Micro-XRF Imaging of a Tibetan Buddhist Thangka Painting

Within the field of Heritage and Conservation Science, painted objects are one of the most complex from the point of view of XRF analysis due to their immanent nature of being a multilayered system that consists of a variety of materials and pigment mixtures. Since the development of open-beam systems, micro-XRF scanning has established itself as a standard technique in the examination of painted objects. Its non-invasive nature and imaging capabilities of surface and sub-surface preparatory and paint layers allows us to directly glimpse over the shoulder of the artist at work. The wide range of elements that can be identified by XRF analysis can be used to gain knowledge on the entire pigment repertoire of artistic workshops, while its fast nature facilitates the study of large quantities of objects.

The following case study on a thangka demonstrates the capabilities of spatially resolved micro-XRF analysis using an M6 JETSTREAM equipped with a polycapillary lens with a smallest spot size of 33 µm.

A thangka is a Tibetan Buddhist painting on cotton or silk, often depicting a deity, scene or mandala. These artworks are traditionally unframed and rolled up when not in use. While most thangkas are small, some can be several meters large and are displayed briefly during religious festivals. They are primarily used for personal meditation or teaching, often featuring intricate compositions with a central deity surrounded by smaller figures [1, 2].

Fig. 1: Tibetan Buddhist thangka painting, mixed media on canvas, 9.3 x 7.8 cm, private collection.

The thangka painting analysed is a small-size painting (9.3 x 7.8 cm) on an unmounted piece of canvas. The aim of this analysis is to identify the pigments and technology used in its creation. Traditional techniques from a religious context follow a very rigid sequence in terms of the application of colors and the details painted [3]. Comparing the technology of the Thangka here allows us to deduce its purpose, whether it was created in a religious context by following established workflows or if it is a replica.

The thangka was measured with a double 60 mm² SDD M6 JETSTREAM system at 35 kV and 800 µA with a spot size of 33 µm, 100 µm pixel size, and 70 ms/ px measurement time. The object was placed on a free flying PE foil to reduce background scattering and any contribution from the table below.

Alike traditional techniques, the canvas was prepared with a thick white priming. This was probably applied in several steps, as in areas in which the priming was lost thin residues remain that are detectable with a low intensity in Ca and S (gypsum) and Pb (lead white) distribution (Fig. 2).

Fig. 2: Elemental distribution images of Ca-Kα , Pb-Lβ, K-Kα, Co-Kα and Cu-Kα and As-Kβ which are inter alia related to priming an initial coloring of the prepared canvas. Combining the data evaluation with microscopic examination allows to allocate elemental signals to certain layers.

The composition was drawn onto the white priming using different materials: Visible below the microscope are black and red lines. A pen underdrawing traced with a black ink is described as traditional technique [3] and could also be identified in a 18th century case from the British Museum [2] but is here complemented for certain figures with red ochre sketches, that becomes clearly visible in the Fe distribution images and was only partly covered with paint thereafter (Fig. 3). This partly integration into the final composition seems to have been done purposely, as the final depiction of the two figures in the lower part of the thangka are based on the visibility of these underdrawing lines, while the Buddha in the upper center of the thangka was painted in a more complex process with multiple layers (Fig. 3). 

Micro-XRF analysis with the M6 JETSTREAM could reveal the remarkable way in which the eyes have been underdrawn: A single outline gives the location of the eyes. In traditional Tibetan Buddhist painting, the “opening of the eyes” is one of the last steps of paintings, as it is an important religious step in the creation of the painting. Here, the depiction of the eyes is in a similar fashion left to the final steps of painting (Fig. 3).

After finalizing the composition, the paint process started traditionally with the application of thin coloring layers applied in a specific color sequence [3]. Painted top to bottom, starting with the sky and a blue wash, that is then covered with single brushstrokes of lighter color [3]. Afterwards, all blue areas are painted, whether it is water, clothes, etc. Green areas are next, followed by light blue, light green, red, orange, pink, brown, pale orange, pale yellow and finally white [3]. Fine details and outlines are afterwards traced in black ink. Finally, fine details of the depiction and the body color of the deities are painted.

In this example, Pb signals are generally present throughout the entire surface and seem to have been used throughout the painting process. The quite unspecific distribution of high-energy Pb-Lβ signals, that show intensity variations that do not keep to the borders of the today visible depiction, indicates the utilization of sub-surface layers applied for initial coloring (Fig. 2. Pb-Lβ). Its distribution correlates with a light blue layer, that probably contains lead white and smalt (Co, Ni, K, Si). This light blue layer is integrated into the final depiction in the sky, water and mandorla surrounding buddha. As in the traditional techniques described in [3], blue underpaintings are among the first steps of painting. These are followed by thin layers of green, which we see in this example in the partly exposed thin paint layers containing lead white and a copper arsenate green (Fig. 2, Cu-Kα).

