window glass is an essential component of most buildings but the transparancy of much glaass means it often goes unnoticed. modern window glass is almost invisible but historic glass usually has a tint or colour and variation in surface texture and thickness, all of which distort a transmitted (and reflected) image. this distortion contributes to the overall aesthetic quality of a historic building. it is important that any architectural conservation decisions, such as the retention and/or replacement of window glass, proceed from a possition of knowledge. The most important aspect of extant glazing is wheather or not it is original. this paper explores the use of a chemical analysis technique to determine the age of historic glass.