Blythe E. McCarthy and Janet G. Douglas

Methods Used for Jade Studies in the Department of Scientific Research

Over the years, a variety of methods have been used to study stone materials belonging to a class loosely termed “jade.” Jade is a complex material, and much of its study at the Freer and Sackler has been aimed at an understanding of its visual appearance and physical characteristics such as mineral composition, color, translucency, and surface alteration.  More extensive studies have focused on learning about early Chinese cultures through the study of their jade objects, such as the study of minor elemental composition which can inform us about the geological sources of their raw materials, and microscopic study of tool marks which provide clues to the working methods used to create them.

The following digest contains many of the basic methods that have been used at the Freer and Sackler and also associates them with the principle investigator when known. Some of these methods are more precise and reliable than others, and some require sampling while others do not.

Specific gravity determination

Scientist: unknown
Years used: 1940s
Specific gravity is the ratio of the density of a material to the density of a reference material. Specific gravity measurements on jades were calculated by dividing the object’s weight in air by its weight in carbon tetrachloride, and multiplying the result by a factor to correct for temperature differences (1.591 at 22oC, 1.588 at 23oC, and 1.570 at 24oC). This technique can be inaccurate for determination of the mineral composition of jade because these materials can contain impurities which can affect the overall specific gravity.

Mohs hardness determination

Scientist: Elisabeth West Fitzhugh
Years used: 1958-1980
Mohs hardness determinations result in a numerical value that indicates the relative position of a mineral’s hardness on the Mohs scale of hardness, which was developed in 1812 by the German mineralogist Friedrich Mohs (1773-1839). The Mohs scale consists of minerals of increasing hardness with values from 1 for the softest (talc) to 10 for the hardest (diamond). Mohs hardness is determined with a test kit with sharpened points of each mineral. The testing is done on a small, inconspicuous area on a jade viewed under an optical microscope. A hardness value is assigned by determining the hardest mineral on the scale that will not scratch the jade being tested. For instance, nephrite has a Mohs hardness of 6-6.5, and jadeite has a Mohs hardness of 6.5-7, thus these two jade materials can be differentiated.

X-ray diffraction

Scientists: Elisabeth West Fitzhugh, Janet G. Douglas
Years used: 1955 – 2007
X-ray diffraction of a small powder sample mounted with adhesive on a single glass fiber was performed using Gondolfi cameras mounted on a Phillips XRG2600 x-ray source with Cu K-alpha radiation at 40 kilovolts and 20 microamps. The sample spindles were placed in a Gandolfi camera with a motor for sample rotation and exposed to x-rays for 12 – 24 hours. The resulting film was developed and compared to previously analyzed reference samples, or d-spacing were determined and compared to x-ray diffraction standard data from the Joint Committee on Powder Diffraction Standards (JCPDS), International Centre for Diffraction Data.

X-ray fluorescence spectroscopy (XRF)

Scientist: Janet G. Douglas
Years used: 1991-2012
X-ray fluorescence spectroscopy (XRF) was performed to determine minor elemental compositions of a selection of jades in the Freer and Sackler collections in a manner which did not require sampling. An OMEGA FIVE x-ray fluorescence spectrometer, a modified Spectrace Model 6000 with a rhodium target x-ray tube and a silicon-lithium (Si(Li)) detector, was used for this purpose. The instrument had an open X-ray beam architecture, and helium was bled into an attached chamber to replace air, with the jade placed exterior to the chamber, on the other side of a Mylar window. X-ray beam positioning on the surface of objects was accomplished using a double beam helium-neon laser alignment system. Various instrumentation limitations along with the uneven and possibly altered jade surfaces analyzed dictate that the elemental compositional data are semi-quantitative.

Analysis conditions used were 15 kilovolt tube voltage, 0.99 microamps tube current, no filter, 200 seconds live time, with a helium chamber. XML peak fitting and quantification was done using the Fundamental  Parameters Program (Version 1.34a) provided by Spectrace, Inc. Elements determined by weight percent include potassium, calcium, titanium, chromium, manganese, iron and nickel. The sum of the magnesium + aluminum content was determined by difference. Details of the technique are given in Janet G. Douglas, “The Study of Chinese Archaic Jades using Non-Destructive X-Ray Fluorescence Spectroscopy,” Acta Geologica Taiwanica 32 (1996): 43-54.

