A descoloração do azul ultramarino em pintura

A seguinte tese de doutoramento sobre a alteração do azul ultramarino em pintura antiga está disponível online:

Jörg Klaas, Die "Ultramarinkrankheit". Studien zu Veränderungen in Ultramarinhaltigen Farbschichten an Gemälden, München, Technischen Universität München, 2010.

Está aqui, com acesso livre.

Resumo:

The mineral pigment ultramarine is a sulphur-containing sodium aluminium silicate with the approximate formula Na₃Ca(Si₃Al₃O₁₂)S. Since the 6th century it is made by grinding lapis lazuli and was used in wall paintings, illuminated manuscripts and easel paintings. The term ‘ultramarine-sickness’ describes colour changes of ultramarine-containing paint layers. The blue colour turns lighter or becomes greyish in appearance. Two theories are discussed: destruction of the aluminosilicate lattice by acids which leads to dissociation of the colour carrying polysulphide anions or an apparent discolouration due to micro-cracks in the binding medium. Using the examples of 18 paintings from 1300–1720 of the Bayerische Staatsgemäldesammlungen a classification of the colour change phenomenons was carried out. Investigations on artificially altered samples and cross-sections of 14 paintings employing light microscopy, SEM/EDX and Raman-spectroscopy give no evidence for a decomposition of the pigment by acids. In the cross-sections a separation of pigment and binders could be observed. The micro-cracks cause a change of the refractive index that is responsible for the glaze pigment ultramarine becoming opaque and lighter.