The Science of Medieval Glass and the Play of Light in Dante

I visited Chartres in summer 2001, but only saw the surface. I have one distinct memory of the feeling immediately after entering, looking up into the cavernous space, full of quiet darkness rather than light. At the time, I did not know what I was missing: it was impressive but did not make a lasting impression.

By only looking for the faith history, I missed seeing a landmark of science history. Those windows of stories and people are not just historically and theologically important, but chemically important. Embedded in those panes of glass are tiny metal particles only nanometers across, remnants of a medieval technology that led to the modern chemical field of nanoscience.

Medieval glaziers conducted empirical experimentation and discovered chemical processes still under investigation in materials science labs today. They did not know what they were doing at the atomic level, but they did it nonetheless. The novel medieval materials redirected and colored light in new ways, redirecting attention to theological matters through beauty.

The glass was made differently depending on geography and economics, so that the northern glass in French cathedrals contrasts with the southern glass made by Venetian glaziers. Venice is famous for colorful glass beads, but in the late Medieval period it was famous for something we take for granted: clear glass panes for windows and mirrors. Chartres glass tinted light and painted with it, but Venice glass redirected, refracted, and reflected it.

Dante’s poetic mind was suffused with images from Venetian glass, and fused these experiences with theology, shaping them into brilliant metaphor. You can hardly get through a canto without reading about light’s motion and action, and the emphasis on light increases at the Commedia continues. The practices leading to what we call science are found as well, with the best example of science in the third part of the trilogy. The weighty theological discourses in Paradiso are preceded by a detailed description of an experiment with mirrors and a candle in Canto II, which is the first recorded example of light’s invariant brightness. Dante had no wall of separation between science and faith.

Therefore, we can read Dante looking for the play of light, with the knowledge of how the atoms are at work beneath the surface and why the Venetian glass had particular chemical advantages, helping us understand where his luminous metaphors come from. In Paradiso, Dante uses three qualities of light to show how God works in the world of matter and form: light projecting its color on a surface, light transmitting through a clear pane of glass, and light filling a piece of crystal, amber, or stained glass. Each of these connects to a theology of creation that is illuminated, but not replaced, by deep chemical knowledge.

Projecting: Painted (Dipinta/Dipigne) Like Light on Mist

I did not encounter projection of light at Chartres because the day was too cloudy, but I found the same experience in a twenty-first-century equivalent. This was not a cathedral but a mobile museum called “Van Gogh: The Immersive Experience.” I bought tickets for this traveling exhibit as a coping mechanism during COVID restrictions. In March 2021, it was an act of hope to purchase tickets for a public indoor event taking place on Thanksgiving weekend. For months I was not sure this would happen—here the play of light was threatened not by wandering clouds but by a wandering virus—but it finally did.

On that day my family and I arrived at an old square warehouse in the Georgetown district of Seattle, an exterior decidedly less charming than Chartres. To get to the main attraction, we passed through a beaded curtain into a room a hundred feet square covered with projection screens. LCD projectors hanging from the wooden warehouse beams ran a half-hour loop of computer-animated Van Gogh paintings, zooming in and out, filling every inch of wall space.

Despite my years of honing a carefully distanced academic posture, the spectacle of the projected color drew me in. When my favorite Van Gogh painting, Starry Night Over the Rhone, was projected thirty feet tall, I leaned on the wall in place of the man in the lower right corner and had my kid snap a photo of me there. I was immersed in the colors.

These traveling exhibits have proliferated across the country because they are cheap to produce and unique to experience. Any large space can be turned into a show with a few small digital projectors, and the immersion is greater than what small home screens can provide. The technology is complex but the projection itself is an inherent property of light, and the core of the experience is as universal as light itself.

More than that, it was our first large secular social activity in more than a year. My lasting impression is of the peaceful normalcy of wandering in a quiet crowd, our attention focused on one thing. The sense of space, and darkness with light, was not dissimilar to my memory of Chartres. It was a weak form of contemplation, but it was contemplation nonetheless, unbroken by commercials or notifications.

These projection shows have proliferated across the globe. One can even be found at Chartres, a twenty-first-century imitation of the play of light from the twelfth-century stained glass inside. These modern projections are creative and convincing, full of every color that LEDs can create. The medieval projection of light through windows inside is not as controlled, but has a similar effect when the sun projects through the windows and paints the stone columns and floor like at the Van Gogh exhibit.