Much of the work using this analytical method is reported in Janet G. Douglas, “Exploring Issues of Geological Source for Jade Worked by Ancient Chinese Cultures with the Aid of X-ray Fluorescence Spectroscopy,” Scientific Research in the Field of Asian Art: Proceedings of the First Forbes Symposium at the Freer Gallery of Art, ed. by Paul Jett with Janet G. Douglas, Blythe E. McCarthy and John Winter, 192-199. Washington: Archetype Publications in association with the Freer Gallery of Art, Smithsonian Institution, 2003.

Color determination using the Munsell Color System

Scientist: Janet G. Douglas
Years used: 1996-1997
The Munsell Color System was developed by A. H. Munsell in the 1930s and identifies three color dimensions: hue, value (lightness), and chroma (color purity). These three attributes are arranged into an orderly scale that can be used to accurately describe color. The color designation is written as hue value/chroma, as in the example  10 GY 6/4. The Munsell color notations  are determined by comparing the sample with small color chips from the Munsell Book of Color^[1] under the conditions  of average daylight, 45° illumination, and normal viewing. Color determination using the Munsell Color system is simple. However, it can be challenging due to the mottled color quality of many jades. The most typical color in the nephrite is recorded. Color names are derived from a standard dictionary of color names based on the Munsell system of color notation. ^[2] Color names, also based on hue, value, and chroma, are designed to be simple and easily understood by most people.

Infrared Spectroscopy (IR) and Scanning Electron Microscopy (SEM)

Scientist: Wen Guang, Fellow, with Janet G. Douglas (done in Beijing, China)
Year: 1997
A Perkin-Elmer 983 infrared spectrometer was used at the Institute of Geology, Chinese Academy of Geological Sciences in Beijing, China, to collect spectra of small powder samples removed from jade objects. The sample microstructure was also imaged using a Hitachi-450 scanning electron microscope. These methods were used for mineral identification and to assess jade quality. Processing of the spectra from 3600 to 3700 cm-1 was used to determine iron to magnesium ratios following methods described in Guang Wen and Zhichun Jing, “Chinese Neolithic Jade: A Preliminary Geoarchaeological Study,” Geoarchaeology, vol 7 (1992), no. 3, 251-275.

Fourier Transform Infrared Spectroscopy (FTIR)

Scientist: Janet G. Douglas
Years used: 1996-2012
Fourier transform infrared spectroscopy (FTIR) was first performed on a Mattson First FTIR (1993 -2000), and subsequently using a Thermo-Nicolet Nexus 6700 spectrometer with a Spectra Tech IR-Plan Advantage Microscope (2001 - 2007).

The Thermo-Nicolet Nexus 6700 spectrometer was coupled with a Thermo-Nicolet Continuum Microscope with an MCT/A detector (2008-2015). The method included removing a small sample to be placed on a diamond microcompression cell, and the spectra were collected in transmitted light, at 4 cm-1 resolution, 128 scans with a scan range of 4000 to 650 cm-1. Spectra were identified by comparison to internal laboratory reference materials and reference spectra in the IRUG (Infrared and Raman Users Group) Spectral Database.

Portable Fourier Transform Infrared Spectroscopy (FTIR)

Scientists: Janet G. Douglas, Xiao Ma
Year: 2014 to current
Diffuse reflectance measurements were made using a portable Agilent Technologies 4100 Exoscan FTIR. The method uses a spectral resolution of 4 cm-1; diffuse sampling head with 100 micron reference cap, 128 background scans, 128 sample scans, with a scan range of 4000-650 cm-1. Spectra were identified using laboratory reference materials, the IRUG (Infrared and Raman Users Group) Spectral Database, as well as online databases such as RRUFF and other online published literature.

Visible-Near Infrared Spectrometry (Vis-NIR)

Scientists: Janet G. Douglas, Matthew Clarke, Xiao Ma
Year: 2015
Vis-NIR measurement was performed using a portable Vis-NIR Fieldspec 4 high resolution Spectroradiometer using a reflectance method that does not require sampling. The method using a resolution of 3nm @ 700nm, 8nm @ 1400/2100 nm, scanning time of 100 milliseconds with a scan range of 350-2500 nm. Three detectors were used including a VNIR detector (350-1000 nm): 512 element silicon array, a SWIR 1 detector (1001-1800nm): Graded Index InGaAs Photodiode, and a SWIR 2 detector (1801-2500nm): Graded Index InGaAs Photodiode.Vis-NIR spectrum were identified using comparison spectra of known reference materials, online databases such as RRUFF as well as other online published literature.

Notes