Insight into Framework Destruction in Ultramarine Pigments

Abstract

We report key evidence on the framework destruction in ultramarine pigments upon color fading. Experiments on faded pigments in a fresco painting environment reveal that the paramagnetic chromophores are set free via sodalite framework destruction and are subsequently degraded. Fading in acidic media produces similar results, although a larger number of beta-cages appear to be destroyed, and H2S is released. The findings are further supported by studies on natural and synthetic ultramarine pigments of various shades via solid-state resonance-Raman spectroscopy, colorimentry, and solid-state 29Si and 27Al NMR spectroscopy. NMR parameters are shown to correlate well with the intensities of Raman signals corresponding to the S3(-*) chromophores. A further correlation is established between the colorimetric parameters, L* (lightness) and C* (chroma), and the paramagnetic shift and paramagnetic linebroadening in NMR spectra for both 27Al and 29Si.

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…The high price and marvellous colour somehow promoted the development of adulterations, and finally the synthesis of artificial ultramarine in 1828 by Guimet, and in the same year, but with a different process, by Gmelin [1,20,28]. Natural ultramarine is stable to light, including lasers [18,29], heat, acids and alkalis [20,30,31]. No oxalates were formed upon exposure of the pigment to oxalic acid [32].…”

“…The zeolitic structure of the mineral allows for ion exchanges [20], and for trapping volatile molecules, such as the S 3 − chromophore [37], and CO 2 whose presence was recently related to natural ultramarine from Afghanistan [38]. Artificial ultramarine discolouration revealed a variation in the aluminium coordination, resulting in the opening of the cage and release of the chromophore [31,39,40]. Such a cage-opening process can be initiated either by high temperatures, acids [31,38,39] or alkalis [31].…”

“…Artificial ultramarine discolouration revealed a variation in the aluminium coordination, resulting in the opening of the cage and release of the chromophore [31,39,40]. Such a cage-opening process can be initiated either by high temperatures, acids [31,38,39] or alkalis [31]. Moreover, as known for sulphursaturated zeolites [41], it appears that ultramarine can have a catalytic effect on the binder breakdown, and that such property can be related to a pigment pre-treatment (heating), which can explain the inconsistent appearance of degradation [42].…”

Thermal analysis of the interaction of inorganic pigments with p(nBA/MMA) acrylic emulsion before and after UV ageing

“…It was demonstrated by using Raman spectroscopy that a higher amount of S 3 -chromophore with small amount of S 2 --responsible for the yellow colour [11] was present in the blue ultramarine than in the violet ultramarine, which contained an higher amount of the red chromophores (S 4 or S 3 Cl) [12]. Furthermore, Del Federico et al [126] investigated the fading of the blue ultramarine pigments in simulated fresco environments, which corresponded mainly to the decrease of the major S 3 -chromophores. Generally, ultramarine blue has a good lightfastness and heat stability (>350 °C) [83], but a low resistance to acids resulting in the conversion of the free chromophores into H 2 S and possibly elemental sulfur [126].…”

“…Furthermore, Del Federico et al [126] investigated the fading of the blue ultramarine pigments in simulated fresco environments, which corresponded mainly to the decrease of the major S 3 -chromophores. Generally, ultramarine blue has a good lightfastness and heat stability (>350 °C) [83], but a low resistance to acids resulting in the conversion of the free chromophores into H 2 S and possibly elemental sulfur [126]. The ultramarine pigment exhibited a low UV absorption and small protective effect on the UV stability of unstabilized polypropylene [137].…”