Getting to Know the Gold
The Conservation of Gold Objects from Panamá and Costa Rica
By Elena Mars, Kress Fellow in Objects Conservation at the CMA
As part of my Samuel H. Kress Foundation fellowship in the conservation department at the CMA, I am studying gold objects made by ancient metalworkers from Panamá and Costa Rica. As is the case with much of the archaeological material of Central and South America, many of these objects were removed from their original contexts without clear documentation of where and how they were found. Without this provenance information, researchers use different analytical techniques to understand the histories of the objects and how they were made.
Most of the goldwork in the CMA’s collection was made through a combination of lost-wax casting and hammering. The lost-wax casting process entails creating an identical model of the object with a beeswax-resin mixture over a core or base material that is typically clay. Additional wax is added to the model to create a circulatory system of sprues, passages through which a liquid material flows into a mold and gates, small openings allowing hot liquid to enter the cavity before passing through and around its internal features until filled. This framework will help the molten metal to flow in and gasses to escape. The wax and core are then covered with a mix of charcoal, sand, and clay that acts as a mold. This mold is then heated at a very high temperature to melt the wax, fire the clay, and prepare the mold for the molten metal. The molten gold alloy can then be poured into the mold, taking on the original wax form. Figure 3 shows a simplified diagram of this process for the CMA’s Lobster Pendant. The actual placement of sprues and gates has not yet been determined.
Through microscopic examination I can confirm that the objects were made with the lost-wax casting technique. Under high magnification I can see microstructures in the metal. As molten metal cools, elements within the alloy can solidify at different times, forming treelike structures called dendrites. Figure 4 shows dendrites in a copper nickel alloy; this image was captured at high magnification with a reflected light microscope, looking at a cut and polished sample of metal. Here, the light gray dendrites are rich in nickel and the dark gray interstices are rich in copper. In cases where the alloy was allowed to cool slowly, the dendrites may be large enough to be seen with the lower magnification of stereomicroscopes. Figure 5 shows the dendrites visible on the surface of this gold figurine.
Another way to understand how these objects were made is through X-radiography, which can show the interior structures of objects. Pendant is a hollow lost-wax casting (fig. 6). In the radiograph two light gray circular spots with dark gray halos can be seen on the tail. These are areas where internal pins were placed to connect the clay core with the outer mold during the casting process. Porosity seen in the figure’s head indicates areas where gas was trapped as the molten alloy cooled. Finally, a major crack is apparent on the tail (fig. 7). The darker gray material around the crack is indicative of a restoration material that is less dense than the original alloy.
Surface flaws typical of the lost-wax casting technique can be seen in many of the CMA’s objects. The spirals emanating from the frogs in figure 8 were made by creating a wax disk and then winding a wax spiral on top of the disk (fig. 9). This wax model was used to make a mold, then the shape was cast into gold as described above.
As the gold flowed into the mold, it did not completely fill in the form of the wax disk, leaving a small open area on the back where the rings of the spiral are visible (figs. 10, 11). This is called a casting flaw and is commonly seen on lost-wax objects.
Due to some inconsistencies of form in the Double Frog Pendant, it is unclear if this object was made by ancient metalworkers. Besides microscopic examination, I am performing elemental analysis to compare the metal alloy with alloys of objects with known provenance.
The gold objects of Panama and Costa Rica were primarily made from gold-copper alloys. Silver is also present as a naturally occurring impurity in the gold. I use X-ray fluorescence spectroscopy (XRF) to understand the ratios of copper, gold, and silver on the surfaces of objects. I can then compare our data with the data of other researchers to see if readings are comparable. This method is also useful in understanding regional trends.
There is much to learn about these objects and the technologies used by their creators. I hope to continue my work with nondestructive techniques and expand my study to collections at other institutions.
On your next visit, take a look at these and other similar objects in gallery 233, and don’t miss the last weekend of the exhibition Fashioning Identity: Mola Textiles of Panamá, closing this Sunday, January 9 in gallery 234!
To learn more about pre-Columbian goldwork in Costa Rica visit:
Metalurgia " Colección de Arqueología " Museo Nacional de Costa Rica
Trabajos de martillado, fundido y moldeado en cobre, oro o aleaciones, dan detalles de manufactura, composición y…
To learn more about the work of other researchers in the field check out:
Harrison, A., and H. Beaubien. “Bringing Context to the Smithsonian Collections of Pre-Colombian Gold from Panama Through Technical Examination and Analysis.” Metal 2010 Proceedings of the Interim Meeting of the ICOM-CC Metal Working Group (October 2012): 266–71, https://core.ac.uk/download/pdf/11341336.pdf#page=267
To learn about hammered gold objects watch: