Egyptian blue

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. 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. 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. 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), 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, and others such as W. T. Russell and F. Fouqué. == Nomenclature ==

made out of "Egyptian blue" faience: Imported to Italy from northern Syria, it was produced 750–700 BC. (Shown at Altes Museum in Berlin) The ancient Egyptian word signifies blue, blue-green, and green. The term for it in the Egyptian language is ḫsbḏ-ỉrjt (khesbedj irtiu), which referred to artificial lapis lazuli (ḫsbḏ). It was used in antiquity as a blue pigment to color a variety of different media such as stone, wood, plaster, papyrus, and canvas, and in the production of numerous objects, including cylinder seals, beads, scarabs, inlays, pots, and statuettes. 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. 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. It was known to the Romans by the name . The first recorded use of "Egyptian blue" as a color name in English was in 1809. == 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: 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. In addition to the way 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, Tite et al. concluded that for fine-textured Egyptian blue, two stages were necessary to obtain uniformly interspersed crystals. First, the ingredients are heated, and the result is 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 possibly were needed to produce a paste that was fine enough for the production of small objects. Coarse-textured Egyptian blue, though, would not have gone through the second stage. Since it usually is found in the form of slabs (in the dynastic periods) and balls (in the Greco-Roman period), these either could have 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, 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, 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. 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. a link in terms of the alkali used for Egyptian blue before and after the introduction of the glass industry might have been possible. == Archaeological evidence ==

In the excavations at Amarna, Lisht, and Malkata at the beginning of the twentieth century, Flinders Petrie uncovered two types of vessels that he suggested were used in antiquity to make Egyptian blue: bowl-shaped pans and cylindrical vessels or saggers. In later excavations undertaken at Amarna by Barry Kemp in 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 in the samples analyzed, which the authors suggest is an indication that use of scrap copper was possible instead of bronze. 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 produced at the site. 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 it had been manufactured on site. These Egyptian blue 'cakes' possibly were later exported to other areas around the country to be worked, as a scarcity of finished Egyptian blue products existed on site. For example, Egyptian blue cakes were found at Zawiyet Umm el-Rakham, a Ramesside fort near the Libyan coast, indicating in fact that the cakes were 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 did not employ separate terms to distinguish the three products from one another. Since Egyptian blue, like faience, is a much older technology than glass, which only begins during the reign of Thutmose III (1479–1425 BC), changes in the manufacture of Egyptian blue undoubtedly 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 only could have resulted from the use of tin bronze scraps as the source of copper, which coincides with the time when bronze became widely available in ancient Egypt. == Occurrences outside Egypt ==

Egyptian blue was found in Western Asia during the middle of third millennium BC in the form of small artifacts and inlays, but not as a pigment. == Roman production ==

microspectroscopic phase distribution map of a paint layer from the church of St. Peter above Gratsch showing several minor, major and trace compounds of Egyptian blue. Around the turn of the eras, Roman sources report that a certain Vestorius transferred the production technology from Alexandria to Pozzuoli near Naples (Campania, Southern Italy). In fact, archaeological evidences confirm production sites in the northern Phlegraean Fields and seem to indicate a monopoly in the manufacture and trade of pigment spheres. Due to its almost exclusive use, Egyptian blue is the blue pigment par excellence of Roman antiquity; its art technological traces vanish in the course of the Middle Ages. == Modern applications ==

Egyptian blue's extremely powerful and long-lived infrared luminescence under visible 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 sixteenth century, long after its use was presumed to have died out. The luminescence in the near-infrared, where neither fat nor hemoglobin show high absorption coefficients, in conjunction with the capacity of Egyptian blue to delaminate by splitting into nanosheets after immersion in water, also indicates it may have several high-technology applications, such as in biomedicine (e.g. bioimaging), telecommunications, laser technology, and security inks. Egyptian blue pigment absorbs visible light, and emits light in the near-infrared range. This suggests that Egyptian blue pigment could be used in construction materials designed to cool rooftops and walls in sunny climates, and for tinting glass to improve photovoltaic cell performance. Egyptian blue can be recreated by heating mixtures of silica, copper compounds, calcium, and alkali. == See also ==