I will publish a multipart essay on the history of optics at the website Jihad Watch later this month. One of the parts will be about the history of glass, a fascinating subject which most of us rarely think about. We often talk about how much we owe to the ancient Greeks, but when it comes to the use of glass, we owe much more to the Romans than to the Greeks.

Today we see huge glass windows in every major city in the world, but many people don't know that the Romans were the first to use glass for architectural purposes, and the first to make glass windows. The Roman legacy of glassmaking survived after the fall of the Roman Empire and was carried in different directions. Under the influence of Christianity, the introduction of glazed windows, particularly in churches, and the further development of painted and stained glass manufacture was one of the most decorative uses. Here is a quote from the book Glass: A World History by Alan Macfarlane and Gerry Martin, page 20:

"There are references to such windows from fifth century France at Tours, and a little later from north-east England, in Sunderland, followed by developments at Monkwearmouth, and in the far north at Jarrow dating to the period between 682 and c.870. By AD 1000 painted glass is mentioned quite frequently in church records, for example in those of the first Benedictine Monastery at Monte Cassino in 1066. It was the Benedictine order in particular that gave the impetus for window glass. It was they who saw the use of glass as a way of glorifying God through their involvement in its actual production in their monasteries, injecting huge amounts of skill and money into its development. The Benedictines were, in many ways, the transmitters of the great Roman legacy. The particular emphasis on window glass would lead into one of the most powerful forces behind the extraordinary explosion of glass manufacture from the twelfth century."

This story is explored further in the book The History of Stained Glass by Virginia Chieffo Raguin.

Page 10: "Stained glass, considered a precious object, was linked in the twelfth and thirteenth centuries to the aesthetics of precious stones and metalwork; it therefore received a place of honour in the building that housed it […]. The importance of stained glass and gems may be explained by a prevailing attitude toward light as a metaphor in premodern Europe. In the Old Testament light is associated with good, and darkness with God's displeasure. The very first verses of Genesis announce to the reader that 'the earth was void and empty, and darkness was upon the face of the deep', then God created light and 'saw the light, that it was good' (Genesis 1:2-3). Light was associated with knowledge and power, 'the brightness of eternal light, and the unspotted mirror of God's majesty' (Wisdom 7:26). Light also functioned as a symbol of God's protection."

Page 32: "Traditionally, stained glass is used as an architectural medium and, as such, it is integral to the fabric of a building; not only, or always, a work of art, but also a screen letting in and modifying the light and keeping out the elements. Its development as a major art form in the Middle Ages was dependent on the needs of a powerful client, the Christian Church, and the evolution of architectural forms that allowed for ever larger openings in the walls of both humble churches and great cathedrals, producing awe-inspiring walls of coloured light. Its exact origins are uncertain. Sheets of glass, both blown and cast, had been used architecturally since Roman times. Writers as early as the fifth century mention coloured glass in windows. Ancient glass was set in patterns into wooden frames or moulded and carved stucco or plaster, but each network had to be self-supporting, which limited the kinds of shapes that could be used. When or where strips of lead were first employed to hold glass pieces together is not recorded, but lead's malleability and strength greatly increased the variety of shapes available to artists, giving them greater creative freedom."

Excavations at Jarrow in northern England have yielded strips of lead and unpainted glass cut to specific shapes from the seventh to the ninth centuries. Benedictine monks played an important role in the spread of stained glass, as in many other things. Often cited as the first Gothic construction, the choir of the Abbey of Saint-Denis, 1140-44, gives an important place to stained glass. We should remember that in the twelfth century, monks were still in some ways the elite class of society.

Raguin, page 63: "The windows they commissioned reflected not only their erudition but also their method of prayer: gathering several times a day in the choir area of the church to pray communally, primarily by singing psalms. The monks remained in the presence of the works of art they set in these spaces. With the construction of his abbey's new choir, Abbot Suger (1081-1155) of Saint-Denis installed a series of windows exemplary of monastic spirituality and twelfth-century visual thinking. Suger, a man of unusual determination and management skills, was a trusted advisor of Louis VII, who reigned from 1137 to 1180. Responding to the call of Bernard of Clairvaux, Louis embarked on the unsuccessful Second Crusade, 1147-49, leaving Suger to act as regent of France in his absence. The abbot's influence with the monarchy consolidated Saint-Denis's place as the site of burial for French kings and the repository of the regalia – crown, sceptre, spurs, and other ceremonial objects – of coronation (coronations themselves, however, were held in the cathedral of Reims). Suger rebuilt the eastern and western ends of the church around 1141-44, using revolutionary vaulting and construction techniques that proclaimed the new Gothic style."

Stained glass was also used in many great medieval cathedrals, for instance Chartres Cathedral and Reims Cathedral in France, Cologne Cathedral in Germany, York Minster in England and Florence Cathedral in Italy. Nothing similar could be found in any other civilization at the time.