During the 18th Egyptian dynasty (1570-1292 B.C.), opaque white, blue and turquoise glasses were opacified by calcium antimonate crystals dispersed in a vitreous matrix. The technological processes as well as the antimony sources used to manufacture these crystals remain unknown. Our results shed a new light on glassmaking history: contrary to what was thought, we demonstrate that Egyptian glassmakers did not use in situ crystallization but first synthesized calcium antimonate opacifiers, which do not exist in nature, and then added them to a glass. Furthermore, using transmission electron microscopy (TEM) for the first time in the study of Egyptian opaque glasses, we show that these opacifiers were nano-crystals. Prior to this research, such a process for glassmaking has not been suggested for any kind of ancient opaque glass production. Studying various preparation methods for calcium antimonate, we propose that Egyptian craftsmen could have produced Ca2Sb2O7 by using mixtures of Sb2O3 or Sb2O5 with calcium carbonates (atomic ratio Sb/Ca = 1) heat treated between 1000 and 1100°C. We developed an original strategy S. Lahlil ( ) * I. Biron * M. Cotte C2RMF, Centre de Recherche et de Restauration des Musées de France, 14 quai François Mitterrand, Palais du Louvre, Porte des Lions, 75001 Paris, France e-mail: sophia.lahlil@culture.gouv.fr Fax: +33-140-202422 M. Cotte * J. Susini ESRF, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France N. Menguy IMPMC, Institut de Minéralogie et de Physique des Milieux Condensés, Universités Paris 6 et 7, IPGP 140 rue Lourmel, 75015 Paris, France focused on the investigation of the crystals and the vitreous matrices using an appropriate suite of high-sensitivity and high-resolution micro- and nano-analytical techniques (scanning electron microscopy (SEM), X-ray diffraction (XRD), TEM). Synchrotron-based micro X-ray absorption near edge spectroscopy (μ-XANES) proved to be very well suited to the selective measure of the antimony oxidation state in the vitreous matrix. This work is the starting point for a complete reassessment not only of ancient Egyptian glass studies but more generally of high-temperature technologies used throughout antiquity.