In the early medieval period, glassmakers lost access to the soda-rich minerals of the Mediterranean. To adapt, craftsmen in Northern Europe began using potash—made from burning wood like beech or fern—creating what is known as "forest glass" ( Waldglas ). This glass often had a distinct greenish or amber tint due to iron impurities in the sand, giving medieval vessels a rugged, organic aesthetic compared to the clear glass of antiquity. The Rise of Stained Glass
Artisans colored the glass by adding metallic oxides to the molten batch: copper for red, cobalt for blue, and manganese for purple. These windows weren't just decorative; they were designed to transform the physical space into a "heavenly Jerusalem," where the shifting sunlight created a spiritual atmosphere through "divine light." Practical Use and Innovation Medieval Glass
The true pinnacle of the craft was reached between the 12th and 14th centuries. During the Gothic era, architects aimed to replace heavy stone walls with light. Glass became "the poor man’s Bible," as vibrant windows depicted biblical stories and lives of saints for a largely illiterate public. In the early medieval period, glassmakers lost access