Pigment problem solved with cotton bud

Coloured minerals and other inorganic compounds have been used in artists' pigments for millennia because they retain their colour even after long periods because of their chemical resistance. Painters and artists have added transition metal oxides, sulfides and carbonates to their palates for generations. Today, the powerful tools of the analytical laboratory can help conservators and researchers identify precisely which pigments were used with minimal impact on the original artefact.

Now, Douglas Goltz, Kimberly Charleton, and Edward Cloutis of C-SCAPE (Centre for Scientific and Curatorial Analysis of Painting Elements) at the University of Winnipeg, Canada, working with Patricia Grinberg of the Chemical Metrology, Institute for National Measurement Standards, National Research Council of Canada, Ottawa, and Cathy Collins of Winnipeg Art Gallery have used graphite furnace atomic absorption spectroscopy and inductively coupled plasma-mass spectrometry to investigate the various pigments in two cultural objects, a painting and a map.

They relied on a touch-taken sample of less than a microgram form each object for their experiments obtained using a "cotton bud". This sampling technique does not visibly affect the object, even under the magnifying glass. The small sample size afforded by this approach is perfectly suited to GFAAS and ETV-ICP-MS, say the researchers.

The first object the researchers examined was "untitled-Arctic Spring" (1950) by Canadian artist George Swinton who also collected Inuit artwork. The pigments in this abstract watercolour have darkened noticeably in recent years, with formerly white regions having become grey or even dark grey. The identification of the white pigments used would be an obvious first step in restoration and subsequent conservation of this painting.

The cotton-bud technique was used to take various samples and GFAAS revealed signals for Ti, Ba, Zn, Pb, and Ca with the largest signals due to barium and zinc. This, the researchers say, suggests the bulk of the white pigment is likely to be lithopone, a compound of barium sulfate and zinc sulfide. ETV-ICP-MS confirmed the presence of large quantities of barium and zinc in the samples. The team points out that both techniques revealed surprisingly high levels of lead and iron in the samples too.

Lithopone was originally developed for use in household paints and problems with it darkening through photochemical reduction of the zinc sulfide component are well known, the researchers add.

The second cultural object the team examined was the Mapp or "Generall Carte of the World" (circa 1669). It is an engraved map executed in black ink on laid paper. The map was illustrated using blue, yellow, and red pigments and it is the red pigments in the object that have darkened, a problem that came to light during cleaning by a conservator.

The team found, using solution nebulization-ICP-MS and ETV-ICP-MS, that the predominant metal in the darkened red pigment areas was lead with trace quantities of chromium present. They also detected relatively high amounts of copper and several other metals including barium and arsenic. The researchers reasoned that a lack of iron or mercury means that neither red ochre (iron oxide) nor cinnabar (mercury sulfide) were used whereas the very high lead concentration would suggest that the red pigment is chrome red, molybdate red, or red lead. Further investigation using Raman spectroscopy revealed the presence of lead oxide and an absence of chromate or manganate ions, providing further evidence that the red pigment is red lead. Red lead oxidises over time to lead(IV) oxide, which is dark brown.
"For conservators this approach gives them another tool for identifying pigments," Goltz told SpectroscopyNOW.com, "Certainly not every museum or art gallery has immediate access to XRF - this approach can be used by anyone. The Q-tip can be carried easily and then stored for later analysis of metals in the lab."