Medieval And Post-Medieval Glass Technology: Seasonal Changes In The Compositon Of Bracken Ashes From Different Habitats Through A Growing Season

Bracken ashes were used as a primary raw material in Northern EU glass manufacture in the late medieval period. However, conflicting reports based on historical sources & modern bracken growth patterns suggest different harvesting times for bracken. This paper explores the chemical composition of bracken ashes from 3 study areas, to explore changes in composition through the growing season which may influence the choice of harvesting time of bracken for use in glass production. The results show that through the season SiO2, CaO, Fe2O3 & MnO increase in concentration & that K2O & P2O5 peak mid-June & gradually decline in concentration. This would give an optimum time to harvest bracken of mid-June when the alkali concentrations were high, & the refractory components & colouring elements low. The results also indicate that glasses produced with ashes which were harvested at different times through the growing season will differ compositionally.