The laminar structure of weathering crusts on soda-lime-silica and on potash-lime-silica glasses is associated with variations in selective leaching due to seasonal variation of temperature and availability of water. The theory has been tested by counting the laminae in the crusts on seven glasses which had been buried or submerged at known dates in the 17th or 18th centuries, and on four glasses of less well known dates between the 3rd and 14th centuries. The agreement of the dates thus calculated with those expected from archaeological evidence warrants acceptance of the theory. Various sources of error in the calculated date are discussed. Further data on additional ancient glass samples will be published in <i>Journal of Glass Studies. 6 refs.