Modern minds that contemplate Van Gogh’s colors projected in a warehouse follow a path that was blazed by medieval minds contemplating light through glass. Dante’s poetry traces the path of light back to its very source, the Creator of Light and the Light of the World. Near the end of his journey, in Paradiso Canto XXVIII, Dante sees something like a “mirrored flame” (specchio fiamma, 4) reflected in Beatrice’s eyes. He turns to see an exceedingly tiny point of “acute” (acuto, 17) light, like the brightness you see when you look straight into an LCD projector’s lens. This point of light is surrounded by a “ring of fire” (un cerchio d’igne, 25) projected onto the vapor around it, then a second, third, fourth, etc. These are the angelic orders that move the celestial spheres. Christian Moevs calls this an image of God’s act of creation: “That point projects the spatiotemporal world from itself as a source of light ‘paints’ (dipigne) a halo onto mist (22-24); note that the halo is a reflected image (like the world) and has no being apart from the source of light. Thus arises the universe as extension in space.”[1]

God projects the universe like the sun projects its rays. When I perceive the universe truly, I let this projection inside, mirroring the act of Creation in my mind. In Convivio, Dante describes how this little light of mine participates in the Light of Creation: “The soul . . . that is the most perfect of all, is the human soul, which through the nobility of its highest power, reason, participates in the divine nature as an everlasting intelligence . . . the divine light shines in it as in an angel.”[2]

These are the words of a man who loved light in all its colors and actions, reflecting, transmitting, even refracting. By the time they reach the Primum Mobile in Canto XXVIII, Dante’s participation in the light has increased to the point where it has saturated and transformed his soul. Moevs writes, “Dante is so punto ["prodded," "pricked"] by the punto [“point”] as virtually to be the punto, the love or light in which all finite being consists.”[3]

Transmitting: Clarity (Chiaro) of Glass and Thought

In Dante’s mind, participation with the light is associated with the clarity of glass. In Canto XXVIII, he writes that Beatrice’s “clear answer” (risponder chiaro) allowed the truth to be seen “like a star in heaven” (come stella in cielo). Some form of the word “clear” or “bright” (chiaro) occurs no fewer than 38 times in Paradiso alone.

Dante only refers to colored glass twice in his writings,[4] but refers more often to transparent glass,[5] and even more often to clear glass mirrors, that Venetian specialty. (Venetians made the best mirrors because they were the first to apply metal leaf as a reflective backing.) This cultural capacity aligns with a chemical circumstance: Venetian glass, the most familiar to Dante, was prized more for its clarity than its color.

The geography and geology of Northern Italy gave Venetians the raw materials for the clearest glass in Europe. Venice cornered the market on both colorful beads and clear circles of glass by leveraging its location. This location gave obvious economic advantages in sea trade and transport, but also access to particular rocks and chemicals that made for the clearest (and therefore most expensive) glass in Europe.

Clear Venetian glass was valuable because it was hard to make. It is true that glass is simply melted sand, but it takes more than a campfire to melt sand. Sand/silica has a melting point (1700°C) higher than copper (1000°C) or even iron (1500°C). Typical glass furnaces could only reach temperatures around 1000°C.[6]

The ancients could not raise the temperature, so they lowered the melting point. A material called a “flux,” when added to silica, interferes with silica’s bonds so that they break apart at lower temperatures.[7] The good news is that a flux will allow silica to melt; the bad news is that you have just added a new material to the mix, which tends to add color.

Iron is so prevalent in the earth’s crust that it is hard to find rocks or sand without it. When iron contaminates melted silica, whether from the silica or the flux, the glass becomes green or even brown. Careful selection of both silica source and type of flux makes for clearer glass, and the Venetians had advantages in both of these.

Years of experimentation showed the Venetians that flint pebbles from local riverbeds gave clearer glass than sand. Northern Italy had several high-quality silica sources: Flint pebbles from Verona gave yellowish glass, but pebbles from Ticino were purer from iron and other elements, giving exceptionally pure silica and exceptionally clear glass.

Venice went farther afield for the flux, importing ash from Syria, which has a high carbon and sodium content but a low iron content.[8] This was made from coastal plants that grew on sandy soil without iron, so they absorbed less iron and gave ash that was almost iron-free. Other ashes were not as clean. Spanish ash, for example, had contaminating metals that would turn glass blue.

Even this solution causes another problem: coastal plants have lots of salt from the ocean, which ends up in the glass. Salt dissolves in water, so wet glass dissolves away and breaks easily. To counteract this fragility, a third ingredient called a stabilizer is added, which is something that will displace the salt and will not itself dissolve.

Venetians added lead as a stabilizer, like many other glassmakers. Mysteriously, their glass was even stronger than typical leaded glass. We now know that one of their ingredients, probably the flint or flux, contained excess calcium that strengthens glass. The importance of calcium (lime) was not discovered until the seventeenth century, yet the calcium content of Venetian glass increased from the sixth to fifteenth centuries,[9] suggesting that unknowingly, trial and error helped them select more calcium-rich materials. Through empirical means, Venetians made stronger glass without knowing why, and stronger glass could be thinner and therefore clearer.

