Colored Light as Theology

Medieval theologians saw colored light as the closest material echo of divine illumination — light filtering through glass could lift the soul from physical sight to spiritual vision.

Making that light demanded extraordinary technical knowledge. Glassmakers melted metallic oxides into the molten glass to fix precise colors: cobalt for blue, copper for red, and manganese for purple. The most prized shade, a deep sapphire blue, relied on recipes guarded as trade secrets and passed from master to apprentice.