Catherine Schmidt Patterson

Sub-micron proximal probe thermal desorption and laser mass spectrometry on painting cross-sections

We demonstrate sub-micron, atomic force microscopy (AFM) proximal probe desorption of organic dyes, and subsequent detection via laser mass spectrometry. A nanothermal analysis (nano-TA) probe tip in contact with a surface is heated (10000 °C s−1) to induce thermal desorption, creating depression sizes ranging from 360–1500 nm in diameter and 20–100 nm in depth. Desorbed material is drawn through a heated capillary via vacuum, and deposits onto a graphite sample bar. Laser desorption, followed by supersonic jet-cooling and either resonant two-photon ionization (R2PI) or non-resonant ionization mass spectrometry is used to characterize the transferred material. Individual, microscopic layers of organic dyes within painting cross-sections were successfully analyzed using this new approach. Separating the AFM thermal desorption step from the detection step allows for the use of analytical techniques appropriate for individual samples of material, desorbed with high spatial resolution.

From lapis lazuli to ultramarine blue: investigating Cennino Cennini’s recipe using sulfur K-edge XANES

Abstract As one of the most desired and expensive artists’ materials throughout history, there has long been interest in studying natural lapis lazuli. The traditional method of extracting the blue component, lazurite, from lapis lazuli, as outlined in Cennini’s Il Libro dell’Arte, involves a lengthy purification process: (1) finely grind the rock; (2) mix with pine rosin, gum mastic, and beeswax; (3) massage in water to collect the lazurite. Repeating the process produces several grades of the pigment, typically referred to as ultramarine blue. Here, we investigate the sulfur environment within the aluminosilicate framework of lazurite during its extraction from lapis lazuli. The sulfur XANES fingerprint from samples taken at the different stages in Cennini’s extraction method were examined. All spectra contain a strong absorption peak at 2483 eV, attributable to sulfate present in the lazurite structure. However, intensity variations appear in the broad envelope of peaks between 2470 and 2475 eV and the pre-peak at 2469.1 eV, indicating a variation in the content of trisulfur (S3−˙) radicals. By studying the effect of each step of Cennini’s process, this study elucidates the changes occurring during the extraction and the variability within different grades of the precious coloring material. The increasing application of XANES to the study of artist’s materials and works of art motivated extending the research to assess the possibility of X-ray induced damage. Direct comparison of micro-focused and unfocused beam experiments suggests an increase of the S3−˙ radicals with prolonged exposure. Analysis indicates that induced damage follows first-order kinetics, providing a first assessment on the acceptable amount of radiation exposure to define the optimal acquisition parameters to allow safe analyses of lapis lazuli and ultramarine pigments.

Nanometer-Scale Proximal Probe Thermal Desorption Coupled with Laser Mass Spectrometry

The analysis of cultural heritage materials presents a number of obstacles which impose limitations on the range of analytical techniques that can be utilized in their analysis: limited, and extremely small samples, complexity of sample structure, the importance of maintaining spatial integrity and, most importantly, the preciousness of the samples. These limitations present particular challenges for the identification of dyes and pigments within microscopic painting cross sections, in which complex mixtures and thin (often only a few microns) layers of organic material are common.

Soft X-ray Absorption Spectroscopy and Imaging of Sulfur in Lapis Lazuli

Since antiquity, lapis lazuli has been highly valued across many cultures for its bright blue color. Due to the material’s significance, there has long been interest in understanding its color variations and determining its geographic origin, whether used as the processed pigment ultramarine in painted works of art (e.g. paintings and manuscripts) or the raw lapis lazuli stone in cultural heritage objects (e.g. jewelry and inlaid decorations). While the most well-known source of lapis lazuli is Afghanistan, there are several other sources, including sites in Tajikistan, Iran, Russia, Canada, and Chile. Naturally occurring lapis lazuli contains the blue mineral lazurite (Na6Ca2(Al6Si6O24)(SO4,S3,S2,Cl,OH)2) with a variety of accessory minerals that are common to many of the known geological deposits—e.g. hauyne (Na3Ca(Si3Al3)O12(SO4)), pyrite (FeS2), calcium carbonate (CaCO3), and diopside (MgCaSi2O6). Much of the current research on the provenance of lapis lazuli has focused on the spectroscopic characteristics, the composition, and the overall distribution of the accessory minerals within the whole rock [1-4]. For example, work in our laboratory has shown that diopside inclusions in lapis lazuli sometimes have a fluorescent response to infrared wavelengths which may be characteristic of the geological deposit [5]. One challenge with basing provenance on the composition and distribution of accessory minerals is that the accessory minerals are mostly removed during the production of the ultramarine pigment, a process that requires crushing, sorting, and soaking the rock in an alkaline solution. Thus, a technique that focuses on in-situ analysis of the lazurite mineral alone, which remains chemically unaltered during this intensive processing, is desirable. Expanding on our previous work, this study therefore focuses on the lazurite component of lapis lazuli.