Change is permanent: thoughts on the fading of cochineal-based watercolor pigments

Abstract

Background: Color 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. Results:We 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.

Photochemistry of Artists’ Dyes and Pigments: Towards Better Understanding and Prevention of Colour Change in Works of Art

“…Them ost common reds used in antiquity to dye textiles are based on hydroxy anthraquinone chromophores (Figure 6). [51,62] They were extracted from the roots of avariety of plant from the Rubiaceae family,being alizarin and purpurin the major occurring species in Rubia tinctoria L. [51] Ther ed extracts were also prepared for use as apigment for painting, by precipitation with aluminium salts (known as madder lake pigments). Thestructure of these metal-dye complexes is still amatter of controversy.Systematic phenomenological studies on the fading of various anthraquinone-lake paints,including cochineal lake pigments,i ndicated differences in colour fading.…”

Characterisation of the different hands in the composition of a 14th century breviary by means of portable XRF analysis and complementary techniques

“…This information is relevant with concern to the geographic provenance of raw matters, being mixite found in copper-bismuth deposits or associated to malachite from the mineralogical point of view. Therefore, according to the indications reported in the work by Berrie et al, [32] we can hypothesise that the mineral used for the blue paint came from one of the historical sources of azurite known in 14th century, such as Harz Mountains (Saxony), Black Forest (Southern Germany), Hungary or Czech Republic.…”

“…There is, however, an interesting additional feature in the analysis of blue initials: the systematic presence of bismuth at trace level amounts (<0.1%). This element has been recently highlighted in the analysis of a painting by Giotto [32] in which the blue areas resulted to be composed by azurite with impurities of malachite and mixite, a rare green-blue mineral with formula BiCu 6 (OH) 6 (AsO 4 ) 3 ·3H 2 O. Despite in situ XRF analysis could not give an absolutely reliable identification of this mineral phase, the contextual presence of Cu and Bi strongly suggests that mixite is accompanying azurite in the blue initials of the Breviario; in fact, a linear correlation between the two elements can be demonstrated on the base of XRF data (Fig.…”

Zur Photochemie von Künstlerfarben: Strategien zur Verhinderung von Farbveränderungen in Kunstwerken

“…[51,62] Sie werden traditionell aus den Wurzeln ver- schiedener Pflanzen der Rubiaceae-Familie extrahiert, insbesondere Alizarin und Purpurin aus Rubia tinctoria L. [51] Die roten Extrakte wurden auch als Pigmente fürd ie Malerei angefertigt, durch Ausfällung mit Aluminiumsalzen (bekannt als Krapplack). [51,62] Sie werden traditionell aus den Wurzeln ver- schiedener Pflanzen der Rubiaceae-Familie extrahiert, insbesondere Alizarin und Purpurin aus Rubia tinctoria L. [51] Die roten Extrakte wurden auch als Pigmente fürd ie Malerei angefertigt, durch Ausfällung mit Aluminiumsalzen (bekannt als Krapplack).…”

“…Die häufigsten roten Farbstoffe in antiken Te xtilien basieren auf Hydroxyanthrachinon-Chromophoren (Abbildung 6). [51,62] Sie werden traditionell aus den Wurzeln ver- [62,63] In Anbetracht der bestehenden Unklarheiten bezüglich der genauen Strukturen von Lackpigmenten konzentrieren wir uns hier auf die Photophysik von Alizarin und Purpurin im Kontext ihrer Photostabilität.…”