Original article: Tracking the transformation and transport of arsenic sulfide pigments in paints: synchrotron-based X-ray micro-analyses

If you have read the novel The name of the Rose by Umberto Eco, you may remember the poisonous book by Aristotle that killed monks living in the convent (there is a new tv series based on this novel!). The book was poisonous because it contained arsenic. Artists have used arsenic pigments, as well as other poisonous materials, since antiquity. Orpiment (As₂S₃) is a yellow pigment with a hue similar to gold (which is the reason why it was called “aurpigmentum” from the latin aurum “gold” + pigmentum “pigment”), while realgar (AsS) is a bright orange pigment. These were the two most common arsenic sulfide pigments used by artists, and they were not only dangerous to the artist but were also known for their instability, leading to the production of chemical reactions throughout the painting stratigraphy.

Orpiment and realgar were obtained from natural sources such as minerals or by synthetic processes well known to artists. Less toxic materials, such as cadmium yellow, substituted these pigments during the 19th century; however, arsenic-based pigments are still found in many paintings and manuscripts preserved in collections worldwide. It is well known that these beautiful pigments lose their brightness and become whitish to translucent with time. The main cause of this degradation is light, which induces an oxidation process that transforms them into arsenic trioxide (As₂O₃) and arsenate (AsO₄^3-). According to Vermeulen, et al., a rearrangement of arsenic pigment structure is produced after light exposure, forming As-As bonds and releasing hydrogen sulfide, contemporary with the formation of As₂O₃. In particular, realgar is transformed into an intermediate phase called pararealgar before becoming As₂O₃.

The oxidation process of arsenic pigments includes a change in the oxidation number of the arsenic. The oxidation number of arsenic is 3+ in both orpiment and arsenic trioxide, and this means that an arsenic atom is donating three electrons to the S or O atoms, respectively. However, as the oxidation process continues, arsenic atoms donate two more electrons to become As⁵⁺ and forms the anion called arsenate AsO₄^3- in the presence of oxygen. This anion can form various new compounds with other materials in the painting stratigraphy.

Katrien Keune and her colleagues studied the degradation of arsenic pigments and how their degradation products are transported and react with other components in the paint stratigraphy. Their paper is a good example of a multianalytical approach for a better understanding of material degradation processes within complex paint mixtures. They studied two objects—a Dutch Golden Age painting and a piece of polychrome furniture—using techniques such as FT-IR, Raman, SEM-EDX, and X-ray synchrotron-based methods.

The study of the painting stratigraphy showed two layers containing arsenic pigments. Raman spectroscopy allowed for the identification of the orpiment and realgar, as well as pararealgar as a degradation product from realgar. SEM-EDX showed the concentration of As in the different paint layers but also in an interface between the ground layer and the paint, suggesting migration of arsenic through the stratigraphy. FT-IR confirmed the presence of gypsum and calcium carbonate and hinted at the presence of arsenate species. The arsenic pigments present in the inner layer were stable; however, when the arsenic layer was at the surface and exposed to light, those pigments showed evidence of photodegradation.

Figure 1. Left: Still Life with Five Apricots by the Dutch master Adriaen Coorte (1704), Royal Picture Gallery Mauritshuis, The Hague, The Netherlands. Right: a) Light microscopic image of paint cross-section, b) backscattered electron (BSE) image, c-e) details of a cross-section under normal light (c), UV light (d) and BSE image (e). Micro-XRF mapping: f) BSE image, g) atomic distribution of arsenic (green), sulfur (red), and lead (blue), h) atomic distribution of arsenic (green), silicon (red), and aluminum (blue).

Synchrotron X-ray techniques such as XRF and XANES allowed for the micro-mapping of the elements present in the cross-sections along with the understanding of the difference in the state of oxidation of arsenic. A synchrotron is a cyclic particle accelerator in which the magnetic field that accelerates the particles can be changed with time. These accelerated particles produce synchrotron radiation, which can be varied from microwaves to X-rays to perform experiments using a specific type of radiation.

The XANES micro-mapping of the Dutch Golden Age painting allowed for the identification of a mixture of As³⁺ and As⁵⁺ in the sample. The presence and formation of calcium arsenate was thus confirmed, as a secondary product of the degradation of arsenic pigments and the interaction with the calcium from the ground layer.

Synchrotron radiation can be selected so precisely that it allows specific identification of elements and their different states of oxidation. XANES mapping showed a higher concentration of As³⁺, corresponding to the unaltered pigment and to the main degradation product, arsenic trioxide, located in the upper pain layers; meanwhile, the As⁵⁺ is less concentrated and mainly located in the ground layer and was attributed to the formation of calcium arsenate (Figure 2).

Figure 2. X-ray energy mapping. a) 11.865 KeV, b) 11.869 KeV, c) 11.875 KeV and d) 11.901 KeV. The As3+ species dominates in the map b and As5+ in the map c. e) Light microscopic image of the sample from the painting. f) Distribution of the As K-half edge position, the blue pixels indicate more reduce species and orange and red pixels more oxidized. 

Something similar was identified in the furniture sample: the presence of lead pigments—used as driers in the ground layer, paint layer, and in one of the varnish layers—led to the formation of lead arsenate due to the interaction of lead pigments and degradation products from orpiment and realgar.

These beautiful but deadly arsenic pigments were not only dangerous for the artists but also are dangerous for the objects themselves as they readily degrade and react with other components producing irreversible damages. The paper by Keune et al. illustrates the complexity of the degradation of arsenic pigments. The oxidation process is irreversible and produces physical modifications to the paint layer, which not only changes color but also becomes friable and can crumble. The degradation also produce an unvarnished-like appearance to the painting surface, due to the migration of degradation products to the varnish layer. This effect cannot be mitigated by varnishing again because the mobility of degradation products reaches also into the new varnish. This is why conservators must understand the degradation mechanism to establish possible conservation strategies to slow down the degradation and avoid factors that may accelerate the process.  

All figures reproduced/adapted with permission from: “Keune, K. et al. “Tracking the transformation and transport of arsenic sulfide pigments in paints: synchrotron-based x-ray micro-analyses”, J. Anal. At. Spectrom., 2015, 30, 813 – Published by The Royal Society of Chemistry.