Egyptian blue, also known as calcium copper silicate, calcium copper tetrasilicate, or cuprorivaite,Cuprorivaite, mindat.org is a pigment with the chemical formula CaCuSi4O10 that was used in ancient Egypt for thousands of years. It is considered to be the first synthetic pigment.
Egyptian blue is produced from a mixture of silica, lime, copper, and an alkali. Its color is due to a calcium-copper tetrasilicate CaCuSi4O10 of the same composition as the naturally occurring mineral cuprorivaite. It was first synthesized in Egypt during the Fourth Dynasty and used extensively until the end of the Roman period in Europe, after which its use declined significantly.
Apart from Egypt, it has also been found in the Near East, the Eastern Mediterranean, and the limits of the Roman Empire. It is unclear whether the pigment's existence elsewhere was a result of parallel invention or evidence of the technology's spread from Egypt to those areas.
After the Roman era, Egyptian blue fell out of use and, thereafter, the manner of its creation was forgotten. In modern times, scientists have been able to analyze its chemistry and reconstruct how to make it.
History
The ancient Egyptians held the color blue in very high regard and were eager to present it on many media and in a variety of forms. They also desired to imitate the semiprecious stones turquoise and lapis lazuli, which were valued for their rarity and stark blue color. Use of naturally occurring minerals such as azurite to acquire this blue was impractical, as these minerals were rare and difficult to work. Therefore, to have access to the large quantities of blue color to meet demand, the Egyptians needed to manufacture the pigment themselves.
The earliest evidence for the use of Egyptian blue, identified by Egyptologist Lorelei H. Corcoran of The University of Memphis, is on an alabaster bowl dated to the late pre-dynastic period or Naqada III (circa 3250 BC), excavated at Hierakonpolis, and now in the Museum of Fine Arts, Boston.Lorelei H. Corcoran, "The Color Blue as an 'Animator' in Ancient Egyptian Art", in Rachael B.Goldman, (ed.), Essays in Global Color History: Interpreting the Ancient Spectrum (New Jersey: Gorgias Press, 2016), pp. 59–82. In the Middle Kingdom (2050–1652 BC) it continued to be used as a pigment in the decoration of tombs, wall paintings, furnishings, and statues, and by the New Kingdom (1570–1070 BC) began to be more widely used in the production of numerous objects. Its use continued throughout the Late period and Greco-Roman period, only dying out in the fourth century AD, when the secret to its manufacture was lost.Chase, W. T. (1971:. "Egyptian blue as a pigment and ceramic material". In: R. Brill (ed.) Science and Archaeology. Cambridge, MMassachusetts: MIT Press.
No written information exists in ancient Egyptian texts about the manufacture of Egyptian blue in antiquity, and it was first mentioned only in Roman literature by Vitruvius during the first century BC.Vitruvius, De Architectura, Book VII, Chapter 11. He refers to it as caeruleum and describes in his work De architectura how it was produced by grinding sand, copper, and natron, and heating the mixture, shaped into small balls, in a furnace. Lime is necessary for the production as well, but probably lime-rich sand was used. Theophrastus gives it the Greek term κύανος (kyanos, blue),Theophrastus, De Lapidibus (On Stones), section 55. which originally probably referred to lapis lazuli. Finally, only at the beginning of the nineteenth century was interest renewed in learning more about its manufacture when it was investigated by Humphry Davy in 1815,Sir Humphry Davy (1815), "Some experiments and observations on the colours used in painting by the ancients", Philosophical Transactions of the Royal Society of London, vol. 105, pages 97–124. Reprinted in: The Collected Works of Sir Humphry Davy, ... (London, England: Smith, Elder, and Co., 1840), vol. VI, pp. 131–159. and others such as W. T. Russell and F. Fouqué.
Nomenclature
The ancient Egyptian word signifies blue, blue-green, and green.
The term for the pigment in the Egyptian language is (khesbedj irtiu), which referred to artificial lapis lazuli ().Pagès-Camagna, S. (1998). "Bleu et vert égyptiens en question: vocabulaire et analyses". In Colinart S, Menu M, eds., La couleur dans la peinture et l'émaillage de l'Egypte Ancienne, CUEBC, Ravello, 20–22 March 1997, Edipuglia, Bari, pp. 51–59. Sometimes, it is referred to in Egyptological literature as blue frit. Some have argued that this is an erroneous term that should be reserved for use to describe the initial phase of glass or glaze production, while others argue that Egyptian blue is a frit in both the fine and coarse form since it is a product of solid state reaction.Nicholson, P. T. & Henderson, J. (2000). "Glass". In: P. Nicholson and I. Shaw (eds.), Ancient Egyptian materials and technology. Cambridge: Cambridge University Press.
