Pigment-size effect on the physico-chemical behavior of azurite-tempera dosimeters upon natural and accelerated photo aging

This paper presents the first comprehensive study on the role played by pigment particle size in pigment-binder interactions, and thus susceptibility to aging of azurite-rabbit glue paint dosimeters upon accelerated UV-aging versus outdoor sunlight exposure. A complementary multi-analytical approach, including spectroscopic and surface analytical techniques, was applied to characterize commercial azurite pigments, as well as blank and aged dosimeters. Results show the crucial role played by azurite particle size on protein-copper complex formation, which governed physico-chemical properties of paints and their aging behavior under different light-aging conditions. On UV-aged dosimeters, finer particles promoted stronger binder structural changes, inducing more severe color changes (Delta E = 6-17). Azurite size also controlled crack formation which was more severe as pigment grain increased due to greater accumulation of binder in inter-pore spaces. Outdoor exposure caused important binder loss (50 -60%) and initial transformation of azurite into malachite. Since analyzed pigment composition and particle size differed from manufacturer data, we recommend their characterization prior to use in conservation works and scientific studies. Based on these results, paints made of coarse azurite particles are preferable since they are less prone to color change. Moreover, a small addition of fine-grained pigments might reduce crack development upon aging. (C) 2017 Elsevier Ltd. All rights reserved.