Egyptian Blue - Composition and Manufacture

Composition and Manufacture

Several experiments have been carried out by scientists and archaeologists interested in analyzing the composition of Egyptian blue and the techniques used to manufacture it. It is now generally regarded as a multi-phase material that was produced by heating together quartz sand, a copper compound, calcium carbonate, and a small amount of an alkali (plantash or natron) at temperatures ranging between 800–1000 °C (depending on the amount of alkali used) for several hours. The result is cuprorivaite or Egyptian blue, carbon dioxide and water vapor:

Cu₂CO₃(OH)₂ + 8 SiO₂ + 2 CaCO₃ → 2 CaCuSi₄O₁₀ + 3 CO₂ + H₂O

In its final state, Egyptian blue consists of rectangular blue crystals together with unreacted quartz and some glass. From the analysis of a number of samples from Egypt and elsewhere, it was determined that the weight percentage of the materials used to obtain Egyptian blue in antiquity usually ranged within the following amounts:

60–70% silica (SiO₂)

7–15% calcium oxide (CaO)

10–20% copper(II) oxide (CuO)

To obtain theoretical cuprorivaite, where there are only blue crystals, with no excess of unreacted quartz or formation of glass, the following percentages would need to be used:

64% silica

15% calcium oxide

21% copper oxide

However none of the analyzed samples from antiquity were made of this definitive composition, as all had excesses of silica, together with an excess of either CuO or CaO. It has been suggested that this may have been intentional. An increase in the alkali content results in the pigment containing more unreacted quartz embedded in a glass matrix, which in turn results in a harder texture. Lowering the alkali content (less than 1%), on the other hand, does not allow glass to form and the resultant Egyptian blue is softer, with a hardness of 1–2 Mohs.

In addition to the way the level of the different compositions influenced texture, the way Egyptian blue was processed also had an effect on its texture, in terms of coarseness and fineness. Following a number of experiments, Title et al. concluded that for fine-textured Egyptian blue, two stages were necessary in order to obtain uniformly interspersed crystals. First the ingredients are heated, and the result is a coarse-textured product. This is then ground up to a fine powder and water is added. The paste is then reshaped and fired again at temperatures ranging between 850–950 °C for one hour. It is possible that these two stages were needed to produce a paste that was fine enough for the production of small objects. Coarse-textured Egyptian blue, on the other hand, would not have gone through the second stage. Since it is usually found in the form slabs (in the dynastic periods) and balls (in the Greco-Roman period) it is suggested that these could have either been awaiting to be processed through a second stage, where they would be ground and finely-textured, or they would have been ground for use as a blue pigment.

The shade of blue reached was also related to the coarseness and fineness of Egyptian blue as it was determined by the degree of aggregation of the Egyptian blue crystals. Coarse Egyptian blue, was relatively thick in form, due to the large clusters of crystals which adhere to the unreacted quartz. This clustering results in a dark blue color that is the appearance of coarse Egyptian blue. Alternatively, fine-textured Egyptian blue consists of smaller clusters that are uniformly interspersed between the unreacted quartz grains and tends to be light blue in color. Diluted light blue on the other hand is used to describe the color of fine-textured Egyptian blue that has a large amount of glass formed in its composition, which masks the blue color, and gives it a diluted appearance. It depends on the level of alkali added to the mixture and therefore the more the alkali, and thus more glass formed, the more the diluted appearance. This type of Egyptian blue is especially evident during the 18th dynasty and later and is probably associated with the surge in glass technology at this time.

If certain conditions were not met, the Egyptian blue would not be satisfactorily produced. For example, if the temperatures were above 1050 °C, it would become unstable. If too much lime was added, wollastonite (CaSiO₃) forms and gives the pigment a green color. Too much of the copper ingredients results in excesses of copper oxides like cuprite and tenorite.