This pigment is attested among Ancient Greek writings, most notably from Theophrastus in De Lapidibus, where is described as having Egyptian, Scythian and Cyprian kinds. It was known to the Romans by the name , derived from the Latin for sky or heaven, . Vitruvius in De Architectura uses this name when outlining its recipe. Pliny the Elder also attests to this name in the Natural History.
The first recorded use of "Egyptian blue" as a color name in English was in 1809.Maerz and Paul A Dictionary of Color New York:1930 McGraw-Hill Page 194; Color Sample of Sunset: Page 93 Plate 35 Color Sample L8
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 multiphase material that was produced by heating together quartz sand, a copper compound, calcium carbonate, and a small amount of an alkali (ash from salt-tolerant, halophyte plants or natron) at temperatures ranging between (depending on the amount of alkali used) for several hours. The result is cuprorivaite or Egyptian blue, carbon dioxide, and water vapor:
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, the weight percentage of the materials used to obtain Egyptian blue in antiquity was determined usually to range within these amounts:
60–70% silica (SiO2) 7–15% calcium oxide (CaO) 10–20% copper(II) oxide (CuO)
To obtain theoretical cuprorivaite, where only blue crystals occur, with no excess of unreacted quartz or formation of glass, these percentages would need to be used:
64% silica 15% calcium oxide 21% copper oxide
However, none of the analyzed samples from antiquity was made of this definitive composition, as all had excesses of silica, together with an excess of either CuO or CaO. 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%), though, does not allow glass to form and the resultant Egyptian blue is softer, with a hardness of 1–2 Mohs.
As with variations in component composition, the processing method employed also affected the texture of the resulting pigment, in terms of coarseness and fineness. Following a series of experiments, Tite et al. concluded that for fine-textured Egyptian blue, two stages were necessary to obtain uniformly interspersed crystals. First, the ingredients are heated, resulting in a coarse-textured product. This is then ground to a fine powder and water is added. The paste is then reshaped and fired again at temperatures ranging between 850 and 950 °C for one hour. These two stages were necessary to produce a paste with particles fine enough for the production of small objects. Coarse-textured Egyptian blue, however, would not have gone through the second stage. As the raw pigment was typically found in the form of slabs (in the dynastic periods) and balls (in the Greco-Roman period), these either could have been awaiting to further processing and refinement in texture, or they would have been ground as-is 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, though, 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, so with more alkali, more glass formed, and the more diluted the appearance. This type of Egyptian blue is especially evident during the eighteenth dynasty and later, and probably is 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 cuprite and tenorite.
Materials
The main component of Egyptian blue was the silica, and quartz sand found adjacent to the sites where Egyptian blue was being manufactured may have been its source, although no concrete evidence supports this hypothesis. The only evidence cited is by Jakcsh et al., who found crystals of titanomagnetite, a mineral found in desert sand, in samples collected from the tomb of Sabni (sixth dynasty). Its presence in Egyptian blue indicates that quartz sand, rather than flint or chert, was used as the silica source. This contrasts with the source of silica used for glass-making at Qantir (New Kingdom Ramesside site), which is quartz pebbles and not sand.
It is believed that calcium oxide was not added intentionally on its own during the manufacture of Egyptian blue, but introduced as an impurity in the quartz sand and alkali. As to whether the craftsmen involved in the manufacture realized the importance of adding lime to the Egyptian blue mixture is not clear from this.
The source of copper could have been either a copper ore (such as malachite), filings from copper ingots, or bronze scrap and other alloys. Before the New Kingdom, evidence is scarce as to which copper source was being used, but it is believed to have been copper ores. During the New Kingdom, evidence has been found for the use of copper alloys, such as bronze, due to the presence of varying amounts of tin, arsenic, or lead found in the Egyptian blue material. The presence of tin oxide could have come from copper ores that contained tin oxide and not from the use of bronze. However, no copper ores have been found with these amounts of tin oxide. Why a switch from the use of copper ores in earlier periods, to the use of bronze scrap during the Late Bronze Age is unclear as yet.