previous post detailing the major grant the UMFA received from the Institute of Museum and Library Services (IMLS) to evaluate and rehabilitate some of the metal objects in our collections. The nearly $100,000 grant funded the evaluation of the nearly 800 metal objects in the UMFA’s Asian and African collection and the remediation of some 200 objects that showed evidence of a common corrosive condition. The UMFA collaborated with the University of Utah chemistry department to accurately identify those objects in need of critical care. Here’s a closer look at how it worked:
Many metal objects in the UMFA collection no longer appear metallic. Sometimes this is intentional; many artists condition or treat the surface of the object to provide it with a protective layer. Coincidentally, these protective layers—known as patinas—can also color the object. This is why many bronze statues have a beautiful, uniform green or bluish color!
The type of material or mineral used to treat the surface dictates the final color of the object. When done correctly, patinas can protect a bronze object for hundreds of years. Often bronze objects that have patinas are very stable and not at any great risk for corrosion if kept in a dry environment. However, many metal objects are excavated from earthen sites which are not protected from moisture and rough surfaces, which can yield small cracks in the patina. These cracks expose the underlying metal to moisture and minerals in the earth, which can react in one of two ways: either the metal is once again stabilized and a “patch” of patina forms where the crack was, or the metal starts to slowly be dissolved through exposure to the moisture and minerals. This unhealthy process, in which the metal is slowly dissolved, is sometimes called “bronze disease.”
This name is a bit misleading, as the corrosion associated with bronze disease won’t spread between objects like a biological disease or infection. Much of the time, the corrosion process can be slowed simply by creating a dry environment to house the object. More severe cases require treatment, which brings us back to the goal of the IMLS grant to evaluate and rehabilitate objects as necessary.
Some types of damage are visible to the eye, but often the coloration of unhealthy bronzes looks very similar to that of healthy bronzes. For example, using our eyes to collect information, we can see about three different colors of blue/green on the surface of the object shown below, but we don’t know which colors suggest unhealthy bronze. To determine if the bronze objects in our collection are healthy or unhealthy, we turn to chemistry. Chemistry gives us what our eyes cannot; it allows us to look closer at the different colors on the surface of this object and see what minerals those colors correspond to. Certain minerals strongly indicate the presence of bronze disease, because they contain the element chlorine. If we find minerals on the surface that correspond to unhealthy bronze, we can safely say that that object needs treatment.
We identified three minerals on the surface of this Peruvian object—malachite, atacamite, and azurite. Of these three, atacamite contains chlorine, which is the “smoking gun” that indicates something is unhealthy in the lighter spots on this object. Now that we know this object needs treatment, the collections department can work their magic and stabilize the surface.
Danielle Montanari gratefully acknowledges support from the National Science Foundation MRSEC, DMR-1121252.
Danielle Montanari is a Ph.D. candidate in the University of Utah’s chemistry department. She has been helping the UMFAs conservator Robyn Haynie answer some questions about the health of our bronze objects.