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Egyptian blue, also known as calcium copper silicate (CaCuSi4O10 or CaOCuO(SiO2)4) or cuprorivaite, is a pigment that was used in Ancient Egypt for thousands of years. It is considered to be the first synthetic pigment. The pigment was known to the Romans by the name caeruleum. After the Roman era, Egyptian blue fell from usage and the manner of its creation was forgotten.
The ancient Egyptian word wedjet signifies blue, blue-green and green.
Egyptian blue is a synthetic blue pigment made up of a mixture of silica, lime, copper, and alkali. Its color is due to a calcium-copper tetrasilicate CaCuSi4O10 of exactly the same composition as the naturally occurring mineral cuprorivaite. It occurs in Egypt during the 3rd millennium BC and is the first synthetic pigment to have been produced there, continuing in use until the end of the Greco-Roman period (332 BC–395 AD). The term for it in the Egyptian language is hsbd-iryt, which means artificial lapis lazuli (hsbd). It was used in antiquity (1) as a blue pigment to color a variety of different media such as stone, wood, plaster, papyrus, and canvas; and (2) in the production of numerous types of objects, including cylinder seals, beads, scarabs, inlays, pots and statuettes. It is also sometimes 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. Its characteristic blue color, resulting from one of its main components—copper—ranges from a light to a dark hue, depending on differential processing and composition. Apart from Egypt, it has also been found in the Near East, the Eastern Mediterranean and at the limits of the Roman Empire. Although undoubtedly an Egyptian invention, it is unclear as to whether its existence elsewhere was a result of parallel inventions or whether its technology had spread to these areas.
History and background
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 semi-precious 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 appropriate the large quantities of blue color that the Egyptians sought, it was necessary for them to manufacture the pigment themselves. It has also been found at Ayanis fortress in eastern Turkey.
The Egyptians developed a wide range of pigment variety including what is now known as Egyptian blue, which was the first of its color at the time of its development. This accomplishment was due to the advancement of Egypt as a settled agricultural society. This stable and established civilization encouraged the growth of a non-labor workforce, including clerics and the Egyptian theocracy. Egyptian pharaohs were patrons of the arts and consequently were devoted to the advancement of pigment technology.
The earliest evidence for the use of Egyptian blue is in the 4th Dynasty (c.2575–2467 BC), limestone sculptures from that period in addition to being shaped into a variety of cylinder seals and beads. 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 utilized in the production of numerous objects. Its use continued throughout the Late period, and Greco-Roman period, only dying out in the 4th century AD, when the secret to its manufacture was lost. There is no written information in ancient Egyptian texts about the manufacture of Egyptian blue in antiquity and was only first mentioned in Roman literature by Vitruvius during the first century BC. He refers to it as coeruleum 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), which probably originally referred to lapis lazuli. Finally, it was only at the beginning of the 19th century that there was a renewed interest in learning more about its manufacture when it was investigated by Sir Humphry Davy in 1815 and others such as W. T. Russell and F. Fouqué.
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 and 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:
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 and 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 of 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.
The main component of Egyptian blue was the silica, and it has been suggested that quartz sand found adjacent to the sites where Egyptian blue was being manufactured was its source, although there is no concrete evidence to support 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 (6th 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 on its own in the manufacture of Egyptian blue, but introduced as an impurity in the quartz sand and alkali. It is not clear from this then as to whether the craftsmen involved in the manufacture realized the importance of adding lime to the Egyptian blue mixture.
The source of copper could have either been a copper ore (such as malachite), filings from copper ingots or bronze scrap and other alloys. Prior to the New Kingdom there is scarce evidence as to which copper source was being used, but it is believed to have been copper ores. During the New Kingdom, there is evidence 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. Some have argued that the presence of tin oxide could have come from copper ores that itself contained tin oxide and not from the use of bronze. However, no copper ores have been found with these amounts of tin oxide. It is unclear as yet, why there would have been a switch from the use of copper ores in earlier periods, to the use of bronze scrap during the Late Bronze Age. It is possible that reserves had run out.
