Few areas of medieval art possess the magic of stained glass windows. The experience of the shimmering majesty of a cathedral flooded with jeweled light is unforgettable and usually reduces chattering tourists to stunned, reverent silence and awe. Yet many of those masterpieces, once thought as eternal as Paradise, have entered collections in Europe and America. Paradoxically, today stained glass is one of the most affordable and underappreciated areas of medieval art.

The origins of stained glass are cloaked in mystery. There is reason to believe that the Romans used colored glass in windows and walls, and some scholars see a connection between stained glass and medieval cloisonné enameling. Whatever the case, the art of making beautiful pictorial stained glass windows erupted quite suddenly in Europe in the 10th century.

To impart color to glass, medieval craftsmen melted it and then added various metallic salts and oxides. Silver tinted glass yellow and gold, copper in various mixtures created brick reds and greens, cobalt made blue, and gold, the most precious of all additives, produced a deep and brilliant cranberry red. By the beginning of the 11th century, colored narrative windows seemed to be a requirement in the construction of churches, and the basic rules of its manufacture were described in a how-to manuscript by the monk Theophilus around 1100: "If you want to assemble simple windows, first mark out the dimensions of their length and breadth on a wooden board, then draw scroll work or anything else that pleases you, and select colors that are to be put in. Cut the glass and fit the pieces together with the grozing iron. Enclose them with lead canes … and solder on both sides. Surround it with a wooden frame strengthened with nails and set it up in the place where you wish." He makes it sound easy!

Details such as facial features and shading were added in grisaille stain and black enamel. According to Mary Shepherd, president of the International Center of Medieval Art in New York, there soon developed various unofficial national styles of stained glass. "French stained glass tended toward extensive use of blue and red, with other colors used as accents, while German stained glass opted for what I call the ‘Crayola box’ effect, with lots of colors. German glass tends to be more expressionistic, with more detailing of faces and hair."

Though most designers of medieval stained glass are unknown, the iconography of a considerable amount of Netherlandish and German glass is derived from the engravings of such printmakers as the Master E.S., the Housebook Master and Martin Schongauer. Such important painters as Albrecht Dürer, Hans von Kulmbach, Hans Baldung, Lucas Van Leyden, Bernard Van Orley, Jan Gossaert and Hans Holbein the Younger directly supplied designs and cartoons for stained glass, occasionally painting glass themselves. The Flemish master Dirck Vellert even became more renowned for his glass designs than for his panel paintings.

With the rise of the merchant classes in the 15th century, stained glass moved into residences for the first time, in the form of finely painted roundels (most no more than 6 to 8 inches in diameter) highlighted in various shades of yellow and orange against a clear background. Unrestricted by leading, the designs were rendered with painterly and atmospheric effects, heightened by occasional careful scratching away of the grisaille glaze to provide bright accents. Because of their secular settings, these roundels lent themselves to a greater variety of subject matter than church windows, encompassing genre scenes, folktales and allegories in addition to religious subjects. Roundels were especially popular in the Netherlands and in England, while multicolored leaded heraldic windows featuring allegorical personifications of figures brandishing family coats of arms became favored in Germany, where their popularity endured until well into the 17th century.