National Gallery Technical Bulletin Volume 25, 2004
The Effect of Relative Humidity on Artists’ Pigments David Saunders and Jo Kirby
The ideal pigment is inert: it does not react with other components in the paint layer within which it is dispersed and is stable to the effects of light, moisture, temperature and other factors in its environment, for example pollutant gases or biological action. In practice this counsel of perfection is rarely reached but is approached by, for example, iron oxide pigments, carbon black and some modern pigments. For most of the materials used as pigments, the conditions under which they are unsuitable became well known to artists and they made their choice or adjusted the method of application accordingly. Pigments sensitive to alkaline environments, for example, were not recommended for use in a lime-containing plaster in buon fresco (note 1), but could be applied bound in another medium after the plaster had dried. In later centuries, however, changing environmental conditions, changes of taste, or simply the passage of time may well have caused discoloration or other alterations to the pigments, in spite of the care taken by the original painter.
In many reports of pigment change in humid conditions, it is clear that the binding medium used may have a considerable influence. Drying oils are not miscible with water; the pigment particles are coated with the oil during grinding and after drying the resultant paint film is coherent and relatively impervious to moisture. Egg tempera films are quite resilient when dry, but the pigment may be less well bound, so less well protected. Animal skin glue or gum paint films usually have a more open structure with less well-protected particles in a medium that retains the ability to take up or lose moisture. Under these conditions, and in any lean paint film (as in a traditional watercolour), the pigment is more vulnerable to damage from environmental factors such as high humidity.