splendent gloom interpreted to reveal the Divine Presence. This association of God with
light, critical to the understanding of medieval stained glass, was grounded in the Gospels
(e.g., John 1:4–9; 8:12) and was further elaborated in the writings of the early church
fathers. While Bernard of Clairvaux (1090/91–1153), the founder of the Cistercian order,
similarly embraced early Christian analogues of God as light, Cistercian glazings
eschewed color, using instead uncolored glass arranged in symmetric interlace patterns.
Whether colored or not, these hovering windows of light altered the very nature of a
church interior from substance to the immaterial, mirroring the transmutation of the
corporeal to the spiritual, thereby creating a vision for the worshiper and a paradise in
which to encounter the Divine. Just as Suger recalled “dwelling” in meditation within
“some strange region of the universe that neither exists entirely in the slime of the earth
nor entirely in the purity of Heaven” (De administratione 33), stained-glass windows
were construed as a mystical conduit for the illumination of the soul. Stained glass, the
preeminent form of monumental painting of the Gothic period, was inexorably tied to its
ecclesiastical and architectural context, the juncture intended to reveal Heaven on earth.
Within this medieval construct of the Heavenly Jerusalem, glass and stone could not be
separated but were conceived of as mutually dependent components of a whole: just as
architectural masonry articulated the shape of a Gothic church, so light transmitted
through its glazing defined and unified its sacred space.
Modern knowledge of medieval stained-glass production is indebted to the step-by-
step instructions provided by a German monk writing under the pseudonym of
Theophilus (ca. 1100). Current use of the term “stained glass” technically refers to pot-
metal glass, that is, molten glass colored by the addition of metallic oxides. Glass itself is
a fluid material composed of sand with additions of potash as a flux. In his manual, De
diversis artibus, Theophilus recommends that scaled designs for windows be drawn on a
flat whitewashed board, which could serve both as a surface for cutting and a workbench
for assembly. The molten glass was blown either into an open cylinder shape (called
“muff”) or crown, a process in which the glass was transferred from the blowpipe to a
pontil iron and spun into a circle. Once a flat surface was obtained, individual pieces of
glass were cut by tracing the desired shape with a hot iron, followed by tracing the
pattern with cold water, thus causing the glass to crack. The piece was subsequently
shaped by edging bits of glass off the rim with the aid of a grozing iron (a handle with a
small hook at the end) and pincers. The pieces of colored glass were in turn painted with
a vitreous paint composed of iron filings and ground glass, bound by either wine or urine.
Painting was used to indicate facial features, drapery folds, or intricate design patterns.
Depending on the amount used, the liquid binder could make a thick black paint used for
detailing or it could create a light grey wash for modeling. The pieces were then fired,
affixing the paint to the glass surface. Theophilus suggests that the painter work in a
three-layered process, beginning by laying down a wash and working up to the details.
With the introduction of silver stain at the beginning of the 14th century, this process
changed dramatically. Composed of silver-oxide, silver stain is a paint that turns a range
of hue from pale yellow to fiery orange depending on the length of time it is fired. The
advantage of this painting technique was the ability to obtain a varied palette without
having to cut and join different colors together with lead strips. Flashed glass (clear glass
laminated with color) also gained popularity in the later Middle Ages, although this
process had always been used for red, as the color is too opaque for light to penetrate
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