Many people who lived during the Middle Ages could not read. Stained-glass windows, in a church, helped people to learn the stories of the Bible. This 13th-Century window, at Chartres Cathedral in France, depicts the story of "The Wedding at Cana." Photo by Vassil, online via Wikimedia Commons. Public Domain.
Why did stained-glass windows became so popular during the Middle Ages? With their filtered light, and jewel-like appearance, such treasures created a mystical environment. As noted by Abbot Suger, whose Abbey at Saint Denis included some of the earliest stained-glass windows in France:
The whole [church] would shine with the wonderful and uninterrupted light of the most luminous windows, pervading the interior beauty. (Abbot Suger on the Abbey Church of Saint Denis and its Art Treasure, Erwin Panofsky Translation.)
But there was another important reason for medieval stained-glass windows. Because religion was an important part of daily life, stories in glass were often inspired by the Bible or by the lives of saints. In the sixth century, Pope Gregory had urged artists to paint Biblical scenes on church walls to educate the public. In the eleventh century, the Synod of Arras did the same thing since this process
enables illiterate people to learn what books cannot teach them. (Gies, Joseph and Frances, Cathedral, Forge and Waterwheel: Technology and Invention in the Middle Ages, page 130.)
Theophilus, as we have seen, described how those jewels of glass were colored in the twelfth century. Because beechwood contains manganese, its ashes (when mixed with sand and fired) produced a range of colors (white, saffron-yellow, shades of red and purple). Actual results (and they could vary greatly) depended on how long the ash-and-sand mixture was worked and how much iron it picked up from clay pots which held it. (See explanatory footnote 1, page 55, On Divers Arts.)
Beginning with Abbot Suger, French churches were built in the Gothic stylewalls of glass as story-telling windows. Glass makers in France, according to Theophilus, expanded their color schemes by grinding-up mosaics and glass vessels from "ancient pagan buildings" to create window hues of green and blue. Ashes from different types of wood could also change the resulting glass colors (just like adding bits of cobalt would turn the mixture blue while copper would turn it green).