By Lisa YardeGlass artists learned to mix sand and wood ash, and introduced color from metallic salts and oxides when the glass was still molten, to produce shades of red, blue, yellows and gold and green. In the hands of medieval glaziers and glass painters, stained glass became admired for both its utility and splendor, the best examples of which are seen in churches and cathedrals.
With its use of natural light to enhance the myriad, jewel-toned colors, stained glass windows have depicted religious scenes since at least the tenth century. A union of form and function, the windows served the dual purposes of making the church a special, sacred building, and teaching the largely illiterate masses about the Bible through its visual representations. A twelfth century monk, Theophilus, wrote On Diverse Arts, in which he explained the technique of stained glass window construction:
...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 [a tool with a hardened steel point]. Enclose them with lead cames [a soft metal used to divide small pieces of glass]...and solder on both sides. Surround it with a wooden frame strengthened with mails and set it up in the place where you wish.
Europe has some of the oldest and finest examples of stained glass windows, in its great cathedrals. Canterbury Cathedral in Kent, England showcases twelfth to fifteenth century glasswork (above) with biblical themes in its western and southern windows, including the Miracles of St. Thomas and images from the Old and New Testaments. Some of the artwork is high and difficult to see, especially the Ancestry of Christ, which depicts biblical figures interspersed with saints and the coat of arms of several noble families in the south window. 
To the north of the country in Yorkshire, York Minster's Great East Window remains the largest medieval stained glass window in the world (above) and features biblical scenes from Genesis and Revelation. It is the size of a tennis court. The Five Sisters Window has glass from 1250, while the South Rose window glass dates from 1490. France's great cathedrals also have beautiful installations of glasswork, the most famous of which (below) is on display at the Gothic cathedral of Chartres, fifty miles outside of Paris. Most of its windows date from the early thirteenth and fourteenth centuries, including subjects such as the Last Judgment in the west Rose Window, prophets and saints depicted in the south Rose Window that dates from the 1230s, and the glorification of the Virgin and Christ.
