There is a strong but largely unsubstantiated belief that fern was used throughout Northwest Europe, and to a lesser extent Southern Europe, as a source of alkali for the manufacture of potash glass by medieval glassmakers. This is because fern is mentioned in surviving documentary evidence relating to glass production, and Medieval glasshouses in NW Europe were often located in forests. This paper presents compositional,archaeological and documentary evidence for the use of the fern as a source of alkali in medieval and post medieval glassmaking. The most compelling evidence can be found in documentary material but this should be treated with some caution. Nevertheless, cumulatively, these sources suggest tentatively that fern ash may have been used at a number of glassmaking centres throughout Europe.