The total alkali content in analyzed samples of Egyptian blue is greater than 1%, suggesting that the alkali was introduced deliberately into the mixture and not as an impurity from other components. Sources of alkali could either have been natron from areas such as Wadi Natroun and El-Kab, or plantash. By measuring the amounts of potash and magnesia in the samples of Egyptian blue, it is generally possible to identify which source of alkali had been used, since the plantash contains higher amounts of potash and magnesia than the natron. However, due to the low concentration of alkali in Egyptian blue, which is a mere 4% or less, compared to glass, for example, which is at 10–20%, identifying the source is not always easy. It has been suggested, nonetheless, that the alkali source was natron, although the reasons for this assumption are unclear. On the other hand, analysis by Jaksch et al. of various samples of Egyptian blue identified variable amounts of phosphorus (up to 2 wt %), suggesting that the alkali source used was in actuality plantash and not natron. Since the glass industry during the Late Bronze Age used plantash as its source of alkali, there might have possibly been a link in terms of the alkali used for Egyptian blue before and after the introduction of the glass industry.
Amarna: In the excavations at Amarna, Lisht and Malkata at the beginning of the twentieth century, Petrie uncovered two types of vessels that he suggested were used in antiquity to make Egyptian blue: bowl-shaped pans and cylindrical vessels/saggers. In recent excavations at Amarna by Barry Kemp (1989), very small numbers of these “fritting” pans were uncovered, although various remaining pieces of Egyptian blue ‘cake’ were found, which allowed the identification of five different categories of Egyptian blue forms and the vessels associated with them: large round flat cakes, large flat rectangular cakes, bowl-shaped cakes, small sack-shaped pieces and spherical shapes. No tin was found present in the samples analyzed, which the authors suggest is an indication that there was possible use of scrap copper instead of bronze.
Qantir: In the 1930s Mahmud Hamza excavated a number of objects related to the production of Egyptian blue at Qantir, such as Egyptian blue cakes and fragments in various stages of production providing evidence that Egyptian blue was actually produced at the site of Qantir. Recent excavations at the same site, uncovered a large copper-based industry, with several associated crafts, namely bronze-casting, red-glass making, faience production and Egyptian blue. Ceramic crucibles with adhering remains of Egyptian blue were found in the excavations, suggesting again that it had been manufactured on site. It is also possible that these Egyptian blue ‘cakes’ were later exported to other areas around the country to be worked as there was a scarcity of finished Egyptian blue products on site. For example, Egyptian blue cakes were found at Zawiyet Umm el-Rakham, a Ramesside fort near the Libyan coast, indicating that cakes were in fact traded, and worked at and reshaped away from their primary production site.
Connections with other vitreous material and with metals
Egyptian blue is closely related to the other vitreous materials produced by the ancient Egyptians, namely glass and Egyptian faience, and it is possible that the Egyptians themselves did not employ separate terms to distinguish the three products from one another. Although it is easier to distinguish between faience and Egyptian blue, due to the distinct core of faience objects and their separate glaze layers, it is sometimes difficult to differentiate glass from Egyptian blue due to the very fine texture that Egyptian blue could occasionally have. This is especially true during the New Kingdom as Egyptian blue became more refined and glassy and continued as such into the Greco-Roman period. Since Egyptian blue, like faience, is a much older technology than glass, which only begins during the reign of Thutmose III (1479–1425 BC), there are no doubt changes in the manufacture of Egyptian blue that were associated with the introduction of the glass industry.
Analysis of the source of copper used in the manufacture of Egyptian blue indicates a relationship with the contemporaneous metal industry. Whereas in the earlier periods, it is most probable that copper ores were used, during the reign of Tutmosis III, the copper ore is replaced by the use of bronze filings. This has been established by the detection of a specific amount of tin oxide in Egyptian blue which could only have resulted from the use of tin bronze scraps as the source of copper, which coincides with the time that bronze became widely available in ancient Egypt.
Occurrences outside of Egypt
Egyptian blue was found in Western Asia during the middle of 3rd millennium BC in the form of small artifacts and inlays, but not as a pigment. It was found in the Mediterranean area at the end of the Middle Bronze age, and traces of tin were found in its composition suggesting the use of bronze scrap instead of copper ore as the source of copper. During the Roman period there was extensive use of Egyptian blue, as a pot containing the unused pigment, found in 1814 in Pompeii, illustrates. It was also found as unused pigment in the tombs of a number of painters. Etruscans also used it in their wall paintings. The related Chinese Blue has been suggested as having Egyptian roots.
Egyptian blue’s extremely powerful and long-lived luminescence under infrared light has enabled its presence to be detected on objects which appear unpainted to the human eye. This property has also been used to identify traces of the pigment on paintings produced as late as the 16th century, long after its use was assumed to have died out. The luminescence, in conjunction with the capacity of Egyptian blue to delaminate by splitting into nanosheets after immersion in water, also indicates that it may have several hi-tech applications, such as in biomedicine, telecommunications, laser technology and security inks.
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