The Making of Ptolemaic Egyptian Blue Pigment in Latin Texts
La fabrication du bleu égyptien ptolémaïque dans les textes latins
Résumé
The text of the Roman architect Vitruvius (7, 11, 1) is the only text which outlines the manufacture stages of this artificial blue pigment used during the Ptolemaic and Roman periods. During the first century B. C., Vestorius, a rich banker of Puozzoli, imported and perfected in Italy the Alexandrine methods. Egyptian blue is a calcium-sodium disilicate of copper, (CaO, Na 2 O) CuO 4 SiO 2 shaped into blue or green balls. Grinding sand (harena) with natron (flos nitri) and adding copper yields a mixture shaped into a ball dried in air and fired in earthenware pots. Yet, the scientific analyses of Egyptian blue show the presence of a high amount of calcium. But Vitruvius's text does not mention any addition of limestone or lime. How was the calcium brought into the mix ? The source for the calcium might be the natural sand like the one extracted near Alexandria. It is a silico-calcareous sand. An other calcium source is the flos nitri, which is usually washing soda or natron, and acts as a flux. Pure natron does not contain lime. However, Pliny (Nat. 31, 114), states that in Egypt, natron is falsified with some lime, a forgery detected by its taste. It can be assumed that the flos nitri described by Vitruvius was imported from Alexandria and contained a fraudulent amount of lime, which seems to be common habit at that time in Egypt.
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