Synthetic Ultramarine Blue degradation phenomenaThe failure of ultramarine pigments, both historically and in modern paintings, is often known as ‘ultramarine sickness’ or ‘ultramarine disease’. Ultramarine (both natural and synthetic) are aluminosilicates which have a sodalite cage structure containing a radical sulphur chromophore (either S3- (for blue) or S2- for green) giving the pigment it’s colour. Synthetic ultramarine has been produced industrially since the 1830’s, and quickly replaced natural ultramarine due to its inexpensive production. The pigment is profoundly polar by nature and it is challenging to produce a stable oil paint that does not separate. Certain common phenomena, such as water sensitivity, darkening, discolouration crumbling and flaking, which have recently been found in paints with the synthetic ultramarine pigment may be explained by this polarity but have yet to be fully explored. Only occasionally, discoloration of the pigment may occur. The loss of colour in the blue pigment has been found to be due to the breakdown of the sodalite cage structure involving the removal of aluminium atoms from the tetrahedral coordination in the ultramarine framework, resulting in the formation of aluminium compounds separate from the pigment structure. This breakdown happens in the presence of an acid, enabling the reaction of the sulphur chromophore leading to its de-radicalisation and subsequent colour loss. In the presence of pollution (such as SO₂), the sulphur chromophore leaves its cage and potentially reacts to become H₂S. The study by René de la Rie et al. (2017) also provides evidence of photo-catalytic degradation of binding media of ultramarine blue paint layers via free-radical processes causing changes in reflectance while the pigment itself remains intact. It is believed that this is the primary process behind the degradation of ultramarine blue paint layers in paintings.
Synthetic Ultramarine Blue degradation phenomenaMills L., Burnstock A., Duarte F., van Keulen H., Megens L., van den Berg K.J., Water sensitivity of modern artists’ oil paints, 2008, in: ICOM Committee for Conservation (ICOM-CC) 15th Triennial Meeting New Delhi, 22-26 Sept 2008, pp 651-659
Cato E., Borca C., Huthwelker T., Ferreira E.S.B., Aluminium X-ray absorption near-edge spectroscopy analysis of discoloured ultramarine blue in 20th century oil paintings, 2016, in: Microchemical Journal, Vol 126, May 2016, pp 18-24
del Federico E., Newman J., Tyne L., O'Hern C., Isolani L., Jerschow A., Solid-State NMR Studies Of Ultramarine Pigments Discoloration, 2016, in: Volume 984 (Symposium MM – Magnetic Resonance in Material Science),
René de la Rie E., Michelin A., Ngako M., Del Federico E., Del Grosso C., Photo-catalytic degradation of binding media of ultramarine blue containing paint layers: A new perspective on the phenomenon of “ultramarine disease” in paintings, 2017, in: Polymer Degradation and Stability, vol. 144, October 2017, pp 43-52
Cato E., Scherrer N., Ferreira E.S.B., Raman mapping of the S3− chromophore in degraded ultramarine blue paints, 2017, in: Journal of Raman Spectroscopy Vol. 48, Issue 12 (2017), pp 1789-1798