Synthetic · Ancient Egypt to the Roman period
Egyptian blue is calcium copper silicate, the same compound as the rare mineral cuprorivaite. Each copper ion sits in a square-planar site within a layered silicate, and this rigid setting makes it absorb red and yellow light while reflecting blue. The same structure gives it an unusual property: when lit with visible light it emits invisible near-infrared radiation, peaking around 910 nm, with a high efficiency and a long lifetime. Ancient batches vary, and only the well-crystallised silicate glows.
It is generally regarded as the oldest synthetic pigment, made in Egypt from early in the third millennium BCE by firing together a silica sand, lime, a copper source such as ore or bronze scrap, and an alkaline flux. Analysis of an early medieval sample points to Campanian sand, a sulphidic copper ore and plant-ash flux, matching Roman accounts of production near Naples. Manufacture spread around the Mediterranean and it was traded widely until knowledge of it faded after the Roman period.
Egyptian blue is chemically very stable and does not fade in light; objects thousands of years old keep their colour. Where it does look altered, the causes are usually physical — coarse, glassy grains can crumble or lose their binder, and grinding it finely makes it paler — or the blue sits beside degrading copper greens and salts. Because its luminescence depends on crystallinity, heavily weathered or poorly fired material may glow weakly even when copper is plainly present.
Its infrared glow is the classic test. In visible-induced luminescence imaging, an object is lit with a visible source that emits no infrared and photographed with a camera modified to record near-infrared: Egyptian blue shines bright white while other blues stay dark, revealing traces invisible to the eye. XRF confirms copper, calcium and silicon, and Raman or X-ray diffraction confirm the cuprorivaite phase. The technique has found it in unexpected places, including Renaissance paintings.
It was displaced long ago by lapis-derived ultramarine and later synthetic blues, and today painters reach for synthetic ultramarine, cobalt or phthalocyanine blue. Egyptian blue itself is made again in small quantities for artists and restorers, and its near-infrared emission has made it a subject of materials research for imaging and sensing — a modern use that has nothing to do with its colour.
FactStrong evidence
Egyptian blue (cuprorivaite) emits near-infrared light peaking at 910 nm with an exceptionally high emission efficiency and long excited-state lifetime.
Source: The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue)
FactStrong evidence
Visible-induced luminescence imaging with a modified digital camera maps Egyptian blue on objects in situ, and the glow depends on the crystalline phase rather than on composition alone.
Source: The spatially resolved characterisation of Egyptian blue, Han blue and Han purple by photo-induced luminescence digital imaging; On the production of ancient Egyptian blue: Multi-modal characterization and micron-scale luminescence mapping
FactStrong evidence
Egyptian blue was long thought to have vanished after antiquity, but it has been found in a few early medieval works and in early 16th-century paintings, including a Raphael.
Caveat: Finds are rare and scattered; whether they reflect continued manufacture or reuse of ancient material is not settled.
Source: Unveiling Raphael's Christ Blessing: Chemical Analysis Reveals Distinctive Underdrawing Techniques and a Rare Early Use of Egyptian Blue in Renaissance Art; Trace compounds in Early Medieval Egyptian blue carry information on provenance, manufacture, application, and ageing
FactModerate evidence
Egyptian blue is chemically stable in all media and lightfast; specimens thousands of years old appear little changed.
Source: Egyptian blue — ColourLex pigment page; Pigments through the Ages — Egyptian blue