Ancient documentation referring to the use of cinnabar in the production of luster decorations has recently been corroborated by archaeological findings. However, luster decorations do not show any trace of the use of cinnabar in their composition because the temperatures involved in luster firing (500degrees-600degreesC) result in the complete decomposition and volatilization of cinnabar. An in situ x-ray diffraction experiment was designed to clarify the role of cinnabar in the production of luster decorations. The high-energy, high-flux radiation ID15B beamline at the European Synchrotron Radiation Facility was necessary to ensure good time/temperature resolution, penetration, and high-quality data. Results show that cinnabar acts as a reducing agent for copper because it provides the formation of copper (Cu+) and silver (Ag+) sulfur-containing phases, and it inhibits the crystallization of metallic silver in the temperature range of interest in luster production. The addition of elemental sulfur also produces these phases, but at temperatures <500degreesC, which are not adequate for the production of copper-containing lusters.