The effects of heat treatment temperature and time on the rates of nucleation and growth, morphology, and color of cuprous oxide crystals developed in a copper lead alkali silicate glass were investigated. On cooling a melt to the appropriate temperature, the crystallization of cuprous oxide starts at about 870°C, i.e. about 360 centigrade degrees below its melting point. The temperatures of maximum crystal nucleation and growth are well separated making the control of the crystallization process easily accessible. The nucleation process is essentially completed in a very short time after heat treatment. The isothermal growth of the cuprous oxide crystals proceeds by diffusion-controlled and coarsening mechanisms in the early stage of crystallization and by the coarsening mechanism alone in the later stage. Cuprous oxide crystallizes out in the form of yellow cubes below 550°C and transforms to the stable red dendritic form at heat treatment temperatures higher than 650°C. Both these forms are known from the first millennium B.C. -- AATA