Egyptian blue

Definition

Egyptian blue is a synthetic blue pigment made up of 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 made in Egypt during the third millennium BC and is the first synthetic pigment 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 ḫsbḏ-ỉrjt, which means artificial lapis lazuli (hsbd). 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. 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 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.

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 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 they needed, it was necessary for the Egyptians to manufacture the pigment themselves.

The Egyptians developed a wide range of pigment varieties, 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 nonlabor workforce, including clerics and the Egyptian theocracy. Egyptian pharaohs were patrons of the arts and consequently were a stimulus for the advancement of pigment technology.

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 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, only at the beginning of the 19th century was interest renewed 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 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 800 and 1,000 °C (1,470 and 1,830 °F) (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, the weight percentage of the materials used to obtain Egyptian blue in antiquity was determined to usually 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% silica15% calcium oxide21% 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.

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 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 is usually found in the form of slabs (in the dynastic periods) and balls (in the Greco-Roman period), 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, 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 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.

Archaeological evidence

Amarna: In the excavations at Amarna, Lisht, and Malkata at the beginning of the 20th 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 in the samples analyzed, which the authors suggest is an indication that use of scrap copper was possible 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. 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 the 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 the Egyptians possibly 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), 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 could only 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 of 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. 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, use of Egyptian blue was extensive, 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.

Modern applications

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 it may have several high-technology applications, such as in biomedicine, telecommunications, laser technology, and security inks.