Evidence for the catalytic properties of ultramarine pigment

Ultramarine blue paint layers in oil paintings can be affected by ultramarine ‘disease’ or ‘sickness’: a phenomenon described by a grey appearance and a loss of detail in the artwork. An explanation for this phenomenon is an interaction between the organic binder network and ultramarine pigment, with the pigment acting as a catalyst for the breakdown of the network. This breakdown results in micro-cracks in the paint film, which influences the appearance of the artwork. To investigate the possible catalytic property of ultramarine pigment, a test reaction – the dehydration of methanol to dimethyl ether – was carried out, with and without ultramarine pigment present in a micro-reactor with in-line gas chromatography mass spectrometry. It was observed that a higher yield of dimethyl ether was obtained in the presence of ultramarine pigment, confirming that ultramarine pigment possesses catalytic properties similar to commercial zeolitic silica-alumina catalysts

Sulfur K-edge micro- And full-field XANES identify marker for preparation method of ultramarine pigment from lapis lazuli in historical paints

Ultramarine blue pigment, one of the most valued natural artist's pigments, historically was prepared from lapis lazuli rock following various treatments; however, little is understood about why or how to distinguish such a posteriori on paintings. X-ray absorption near-edge structure spectroscopy at the sulfur K-edge in microbeam and full-field modes (analyzed with nonnegative matrix factorization) is used to monitor the changes in the sulfur species within lazurite following one such historically relevant treatment: heating of lapis lazuli before extracting lazurite. Sulfur signatures in lazurite show dependence on the heat treatment of lapis lazuli from which it is derived. Peaks attributed to contributions from the trisulfur radical-responsible for the blue color of lazurite-increase in relative intensity with heat treatment paralleled by an intensified blue hue. Matching spectra were identified on lazurite particles from five historical paint samples, providing a marker for artists' pigments that had been extracted from heat-treated lapis lazuli

Sulfur K-edge Micro- and Full-field XANES Identifies Marker for Preparation Method of Ultramarine Pigment from Lapis Lazuli in Historical Paints

Ultramarine blue pigment from lapis lazuli is one of the most valued natural artist’s pigments used throughout history for its brilliant hue. Historical recipes describe various pigment preparation methods, however, little is understood about why such treatments were performed or how to distinguish them a posteriori on historical paintings. One such historically relevant treatment is heating of the lapis lazuli rock prior to extracting lazurite. X-ray absorption near-edge structure spectroscopy (XANES) at the sulfur K-edge in microbeam and full-field modes (analyzed with non-negative matrix factorization, NMF) is employed to monitor the changes in the sulfur species within lazurite following treatments. Sulfur signatures in lazurite show dependence on the heat-treatment of lapis lazuli rock from which it is derived. In particular, peaks at 2469.0 and 2471.2 eV, attributed to contributions from the trisulfur radical (S3•⁃), which is responsible for the blue color of lazurite, increase in relative intensity with heat of treatment ≥ 600 °C, paralleled by an intensified blue hue. The peak at 2472.5 eV (presumed as S8), on the other hand, decreases in relative intensity. The sulfur signature following heat-treatment is not only unique but also retained following accelerated aging of each pigment mixed as a paint with linseed oil. Further, XANES spectra using an attenuated focused microbeam were gathered on lazurite particles from five historical paint samples. In each, profiles matching that of pigment derived from heat-treated rock were identified, providing a marker for artists’ pigments that had been extracted from heat-treated lapis lazuli