Dr. Jörg Klaas: Die "Ultramarinkrankheit"

Studies on colour change in ultramarine containing paint layers.

The mineral pigment ultramarine is a sulphur-containing sodium aluminium silicate with the approximate formula Na3 Ca (Si3 Al3 O12) 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.