Jade was the material most highly prized by the ancient Chinese. Its polish, brilliance, subtle and translucent colors, and extreme toughness have long been associated with the quality of virtue and the concepts of the soul and immortality. The Freer|Sackler’s collections of jades include works of exceptional artistic quality, as well pieces of great cultural, historical, and sociological importance. Searching Open F|S, our fully digitized collection, is a superb way to become familiar with this unparalleled group of jade objects.
Next year, we will further expand access to our holdings with an online jade catalogue. As we prepare to make the evolving field of research on Chinese jades available to the public, we decided to test objects not previously examined for mineral composition to ensure our reports are complete and accurate. Jade is a chemically complex material, and studying its mineral composition is one way to understand more about the choices made by artisans in ancient China. The material properties can provide insights into jade sources, how jades were worked, and how they have changed since they were made.
Xiao Ma, a recent intern at the Smithsonian’s Museum Conservation Institute (MCI), and I spent a few days studying the jades using two noninvasive methods: portable Fourier Transform Infrared spectroscopy (FTIR) and Visible-Near Infrared spectroscopy (Vis-NIR). Both techniques deliver data directly from the surface of a jade in just a few minutes, and because no sample is required, there is no risk of damage to the object.
How do FTIR and Vis-NIR work? The light we typically see is within the visible portion (400–760 nm) of the electromagnetic radiation spectrum, or the part that human eyes can detect. The light we cannot see, above and below the visible portion, plays an important role in analyzing the composition of materials. For instance, the FTIR technique uses mid-infrared light (2500–25000 nm) to interact with the molecules that make up a jade object. As the infrared light is reflected back to a detector, FTIR measures how well the jade absorbs it at each wavelength, helping us create a molecular fingerprint.
Vis-NIR spectroscopy uses light in the visible region (400–760 nm) and overtones in the near-infrared region (760–2500 nm) to measure light that a material absorbs and scatters. Using this method, we placed jade objects near a light source; the light reflected from the object was collected by the Vis-NIR’s photodetector. The Vis-NIR spectrometer is particularly useful for analyzing mineral composition, as it is highly sensitive to electron transitions in both the visible and near-infrared regions.
We analyzed a total of 103 jades, including bi disks, cong tubes, pendants, ornaments, beads, and axes. Besides the most common jade material, nephrite (tremolite/actinolite), we also found other minerals, such as serpentine, quartz, and diopside. Our research methods and findings promise to serve as guidelines for museums to quickly and noninvasively analyze jade collections. Stay tuned to see more discoveries in the online catalogue when it is released next year.