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 Ca 2 Sb 2 O 7 by using mixtures of Sb 2 O 3 or Sb 2 O 5 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