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August 22, 2017

A Study in Blue Pigments Part 2 - Egyptian Blue

Egyptian blue, also known as calcium copper silicate, is a very stable synthetic pigment used in ancient Egypt for thousands of years. 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 such as stone, wood, plaster, papyrus, canvas, and in the production of numerous objects, including cylinder seals, beads, scarabs, inlays, pots, and statuettes. In Egyptian belief, blue was the color of the heavens, and hence the universe. It was also associated with water and the Nile. Thus, blue was the color of life, fertility and rebirth.

The oldest known example of the exquisite pigment is said to be about 5000 years old, found in a tomb painting dated to the reign of Ka-Sen, the last pharaoh of the First Dynasty. In the Middle Kingdom , it continued to be used as a pigment in the decoration of tombs, wall paintings, furnishings and statues, and by the New Kingdom, began to be more widely used in the production of numerous objects. Apart from Egypt, it has also been found in the Near East, the Eastern Mediterranean, and the limits of the Roman Empire. After the Roman era, Egyptian blue fell from use and the manner of its creation was forgotten. 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 B.C. He 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. Today, we know that Egyptian Blue is made by heating a mixture of a calcium compound (typically calcium carbonate), a copper-containing compound (metal filings or malachite), silica sand and soda or potash as a flux, to around 850-950 degrees Celsius.

In an exciting development, it was reported in 2009 that Egyptian blue shows exceptional luminescence in the near-infrared region when red light is shone onto it. 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.

Scientists now believe that its unique properties may make Egyptian Blue suitable for a variety of modern applications. Its long luminescence lifetime and intense IT emission make it a promising candidate for use in biomedical applications. For example, the long luminescence lifetime and the greater penetration depth in human tissue of infrared compared to UV or visible photons raises the possibility of obtaining more detailed and highly resolved biomedical images by using the pigment as an imaging agent. Egyptian Blue may also one day be utilized for communication purposes, as its beams are similar to those used in remote controls and telecommunication devices. As an ink solution, Egyptian blue also represents an attractive alternative to the expensive lanthanide compounds currently used in security inks. While the use of Egyptian blue in modern high-tech applications is still in its infancy at this stage, it does seem that its future is a bright one.

Stay tuned for our next installment on blue pigments when we discuss the history of Ultramarine.