As part of the Infinite Blue exhibition, conservators have been charged with identifying different kinds of blue media in the artworks that are part of the show. We have used several techniques to investigate this question, such as multiband imaging which is highlighted in previous posts, the Fabergé watch-clock, and Eisen Keisai woodblock print.
Nancy Rosoff, Andrew W. Mellon Senior Curator of Arts of the Americas, also asked if we could identify the type of stone used in two pre-Columbian objects, a Chimú necklace (top) and Wari figure (bottom left), which will be part of the show in May.
The blue/green beads in the necklace and stone from which the Wari figure is carved, had been previously identified as turquoise, but we sought to see if we could confirm their identification using analytical techniques. Turquoise is a mineral that can be found in many parts of the world, including the pre-Columbian Andes, where both artifacts are from. The stone consists of copper, aluminum and phosphorus, with the chemical composition CuAl6(PO4)4(OH)8·4H2O. The appearance of the stone varies widely depending on where it is sourced, and the ratio of minerals in its composition, making it sometimes tricky to identify visually.
For example, the beads on the necklace range in color from olive green to bright pale blue, some with veining and some without. Above on the right is a magnified view of the beads.
Chrysocolla and serpentine are two other kinds of blue/green stone that are found in objects from the same region, which were used to imitate turquoise. They have different chemical compositions from turquoise however, which meant that if we could define the elements present in the artifacts, we could identify the stone more precisely.
An XRF spectrum from the necklace beads, showing the presence of aluminum, phosphorus and copper, as well as iron and arsenic. 15kv, 32µA, 60s, vacuum
To do this, Dawn Kriss and I used portable X-ray Fluorescence (p-XRF), a non-destructive technique available in the lab. With this process, chemical elements present in a sample are identified using an x-ray source, based on their fluorescence. Using this technique, we found that both the blue/green beads in the necklace and the Wari figure are primarily comprised of copper and include aluminum and phosphorus. These compositions are indeed consistent with turquoise! In addition, there was no evidence of silicon, which would be present if the stone were chrysocolla, Cu2H2Si2O5(OH)4. It also allowed us to rule out serpentine, Mg3Si2O5(OH)4 because serpentine does not contain copper. The other colored beads on the necklace were identified as shell, which is primarily comprised of calcium carbonate. The specific type of shell cannot be distinguished with XRF, but past curators have identified them as spondylus (orange) and clam (purple).
Posted by Sara Levin