All these advantages added up to make Venetian windows the clearest in the world and Venetian mirrors the most reflective. The brilliant action of light through and from this glass gave Dante a clear metaphor for the power and abundance of God’s creative light.

Dante wrote that the very purpose of creation was to be a mirror, reflecting God’s goodness throughout the universe and eternity. In Canto XXIX, Beatrice explains that God created the universe not that he should gain (aquisto) good for himself, but that his own splendor (splendore) should re-reflect (risplendendo), shouting “I subsist!” (“Subsisto!”).[10] Creation is intended to be a kenotic mirror, pouring out light to be scattered widely and generously, like seed from a sower. Dante’s enduring text is a mirror that reflects God’s light around the world today.

The same glass that gave Dante metaphors for theological clarity led to scientific clarity. Dante described a scientific purpose for Venetian mirrors in Canto II’s experiment with a candle and mirrors, making him the first to record the scientific truth that light’s brightness does not vary with distance.[11] Even so, I suspect that Dante would be surprised at just how important the clear, strong Venetian glass turned out to be for science. Mirrors, glass lenses, eyeglasses, and chemical vessels led to all sorts of “scopes” and “meters”: microscopes, telescopes, thermometers, and barometers.[12] I can trace a straight line from Venetian glass to the instruments and glassware lining the shelves of my own research lab.

Filling: From “Be” to “Shine” (Dal Venire a l’Esser)

The numbers recorded by these laboratory instruments, and the solutions held by those bottles, like the stars in the heavens, declare the glory of God. But clarity is only one aspect of what glass can do, and clarity of Venetian glass is only one metaphor for God’s enlightenment of the soul. A different kind of glass made by different people in a different place produced glass with color rather clarity, demonstrating a different aspect of the action of Dante’s light.

The clarity that is so useful in a lens is less useful when installed as a window in a church. One sanctuary in my city was built with a side wall of windows, intended to remind the Church of its obligation to serve the world. In practice, those windows opened onto a busy street and heads would turn reflexively whenever a car passed outside. These windows were more distraction than reminder. Soon the church installed retractable curtains at some expense to block out the scene.

Clarity is not always a theological virtue either; someone who says “Scripture is clear” on an issue can often be wrong. David Bentley Hart points out the pitfalls of a literalist hermeneutic of clarity:

[For literalist views] all the words of the Bible must be understood as direct locutions of God, passing through their human authors like sunlight through the clearest glass, and the canon of the New Testament . . . must be understood as a flawlessly immediate communication, in its every historical and lexical detail, of the teaching of the Holy Spirit and of the faith of the apostolic church.[13]

Excessive literalism is idolatry of the text. The Word of God is fully made clear not in the scriptures but in Jesus Christ himself, who plays in ten thousand places.

Dante describes creation without clarity, with the metaphor of light moving through differently colored materials, translucent rather than transparent. Later in Canto XXIX, after God’s “Subsisto!”, Dante describes the instantaneous creation of matter and form as light illuminating and filling a piece of glass (vetro), amber (ambra), or crystal (cristallo). In this way everything proceeded immediately “from coming to being,” (dal venire a l’esse) or as Anthony Esolen translates, “from ‘be’ to ‘shine.’”[14] This immediate fullness of being is similar to Dante’s previous image of a universe projected, but adds the fourth dimension of time to the three dimensions of space. Everything (tutto) was made together (concreato) with the declaration, “Let there be light!”, and the immediately the universe was full with matter and form.

Light fills clear glass, but the direct image of the sun overloads our poor retinas. When light fills colored glass, it is dimmed to something we can take in. Behind stained glass, the outside world is veiled, but the glass itself still communicates. The colors in the glass give the window form and tell a story. The artisans at Chartres and other inland cathedrals far from Venice used the thick, colored glass at hand like a (literally) illuminated manuscript.

The reason why these places developed the art of colored glass is also rooted in their geography and geology. Inland French communities had little access to Syrian ash, so they made their ash from local trees. The famous “Jesse Tree” window in Chartres depicts a tree and is itself made from trees. Inland, northern trees grow in distinct soils and accumulate distinct elements: more potassium and calcium, less sodium. This makes them stronger than the “saltier” Venetian glass.

This northern glass was so different that it was named “forest glass” (waldglas), often colored dark green or even brown from contaminants like iron.[15] The good news for glaziers is that this potassium-rich ash lowered the melting point of the silica mix (probably due to the calcium), so that “forest glass” was easier to make, harder, heavier, and better for cutting and engraving.[16] It just was not as clear or thin as the Venetian vetro.

