Conserving conservation: Spectroscopic insights into fading

Ezine

Published: Jan 5, 2011
Author: David Bradley
Channels: Infrared Spectroscopy

thumbnail image: Conserving conservation: Spectroscopic insights into fading

Pigment problems

Improving storage and exposure conditions in conservation of artefacts is crucial to suppressing the fading and degradation of dyes and other components of paintings. Researchers have now used several analytical techniques, including attenuated total reflectance infrared spectroscopy, Raman spectroscopy, reflectance UV-Vis spectroscopy, X-ray fluorescence spectroscopy and optical microscopy, to investigate different conditions on common pigments.

The vast range of dyes, pigments, paints and other colouring agents of disparate composition and origin found in historical documents, paintings and other artefacts mean that no systematic studies of the deterioration of these pigments has been possible, despite the pressing need to catalogue the spontaneous processes that might occur for the sake of conservation. Nevertheless, it has been possible to classify the various agents of attack on artefacts - they are either exogenic or endogenic and include exposure to light (including sunlight), humidity and moisture, atmospheric oxygen and temperature changes, air pollutants commonly found in urban and industrialised areas and invasion by pests and microorganisms such as fungi and bacteria.

Photo fading

Photochemical fading, photofading, can occur in the air or under anoxic conditions and has been known for at least two centuries. Of course, the presence of oxidising species activated photochemically might suggest that photofading would be inhibited by the exclusion of oxygen but this is not necessarily so. Oxygen-free and nitrogen atmosphere devices designed to protect priceless artefacts from fading do not fulfil their role in all cases as dyes can undergo photochemical changes even in the absence of oxygen or its damaging radicals, even this has been known since the 1950s.

Monika Koperska, Tomasz Lojewski and Joanna Lojewska of the Faculty of Chemistry, Jagiellonian University, in Cracow, Poland, point out that while oxygen-free "cassettes" are commonly used by museums for the storage of artefacts little is known about degradation of such artefacts in this environment as opposed to the open air. They have now initiated the formation of a database on dye degradation that involved studying five commercial dyes - indigo, dragon's blood from the fruit or bark of the rattan palm (Calamus draco), curcumin (from turmeric root), madder lake (root extract) and carminic acid (Coccus cacti). All of these dyes have been used extensively in oil and watercolour paintings

The researchers exposed each dye substance to accelerated ageing using high exposure to visible light under anoxic and oxygen-rich conditions and analysed the effects with a raft of techniques including attenuated total reflectance infrared spectroscopy. An estimate of the level of degradation was based on colour differences determined by visible spectra and changes in FTIR and Raman vibrational band intensities. "According to [those], only indigo, dragon's blood and curcumin show greater stability in anoxic conditions in comparison with oxygen-rich ones while madder, carminic acid undergo greater degradation," the team concluded.

Conservation piece

"Even though the conclusions are drawn on the small sample set which is far too low for any statistical judgements, the practical solution for the conservators is that before any decision on such a kind of preservation and exposure methods, the materials included in paintings should be recognised," the team concludes. "This also means that a database of the ageing effects occurring in them should be created in the literature available for conservators."