From chemical analyses of ancient glasses found in Europe, Western Asia, and Africa from roughly the 15th century BC until the 12th century AD, five main compositional categories have become apparent. With the possible exception of lead glasses which are only occasionally encountered in finds from this period, each of the main categories was prevalent over a wide geographic area for a period of at least several centuries. The categories are described in terms of the expected ranges of concentrations of the five elements, magnesium, potassium, manganese, antimony, and lead, which most clearly differentiate the groups.