BY ANTHONY KING
The world’s oldest synthetic pigment – Egyptian blue – has been recreated using various recipes. Egyptian blue was used for colouring frescoes and small objects first around 5,000 years ago in ancient Egypt and later in ancient Rome.
The mineral responsible for its colour is cuprorivaite (CaCuSi4O10) but there is limited evidence for how it was made in ancient times. It likely involved a series of precursors that included a silica source, lime, an alkaline flux and a copper source. Egyptian blue was widely used in the ancient world as an alternative to expensive blue stones such as turquoise and lapis-lazuli.
Now, researchers in the US have created a dozen recipes for the pigment to explore various compositions. This involved mixtures of silicon dioxide, copper, calcium and sodium carbonate.
The manufactured pigments were compared with two ancient Egyptian artifacts. The colour of the Egyptian blue varied and just small differences in the process yielded quite different results.
Results confirmed that the blue colour originated mainly from cuprorivaite; in most cases as little as 50% in weight of this phase delivered a deep blue colour. Particle size mattered. ‘Larger particles are both red- and blue-shifted, making them appear more violet relative to the very same material when ground finer,’ says Edward Vicenzi at the Smithsonian Institution who helped lead the research. A copper-containing glass phase, only produced when soda was present, gave the pigment a green hue.
‘The synthesis firing temperature was held constant while using different types of starting materials, like using multiple sources of copper, and the presence or absence of a flux agent to lower the melting temperature,’ notes Vicenzi. ‘We also tested slow cooling versus more rapid cooling from high temperature, and we evaluated how the chemical phase mixture affected the colour of the pigment.’
Colour swatches generated could be used to match ancient examples and help select a hue for repairing an artefact with pigment loss, Vicenzi says.
Copper sources in the recipes included copper oxide and copper carbonate minerals azurite or malachite; calcium was either calcium carbonate or calcium oxide. The material was heated to 1000°C. After cooling, the pigment samples were analysed using X-ray diffraction, electron beam X-ray microanalysis and X-ray nano-computed tomography. Colour was measured using reflectance visible/near-infrared spectrometry.
‘Egyptian blue itself is very well studied,’ says Admir Masic, a chemist and archaeologist at Massachusetts Institute of Technology, US. ‘Some of the characterisation tools they applied here are really cutting edge, which is wonderful, as it allows us to explore the material in more depth.’
‘What is most interesting is the comparison with archaeological Egyptian blue examples, as well as substantiating that grain size matters,’ says geochemist Alexandra Rodler-Rorbo at the Austrian Archaeological Institute in Vienna. She has analysed the chemical composition of Egyptian blue samples and used lead isotope ratios to understand the origins of raw materials, as well as the production and trade of the pigment amongst ancient societies.
‘It would be interesting to see how element partitioning is influenced by the different experiments, specifically variation in starting materials and temperature/duration of firing,’ says Rodler-Rorbo. The samples are now on display at Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, US, and will become part of a new ancient Egypt gallery.
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