Egyptologists have long noted that the surfaces of many ancient Egyptian objects made of gold bear a distinctive coloration that ranges from a pale reddish hue to a dark purple. This effect is observed on solid cast figures as well as on hammered sheet metal and gold leaf, such that its origin would seem to be independent of the technology used for fabrication. A typical example is the gilded face mask on the mummy of Ukhhotep . While the effect has been recognized for more than a century, its cause remained a subject of speculation until recently. Over the years, numerous hypotheses have been advanced to explain the phenomenon, including tarnishing of a debased gold alloy, remanent colloidal gold following selective corrosion and removal of alloying elements such as silver and copper, deposition of organic films, and adventitious or deliberate addition of iron-bearing minerals such as hematite or pyrite to the gold alloy. Notably, Alfred Lucas, one of the foremost early researchers in the study of ancient Egyptian technology, correctly surmised that the vast majority of such colorations resulted from fortuitous tarnishing of silver-bearing gold and also recognized correctly that a smaller group of objects bearing a distinctly different red coloration represented another phenomenon altogether.
The idea that this coloration derives from a corrosion process and not a deliberate patination is prompted partly by the fact that nearly all native gold occurs as an alloy of gold and silver known as electrum, and partly by occurrences of the coloration in what are sometimes observed to be seemingly irregular distributions on the surfaces of objects. The most notable examples of this kind are the gold-leaf decorations on the wood sarcophagus enclosures from the tomb of Tutankhamun, where areas of bright gold leaf are seen juxtaposed against areas of a dark purple coloration along irregular borders that would seem to have no relationship to an intended design.
Early attempts to analyze the red colorations often were confounded by the extremely small thicknesses of the layers, such that samples obtained by scraping–no matter how judiciously performed–were usually overwhelmed by contamination from the substrate alloy. However, analysis in situ by x-ray diffractometry and x-ray fluorescence spectrometry has provided a rapid and straightforward way of characterizing the films and has shown them typically to be composed of one or more silver-gold sulfides. The species responsible for the predominant reddish purple coloration is most often indicated to be AgAuS, a compound sometimes found in nature as the mineral petrovskaite. In addition, synthetic gold-silver alloys having a silver content between approximately 8 and 11 weight percent silver have been observed to develop red-purple tarnish films identical in appearance and composition to those found on ancient Egyptian silver-gold objects when exposed to sulfide ion for extended periods at elevated temperatures. With increasing silver content and prolonged exposure to sulfide ion, both historical gold-silver objects and modern synthetic gold-silver surfaces develop black tarnishes that include another phase, Ag3AuS2, which also occurs in nature as the mineral uytenbogaardtite. Taken together, the evidence suggests that the red colorations derive largely–as Lucas first conjectured–from fortuitous tarnishing of native electrum having silver-gold compositions appropriate for the formation of the AgAuS phase.
Red sulfide tarnishes have been identified on historical gold-silver objects from other cultural contexts, including goldwork from the Royal Cemetery at Ur and nineteenth-century European jewelry. That these tarnishes occur predominantly on ancient Egyptian objects likely reflects the high sulfide ion activity associated with the typical contexts of sealed burial chambers as well as the unparted gold-silver alloys used in antiquity.
As a footnote to the discussion, it should be added that not all red-purple colorations on historical gold objects belong to the sulfide-tarnish group described here. Indeed, as Lucas also observed, a small number of gold pieces from the tomb of Tutankhamun bear a bright, translucent red coloration on their surfaces distinctly different in appearance from the darker and more opaque examples. The origin of the color on these unusual objects has not been determined, but may well reside in the deliberate or accidental addition of iron-bearing compounds to the gold, as synthetic samples of such composition have yielded similar appearing surfaces. There also occur archaeological gold objects that bear reddish accretions of hydrated iron oxides, such as lepidocrocite, presumably deposited as residues from groundwater during burial, as well as the gold masks and other objects from Precolumbian South America that exhibit deliberately applied coatings of the red mercuric sulfide mineral cinnabar . Finally, we should mention that the addition of copper to gold in several types of Egyptian objects during the reign of Akhenaten appears to have been done for its rutilizing effect, and that during the Third Intermediate Period copper-rich gold inlays were used with precious-metal inlays of other compositions and hues for the embellishment of large figural bronzes.