Friday, June 15, 2012

Art and Chemistry (again...)


In our second writing assignment we were asked to do a case study on a piece of art or analytical technique used in art restoration, conservation, etc. I happened to find a study linking two of my favorite topics: chemistry and Buddhism. The article was about a Buddhist monastery built between 1470 and 1472 AD in Lo Manthang, Nepal. Constructed with wood and rammed mud, the temple has undergone multiple restorations, particularly along the northern wall, due to several earthquakes over the centuries. A new conservation effort began in the early 21st century coordinated between Italian conservators and local Nepalese carpenters in order to repair the building and clean the interior wall murals. These paintings have intricate depictions of the Buddha and other central figures flanked by attendants and surrounded by many smaller designs, as seen below.
a)

b)
Figure 1 a) Interior of Thubchen Lakhang Temple b) East wall mural during cleaning (1)

A study was conducted by Mazzeo et al to characterize the pigments used in the mural paintings through micro-Raman spectroscopy. This technique has some tricky theory behind it but basically, when light hits a molecule it’s photons can be altered due to an increase or decrease in energy from the rotations and vibrations of the molecule’s bonds. The important thing to know is that Raman micro-spectroscopy requires only small samples for analysis and the precision of the technique enables the analysis of individual sections heterogeneous materials like paint. In addition, scanning electron-microscope energy-dispersive x-ray (SEM-EDX) was used to analyze the cross sectioned samples after preparation of samples with a thin layer of graphite. The EDX spectrometer determined elemental composition of the samples and provided a semi-quantitative analysis based on the intensities of the x-rays.
The results were compiled to provide information on both the composition of the different layers of pigment and the method through which they were applied. The blue pigments were determined to be high quality azurite, occasionally mixed with haematite or lazurite. There were also traces of vermilion, a red pigment, and malachite, a green pigment, in some of the blue samples. Excluding one sample, all of the blue paint was applied over a very thin layer of black paint, revealing one ancient painting technique. Haematite, or anhydrous ferric oxide (Fe2O3), and vermilion, HgS, were identified, both together and separately, in the various red and orange-red paint samples. Vermilion was also distinguished on one sample from along with pararealar (arsenic (II) sulfide) and orpiment (arsenic (III) sulfide). These particles were used to hold gold leaf in place on the Buddha’s face by a technique known as gilding in which thin gold or silver pieces or powder are attached to an artwork. The particular gilding technique used was very unusual compared to other murals, which employed oil mordant to affix the metallic leaf. The brown layer in one paint sample was unable to be characterized due to fluorescence on the Raman spectrum. Further identification of this pigment and an unidentified yellow ochre pigment are needed. Malachite (CuCO3*Cu(OH)2) and brochantite were determined in the green paint along with trace amounts of azurite and vermilion. The brochantite particles were found only on the outermost layer of the green paint and may be a result of the interaction of sulfur in the atmosphere with the original malachite pigment. Mural preparation and application methods, previously theorized by locals, were confirmed with the identification of two preparation layers beneath the exterior paint.
The ultimate goal of the researchers involved in the study was to aid in the authentication, conservation, and restoration of these ancient Nepalese wall paintings. The major issues in the conservation of these mural paintings are the clay dripping down the walls from the roof of the temples and the air pollutants that may be affecting the pigments. This research is the first step in the long arduous process of preserving the artwork, especially since the materials and techniques used in the initial creation were not well known previously. From this starting point, more studies should be conducted to gain more information about the murals to further guide conservators in how best to maintain this Nepalese monastery for another 500 years.

Works Consulted

1. Mazzeo, R.; Baraldi, P.; Lujan, R.; Fagnano, C. J. Raman Spectrosc. [Online] 2004, 35, 678-685. DOI 10.1002.

2. Science of Micro Raman Spectroscopy. http://www.microspectra.com/support/technical-support/raman-science/35-technical-support/126-science-of-micro-raman-spectroscopy (accessed 6/14/12). Part of Craic Technologies.

3. Raman Spectroscopy. http://en.wikipedia.org/wiki/Raman_spectroscopy. (accessed 6/13/12).

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