Mixtures with Prussian Blue – colour permanence of green and blue-greenish paint layers in painting and in conservation-restoration

Abstract

Pigments individually characterized with an acceptable lightfastness do not always retain that characteristics when mixed on a palette by a painter or conservator-restorer. The awareness of this phenomenon is useful both while interpreting the appearance of a paint layer of pictures suggesting that some changes unintended by the artist have occurred, and while performing reintegration of damaged paint layers. Earlier preliminary research, in part already published, of several of such mixtures reported as unstable in painting manuals allowed to identify some of them containing Prussian Blue prone to distinct alterations. The subsequent tests and further observations made over the period of several years prompted the author to the conclusion, that the problem extends beyond the limits of such a simple ascertainment. Further examinations reported in this paper concerned several of such mixtures used to obtain green and greenish-blue hues tested in perspective of their potential use in painting restoration, with two different binders and with the admixture of white pigment or extender in order to allow for the assessment of lightened tone. The examination consisted on accelerated ageing (light exposure) of the obtained paint layers together with documentation of their condition at several stages (photographic and colorimetric records). Attempts were made for a preliminary identification of the mechanisms of changes based on the analysis of the results of IR- and Raman-spectroscopy of several samples. The direction of further research has been established, that would possibly allow for a more comprehensive interpretation of the observed phenomena. In the same time certain conclusions important for restoration practice, (possible to enunciate at this stage of research) have been presented.