Abstract
A number of historical texts are investigated to ascertain the optimum conditions for the preparation of synthetic ultramarine, using preparative methods that would have been available to alchemists and colour chemists of the nineteenth century. The effect of varying the proportion of sulphur in the starting material on the colour of the final product is investigated. The optimum preparation involves heating a homogenised, pelletised mixture of kaolin (100 parts), sodium carbonate (100 parts), bitumen emulsion (or any ‘sticky’ carbon source) (12 parts) and sulphur (60 parts) at 750°C for ca. 4 hours. At this stage the ingress of air should be limited. The sample is allowed to cool in the furnace to 500°C, the ingress of air is permitted and additional sulphur (30 parts) is introduced before a second calcination step is undertaken at 500°C for two hours. The products obtained from the optimum synthesis have CIE ranges of x = 0.2945-0.3125, y = 0.2219–0.2617, Y = 0.4257−0.4836, L* = 3.8455–4.3682, a* = 4.2763–7.6943, b* = −7.6772–(−)3.3033, L = 3.8455–4.3682, C = 5.3964–10.8693, h = 315.0636–322.2562. The values are calculated using UV/visible near infrared spectra using Lazurite [1], under D65 illumination, and the 1931 2° observer.
Highlights
The colour blue crops up in art through history in the works of great artists, sculptures and ancient civilisations and a fast, vivid blue was sought by artists
Highly prized was the beauty of the blue derived from lapis lazuli, that it excited much comment and historical texts are littered with attempts to recreate artificial lazurite as ‘ultramarine’ and in the 19th century, alternative synthetic blue pigments, such as cobalt blue, cobalt aluminium oxide (CoO.Al2O3) and Prussian blue (iron(III) hexacyanoferrate(II)), were developed
To synthesise a historically accurate sample of ultramarine, kaolin (100 parts) should be heated overnight at 600 uC, if the clay is left for an extended period of time, over 3 hours in air, it should be reactivated with heating
Summary
The colour blue crops up in art through history in the works of great artists, sculptures and ancient civilisations and a fast, vivid blue was sought by artists. Formed from the ground, semi-precious stone lapis lazuli (found in treacherous mountain locations), Lazurite was exceedingly expensive, costing up to 11000 French Francs per kilogramme FrF/kg [2]. Despite these drawbacks, Lazurite was undoubtedly the most favoured blue colour: the pigment worked well in water and oil, did not fade, and gave a very consistent shade. The colours were inconsistent and poor quality blue pigments like smalt, a cobalt doped glass, could damage the appearance of the canvas Cheaper alternatives such as azurite have been employed and the expense and preparation of the blue pigment used can often inform one of the importance and standing of the artist and benefactor. In a depiction of Christianity, most of the blue will be painted in a lesser pigment, such as azurite, whilst the robes of Mary and sometimes Peter, will be depicted with ultramarine, showing respect for the figures and the religiosity of the time in which the work was painted [3]
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