area and centuries of time, the medieval glass (of Si/K/Ca composition) is characterised by a variety of compositions. This may be explained to some extent by examining how the melting temperature of glass depends on the relative proportions of its components, which for simplicity, are reduced to three. In practice glass contains many more components that complicate the system. The study of such ternary systems, together with analysis of trace elements is useful to archaeologists for establishing the provenance of glass. It is believed that in pre-Medieval times the batch of raw materials was heated to a temperature where it partially melted, the unmelted parts removed and washed of non-reactive components, and added to the next batch. Because of the strong way that the Si/Na/Ca compositions affect the melting temperature, the resulting glass was of a fairly uniform composition regardless of the recipe of raw materials used. The melting temperatures of the Si/K/Ca glasses are not so strongly affected by composition, resulting in glasses of more varied composition, so the self-limiting features of the Na system that allowed the traditional partial-batch method to produce consistent compositions, ceased to apply, and a new way of controlling consistency had to be developed. The wide variety of compositions, together with historical accounts of glassmaking, suggest that the new method involved melting a complete batch of raw materials, removing the unreactive components as scum. From approximately 1400 AD, in an effort to compete with the quality of Venetian glass, it was found that calcium oxide (CaO) added as flux to the sand-potash mix in the form of shells, limestone, or marble gave a clearer glass, by virtue of reducing the amount of potash required along with its attendant colorants. Comparative compositions Typical compositions of some historical and ancient glasses - the components are given in weight per cent; in addition to those listed the ancient glasses also would have contained up to one per cent iron oxide and up to three per cent aluminium oxide, in addition to any colorants and opacifiers Control of colour Experimenting with the new technology, the forest glassmakers found it difficult to achieve the high standards of clarity and colour of the Roman methods, due mainly to the great variability of colour-controlling elements in the raw materials. European sand and soil is generally higher in iron and manganese. Iron gives a blue-green tinge to glass under usual furnace atmosphere