Degradation of Cadmium Yellow Paint: New Evidence from Photoluminescence Studies of Trap States in Picasso's Femme ( Époque des "Demoiselles d'Avignon").

Paints based on cadmium sulfide (CdS) were popular among artists beginning in the mid-19th century. Some paint formulations are prone to degrade, discoloring and disfiguring paintings where they have been used. Pablo Picasso's Femme (Époque des "Demoiselles d'Avignon") (1907) includes two commercial formulations of CdS: one is visibly degraded and now appears brownish yellow, while the other appears relatively intact and is vibrant yellow. This observation inspired the study reported here of the photoluminescence emission from trap states of the two CdS paints, complemented by data from multispectral imaging, X-ray fluorescence spectroscopy, micro-FTIR, and SEM-EDS. The two paints exhibit trap state emissions that differ in terms of spectrum, intensity, and decay kinetics. In the now-brownish yellow paint, trap state emission is highly favored with respect to near band edge optical recombination. This observation suggests a higher density of surface defects in the now-brownish yellow paint that promotes the surface reactivity of CdS particles and their subsequent paint degradation. CdS is a semiconductor, and surface defects in semiconductors can trap free charge carriers; this interaction becomes stronger at reduced particle size or, equivalently, with increased surface to volume ratio. Here, we speculate that the strong trap state emission in the now-brownish cadmium yellow paint is linked to the presence of CdS particles with a nanocrystalline phase, possibly resulting from a low degree of calcination during pigment synthesis. Taken together, the results presented here demonstrate how photoluminescence studies can probe surface defects in CdS paints and lead to an improved understanding of their complex degradation mechanisms.

Analyzing the Heterogeneous Hierarchy of Cultural Heritage Materials: Analytical Imaging.

Objects of cultural heritage significance are created using a wide variety of materials, or mixtures of materials, and often exhibit heterogeneity on multiple length scales. The effective study of these complex constructions thus requires the use of a suite of complementary analytical technologies. Moreover, because of the importance and irreplaceability of most cultural heritage objects, researchers favor analytical techniques that can be employed noninvasively, i.e., without having to remove any material for analysis. As such, analytical imaging has emerged as an important approach for the study of cultural heritage. Imaging technologies commonly employed, from the macroscale through the micro- to nanoscale, are discussed with respect to how the information obtained helps us understand artists' materials and methods, the cultures in which the objects were created, how the objects may have changed over time, and importantly, how we may develop strategies for their preservation.

Characterization of lapis lazuli pigments using a multitechnique analytical approach: implications for identification and geological provenancing.

Many of the Raman spectra obtained from areas painted with ultramarine pigments in illuminated manuscript leaves from the 14th century Italian manuscript the Laudario of Sant'Agnese, in the collection of the J. Paul Getty Museum, also contain strong bands not typically associated with this pigment. The source of these features was investigated using a multitechnique analytical approach. Techniques employed include Raman microspectroscopy, scanning electron microscopy with energy-dispersive spectroscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry. The results indicate the presence of diopside (CaMgSi(2)O(6)), a mineral commonly associated with lapis lazuli in nature, and suggest that transition metal dopants in the diopside may be responsible for the Raman features, likely the result of fluorescence with vibronic coupling. The implication of this result with respect to using Raman spectroscopy as a fast, noninvasive, and nondestructive method for determining the geological provenance of natural lapis lazuli pigments used in art is discussed.

Characterization of Pararealgar and Other Light-Induced Transformation Products from Realgar by Raman Microspectroscopy.

Naturally occurring arsenic sulfide minerals are brightly colored and consequently have been used as artists' pigments since ancient times. Orpiment, As2S3, is yellow and is often found associated with realgar, As4S4, which is red to red-orange. When exposed to sunlight, unprotected realgar develops a surface coating of friable yellow material that, until recently, had generally been assumed to be orpiment. However, it has been determined that this material is pararealgar, a light-induced polymorph of realgar. We have identified pararealgar in a major work by the Renaissance master, Tintoretto. The accurate identification of pigments in artistic and historic works is of great importance for determining the construction, history, and future preservation of these works. We report here the characterization by Raman microspectroscopy of pararealgar and the various intermediate species involved in the light-induced transformation of realgar to pararealgar. The relative merits of Raman microspectroscopy and X-ray diffraction for the accurate and efficient characterization of these arsenic sulfide compounds are discussed.