Laser Engraving Gulf Pearl Shell – Aiding the Reconstruction of the Lyre of Ur

Abstract

The Lyre of Ur was one of two bull's-headed harp instruments that British archaeologist Sir Leonard Woolley recovered from a mass grave during his excavations of Ur's Royal Cemetery in the 1920s and 1930s in Iraq. Re-created unplayable models of the lyre are held in Pennsylvania, London, and, until recently before it was vandalized, the Baghdad Museum. It is believed that the original lyre is approximately 4,750 years old. An attempt to re-create an authentic playable version is underway. The instrument is to be ornamented by a golden-sheeted bull's head and geometrically inlaid lapis lazuli as well as engraved gulf pearl shell plaques depicting Sumerian images, which are to be placed on the front of the lyre. Authentic cedarwood from Baghdad was used to create the main framework of the lyre. Due to the intricate designs required for the shell plaques, an investigation to laser-engrave them was undertaken. The main objective when laser-engraving the pearl shells (which are chiefly calcium carbonate) was to achieve sufficient depth without compromising image quality so that the plaques could be backfilled with bitumen in order to replicate the originals. Moreover, it was imperative to re-create the images without damage to the surrounding pearl either by scorching or redepositing removed material. Experiments were carried out to engrave the shells utilizing a Synrad CO2 laser, which has galvanometer-controlled mirrors to direct the beam that enables it to be scanned in an XY direction across the surface of the material. An image software package was used in conjunction with the laser so that the images could be manipulated in terms of size and positioning. It was necessary to remove excess material produced between successive laser passes using a soft brush to allow full penetration of the laser beam to the newly created lower surface level. Successful engraving was achieved using multiple laser passes and an identified optimum-processing window.