Surface Analysis of Fine Art Paintings: Studying Degradation Mechanisms with a Systematic Approach

Abstract

A large body of work has been established in the field of pigment alteration covering several artistic eras [1][2][3]. While these studies are fundamental in understanding the physical and chemical properties pertaining to color as seen in paintings, they do not cover the important facet of molecular alteration due to degradation of the binding medium of these paintings. By using surface analytical techniques proven in the study of cultural heritage materials it is possible to elucidate the effects of degradation inherent to binding media and also add to the knowledge of pigment-binder interactions. For this study, chemical information gathered by both TOF-SIMS and XPS, a complete characterization of both short-range (XPS) and long-range (TOF-SIMS) molecular alteration related to egg tempera degradation is presented herein. By utilizing gas cluster ion beam (GCIB) technology with XPS it was possible to elucidate any changes in chemical oxidation and reduction as a function of depth for a given material and its exposure to its surroundings.Thin films were prepared by spin casting freshly made egg tempera (1:1 w/w egg yolk:water) onto clean 1 × 1 inch silicon wafers. Samples were then placed as a set inside of a weathering chamber to simulate environmental degradation effects. Detailed in Figure 1, the chamber consists of an N2 purge, heat source, UV light source, and a relative humidity (RH) source. The variables to start the degradation process were to be tested across two major sources: heat/humidity exposure and UV exposure. The three trials set forth in the experiment were to expose the egg tempera thin films to the following conditions: i) 60 °C, >80% RH, and a dark chamber; ii) 20 °C, 0% RH, and UV exposure; and iii) 60 °C, >80% RH, and UV exposure.XPS and GCIB depth profiling were conducted with a K-Alpha+ with MAGCIS (Thermo-Scientific, Inc.) located in the Surface Analysis Facility at the University of Delaware. Monochromated Al Kα x-rays (1486.6 eV) with a spot size of 100 µm and pass energy of 20 eV for all high resolution scans. GCIB profiling was done with a 4 kV Ar2000 cluster for 60 s each level of sputtering. All curve-fitting was done in CasaXPS using Shirley-type background correction. The sputter rate of the egg tempera was measured using atomic force microscopy and found to be 4.5×10 -3 nm/Ar atom. TOF-SIMS was conducted with a TOF-SIMS IV (ION-TOF GmbH), upgraded to TOF-SIMS V capabilities, located in the Surface Analysis Facility at the University of Delaware. 25-kV Bi 3 + primary ions were directed towards the sample surface in "high-current bunched mode" with secondary ion extraction with ±2 kV into the TOF mass analyzer and given a 10 kV post-acceleration. All spectra were collected over an area of 250 µm 2 with primary ion beam dosage taken to the static SIMS limit of 1×10 12 ion/cm 2 . All data processing was completed in Matlab, using the correlated optimized warping (COW) package and the PLSToolbox. After COW alignment of the TOF-SIMS data to remove sample variance due to miscalibrati...