Fig. 3: Fe-Kα distribution showing the drawing stage of the three figures depicted. In the case of the smaller figures, these red ochre lines are still clearly visible and integrated into the final depiction, while the compositional drawing of the Buddha is covered by a more complex paint application that testifies to the higher importance of this figure.

The Cr distribution, here overlapped with the Cu map, partly shows the second green pigment present, which was only added during a conservation treatment, as it is present in cracks and areas of losses of the original paint (Fig. 2, Cr-Kα and Cu-Kα). In the original painting, Cr is present as a red-yellow pigment covering the entire painted bright red frame and gives it a duller color. Why the object was reworked later cannot be said. What becomes clear, however, is that Cr-based pigments are not used in the original composition.

istorically, the paint was self-made by mixing pigments created out of minerals, precious stones, bark, leaves, flowers, gold, silver, or copper with a binding agent [3]. Today, tube colors are often used. Original pigments used for the second step of paint of the thangka (Fig. 4), in which fine details were created, contain vermilion (HgS), red and yellow ochre (Fe, Mn), and copper arsenate (Cu, As). Interesting here is the use of ultramarine (Si, S, and K) for bright blue areas (Fig. 2, Si-K) in contrast to the slight blue hue used for initial blue paint application based on smalt (Fig. 2, Co-Kα). Gold areas are made of a silver-containing shell gold alike traditional recipes (Fig. 4, Au-Lα and Ag-Lα) [3].

The pigments, identified with the M6 JETSTREAM, offer insights into the dating of the painting: Smalt was used at least in Europe only until the 18^th century [4]. It might have remained longer in use in other parts of the world, as especially the utilization of copper arsenate green (Fig. 4, As-Kα) hints at the creation of the thangka in the 19^th century [4]. Due to the use of lead white, a terminus ante quem can be set to the first half of the 20^th century [4]. Interesting to note is that though Cr-green pigments were already available on the market in 19^th century [4], these were not used in the present case. The canvas itself is industrially produced, as the consistent weave structure shows, which likewise fits into the 19^th century.

Fig. 4: Elemental distribution maps revealing the original materials used. From left to right: Lower energy lead (Pb-M) signals showing the fine details painted with mixtures of pigments and lead white for the final depiction. Mercury (Hg-Lα) signals from vermilion red. Arsenic (As-Kα) correlating with green areas, where copper arsenate was used. Potassium (K-Kα) partly in parts hinting at the use of ultramarine. Gold (Au-Lα) and traces of silver (Ag-Lα) from the shell gold decoration.

Areas of damage can be found all over the object. Due to the fragile structure of the thangka, paint and priming layers have been lost along the edges and in the lower part of the painting. In these areas, an enrichment of Zn and Cl is notable.

No matter the actual date the object was created, the thangka reveals an intricate complex painting technique that resembles traditional processes of Tibetan Buddhist paintings despite its small size. Using a spot size of 33 µm, spatially resolved micro-XRF analysis using the M6 JETSTREAM can visualize even the very fine details of the underdrawing or the paint layers, such as the rainbow that is decorating the mandorla of Buddha. 

The elemental distribution not only provides insight into the creation date of the thangka, but also to the layer sequence, the painting technique, the state of preservation and later additions. By providing users with all of this information micro-XRF scanning enables a holistic study of art objects. Combined with microscopic examinations, the genesis of the thangka can be deciphered, even if the object has been extensively reworked.

1. Dyer, J.; Derham, A.; O’Flynn, D.; Tamburini, D.; Heady, T.; Ramos, I. Studying Saraha: Technical and Multi-Analytical Investigation of the Painting Materials and Techniques in an 18^th Century Tibetan Thangka. Heritage 2022, 5, 2851–2880. https://doi.org/10.3390/heritage5040148.
2. Gopa Bhaumik, Mahesh Chandra Govil: Conserving Thangka − A technical approach unto the preservation of Buddhist Thangka through automation. Digital Applications in Archaeology and Cultural Heritage, Volume 18, 2020. https://doi.org/10.1016/j.daach.2020.e00149.
3. Cotton9. Art Of Thangka Painting. https://www.cottage9.com/art-technique/art-of-thangka-painting/ (Accessed 12.08.2024).
4. Eastaugh, Nicholas; Walsh, Valentine; Chaplin, Tracey: Pigment Compendium. A Dictionary and Optical Microscopy of Historic Pigments. 2008, 1st ed. Oxford: Elsevier.