Glass is one of several early modern industries where the development from small-scale workshop to large-scale industry offers a valuable insight into wider socio-economic trends. Previously, medieval and early modern forest (wood ash) glass has been studied using a range of analytical techniques. However, characterisations of production centres and exchange systems for forest glasses are difficult to verify, in part because very few examples of raw glass from furnace sites have been investigated. Compositional analysis can provide evidence for the raw materials used and can sometimes provide compositional groupings specific to sites. However, strontium and neodymium isotope determinations can actually provenance the glass by linking the geological ages, or sources, of raw materials to production sites. Using EPMA fifty-six raw glass samples from four sites in two English production areas in operation during the 14^th and 16^th centuries have been analysed. These analyses have shown that the main products of these sites were potash-rich, wood or bracken ash glasses. However, the chemical composition does not provide sufficient resolution to identify differences between the two English regions or the sites within these regions. A subset of five raw glass samples from each of these sites have also been analysed using TIMS to determine strontium and neodymium concentration and isotope ratios. The results allow glasses produced in each region to be differentiated, and can even be used to divide glass production sites within the regions.