title: Spectroscopic studies on the darkening of lead white.
abstract: Lead white is an historically important paint used by artists since antiquity. The darkening of lead white has been well documented in works of art such as paintings. In this paper, mid-infrared (MIR) and visible spectroscopy were used to examine spectral changes accompanying the darkening of lead white paint as a result of exposure to H2S(g). Laboratory-prepared paint and a commercial lead white paint were used to observe the darkening reaction over time. Structural changes to the pigment, 2PbCO3.Pb(OH)2, in lead white were readily detected using MIR by applying a thin film on a KBr pellet. Spectral changes at 3541 (O-H stretch), 1400 (C-O), and 680 cm-1 (C-O) were the most significant over time as the paint darkened. Visible spectra were also collected to provide a semi-quantitative measure of color change with structural changes observed by MIR. Experiments in the visible region were also conducted to compare the spectral response as % reflected and % transmitted light as lead white darkened. The effect of different binding agents (egg tempera, linseed oil, water, and gum Arabic) on the rate of darkening of lead white was also examined. Other sulfur-containing pigments such as orpiment and realgar were also tested for their ability to darken lead white. By applying paint as a thin film inside a sealed cuvette, darkening of lead white was observed in the visible spectra (800 nm) when either powdered orpiment or realgar was in placed in the cuvette for 24 h.
authors.affiliation: C-SCAPE: Centre for Scientific and Curatorial Analysis of Painting Elements, Department of Chemistry, University of Winnipeg, 515 Portage Ave., Winnipeg, Manitoba, Canada, R3B 2E9.
authors.forename: Douglas
authors.initials: D
authors.lastname: Goltz
authors.forename: Jennifer
authors.initials: J
authors.lastname: McClelland
authors.forename: Angela
authors.initials: A
authors.lastname: Schellenberg
authors.forename: Michael
authors.initials: M
authors.lastname: Attas
authors.forename: Edward
authors.initials: E
authors.lastname: Cloutis
authors.forename: Cathy
authors.initials: C
authors.lastname: Collins
mesh_terms: D003116:Color; D003124:Colorimetry / Q000295:instrumentation / Q000379:methods*; D006862:Hydrogen Sulfide / Q000032:analysis* / Q000737:chemistry*; D007854:Lead / Q000032:analysis* / Q000737:chemistry*; D010150:Paint / Q000032:analysis*; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013057:Spectrum Analysis / Q000379:methods*
chemical_list: D007854:Lead; D006862:Hydrogen Sulfide