The technique of fission-track dating, used by geochemists to date certain natural glasses and minerals has been successfully applied to the dating of a few man-made glasses. The samples range in age from 10 to 125 years. The most important requirement for dating is that the glass contain appreciable concentrations of uranium. The samples studied all contained in excess of 0.5 per cent UO₃. Glasses as old as 2000 years could be dated with reasonable accuracy if they contain as much as (approx.) 0.01 per cent uranium. The technique makes use of a microscopic count of etch pits on a small fractured or polished surface. The pits are developed by mild etching with HF, and appear wherever a uranium atom has undergone spontaneous fission near the surface. In order to calculate the age of the glass (really the time that has elapsed since the glass was last heated) either the uranium content of the glass must be known or, preferably, a sample of glass must be irradiated in a reactor to induce further fission. The interesting case of the Cape Posilipo mosaics, which might contain uranium-colored tessetae, is also discussed.