Gallo-Roman glass and that manufactured during the several centuries after the disintegration of the Roman empire has much the same composition in examples found in northern and western Europe. The components calcium oxide and alumina (CaO and Al₂O₃) increase or decrease together. Compositional variation between different samples from this geographic area are not much greater than those at a single site of manufacture. Contamination during the manufacturing process is unlikely to account for the CaO and Al₂O₃ contents in glass from this period according to analyses of glass adhering to crucibles or fusion pots. CaO and Al₂O₃ were apparently added intentionally to the glass composition. A slight tendency to an increase in these oxides is accompanied by a decrease in Na₂O content which might be due to a loss through volatilization during reworkings. Such a trend is apparent in the 8th-9th-century samples. The remarkably constant composition of glass found in a wide area for objects produced over a long period of time suggests that a limited number of production sites existed for either the raw materials or the confection of raw glass which was fashioned at various other sites.