Abstract
BackgroundColor change in artworks has been commented on for centuries. Fading of watercolor pigments is a notable alteration. Pigments based on carminic acid are among those particularly prone to color loss, but the mechanism and factors are not well understood.ResultsWe painted out three pigments prepared from the aqueous extract of carminic acid (CA) from Dactylopius coccus: the uncomplexed, and aluminium- and tin-complexed lakes. These were applied in a 2% gum Arabic solution to papers that were acidic, neutral or alkaline pH and exposed to accelerated light aging in a weatherometer. The comparative rate of fading was dependent on the complexation and on the pH of the substrate. On alkaline paper, the Al complex was least light stable, on neutral paper the three colorants responded similarly, and on acidic paper the tin complex was the least light stable. This is discussed in light of the published information on reactions and mechanism of color loss of CA.Graphical abstractCochineal-based pigments fade on exposure to light. In watercolor paints, the rate depends not only on the complexation of the ligand but also the pH of the paper substrate
Highlights
Color change in artworks has been commented on for centuries
He noted that red lead and orpiment turn black in air; he provided a list of pigments that ought not be used in fresco, and he commented that red lakes are unstable, noting that the method used for preparation of the pigments has a large effect on the stability of the product: he said that the lake pigment made from cloth shearings does not last at all compared to that made directly from the resin, due to the presence of alum [1], an observation which will be discussed further
Our increasingly sophisticated methods for chemical analysis can be applied to understanding color use and unforeseen color change in artifacts
Summary
Color change in artworks has been commented on for centuries. Fading of watercolor pigments is a notable alteration. 1400 on the alteration of color, giving examples of inherent vice, problems due to incompatibility of materials, and the effects of methods of preparation of pigments causing color change in works of art He noted that red lead and orpiment turn black in air; he provided a list of pigments that ought not be used in fresco, and he commented that red lakes are unstable, noting that the method used for preparation of the pigments has a large effect on the stability of the product: he said that the lake pigment made from cloth shearings does not last at all compared to that made directly from the resin, due to the presence of alum [1], an observation which will be discussed further. Without knowledge of the original pigment’s color we cannot be sure if we are observing change, we are lacking information needed to understand the artist’s original intent, and it is challenging to specify suitable display conditions and/or treatment options
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