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The Effect of Relative Humidity on Artists' Pigments

David Saunders and Jo Kirby
Technical Bulletin Volume 25, 2004


Presents an overview of the changes observed in the colour of pigments exposed to high humidities, light, and saline environments. The phenomenon of colour change in lead, vermilion, azurite, Prussian blue, and red and yellow lake pigmented paint films is reviewed in detail by describing particular examples and referring to previous studies. Differences between environmental conditions of paint films on wall paintings and easel paintings are discussed. Experimental studies were carried out on both linseed oil and gum- or glue-bound paints. Saturated salt solutions were used to maintain relative humidity (RH). Control samples were kept in the same humidity chambers and were covered from light exposure. Colour measurements were taken in order to quantify the change in the paint films over time. Those pigments that were largely unaffected by RH on light exposure were lead white, Naples yellow, malachite, and azurite. Pigments that exhibited a significant change in colour with an increase of RH during light exposure included lake pigments, dyestuffs, and vermilion. Wet-processed vermilion was observed to undergo more change than dry-processed vermilion. Some pigments, such as lead tin yellow types I and II, exhibited different behaviour in oil and egg films. Limitations to the experimental procedure are discussed.


artists' materials, colourimetry, discolouration, pigment, relative humidity

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Saunders, D., Kirby, J. 'The Effect of Relative Humidity on Artists' Pigments'. National Gallery Technical Bulletin Vol 25, pp 62–72.

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