So, in northern Europe, windows were for more for looking inward than outward, as light filled angular shapes rather than clear disks. An empirical culture of experimentation developed here as well, with glaziers adding materials to the class and therefore adding colors.[17] They would mix in powdered rocks or metals, filling the glass with colorful ions. For example, copper oxide will color glass green or red, depending on the temperature.[18] The first colors to be discovered led to a distinctive palette for the oldest surviving stained-glass windows in Bourges and Augsburg: green, orange, yellow, and translucent white, some red, and less blue.

More colors developed over time, including the blues so prominent at Chartres and Notre Dame Paris. UNESCO has defined one deep blue color, reported to be a combination of cobalt with manganese, as “bleu de Chartres.”[19] Some enterprising glazier(s) found that gold and silver, under certain conditions, would saturate glass with vivid reds and yellows. As scientific investigation was systematized, we found that there was far more going on underneath the surface as these colors were made, unknowingly employing chemical reactions that are still being investigated in labs today.

The tools of science eventually traced this color back to its molecular origin. In 1659, Johann Rudolf Glauber published an account that mixing gold chloride from aqua regia (discovered by alchemists) with tin hydroxide would produce a strikingly purple pigment.[20] This pigment colored Meissen and Sevres porcelain, for example. When added to glass it makes ruby-colored “cranberry glass,” showing that this could be the chemistry beneath the surface of some medieval panes.[21]

In 1925, the chemist Richard Zsigmondy would win a Nobel Prize for showing that this special form of gold is a “colloid” of small particles, which when uniformly shaped and sized will interact with visible light to impart red and purple color to a solution or to glass. Because gold was so precious, this is probably very rare, because the same red color can be achieved with copper, which also forms nanometer-sized, color-imparting particles.

Now these are called “nanoparticles” and can be made from all sorts of elements in the field of nanotechnology. The original nanoparticles were spheres; now we can make multiple shapes and colors using elements unknown to alchemy.[22] It only takes a few ingredients to coax gold into nanoparticles, using a protocol simple enough to be repeated in a high school classroom.[23] This current area of intense research has its roots in the medieval glassworks.

When light passes through Chartres glass, it accomplishes a different purpose from a clear window or lens. It provides a glimpse of the past, both of the Medievals who made the window and of the ancient stories depicted there. In “The Windows,” George Herbert writes, “thou dost anneal in glass thy story, // Making thy life to shine within.”[24] Herbert is speaking not of literal windows, but of those who preach the Word. And the greatest preacher, the one who is the Word, gave the ultimate example of light filling flesh with life.

The Transfiguration, as represented in the Life of Christ window at Chartres, fills with light that shines in Christ’s white head, surrounded by a red mandorla. As Bishop Berkeley wrote, “We are sprung from the father, irradiated or enlightened by the son, and moved by the spirit.”[25] The three disciples who saw this glory on the Mount of Transfiguration were themselves filled with and transformed by God’s light. Peter describes how this “majestic glory” (2 Peter 1:17) speaks, with the voice of the Father, of the fulness of God in Christ. Therefore “be attentive to it, as to a lamp shining in a dark place, until day dawns and the morning star rises in your hearts” (2 Peter 1:19).

This is how light works in Dante, too. From his vantage point in the Primum Mobile, Dante sees that time “is infinitely more than a mere succession of corporeal movements. It is the procession of the Light and Love of Eternity into the temporal life of man."[26] Like light through glass, this light does not compete with its medium, but completes it, whether colored or clear, imparting the brilliance of heaven to the earth.

Conclusion: The Double Rainbow (Iri da Iri)

All of these actions of light are brought together in Canto XXXIII, in Dante’s final image, where he at last sees God. No one image will suffice. Two circles, mirrored as a double rainbow (come iri da iri parea reflesso), with a flame and the figure of a man. The light reflects and re-reflects as through windows of glass, and as it did on the Mount of Transfiguration.

In rainbows, all the actions of light come together as one. Rainbows are the products of light through transparent drops of water, each acting as a mirror-prism, together producing a coherent, colorful circle of light. Light is at play, reflecting, refracting, dispersing, converging. The colors are always in the light but the rain separates them into distinct bands we can see.

A similar effect is found in Chartres on a sunny day, where the stories of Scripture are projected in a dazzling, glorious display, too bright to comprehend. In the Transfiguration window, Christ’s red mandorla, full of copper or gold nanoparticles, is surrounded by bands of yellow, brown, then green, leading to the blue glass. Somewhere in there are nanoparticles of precious metals beside atoms drawn from the Frankish soil, purified and formed into a representation of Christ.

In this stained glass at Chartres, the bands of colors bridge God and creation. In Dante as well, the rainbow is a divine messenger, “the bridge of communication between Heaven and earth, between gods and humans.“[27] The point of this image, and of the entire Commedia, is to draw our hearts to sit with Dante in the time and place beyond time and place, to contemplate the glory and goodness of God, illuminating the cosmos with life.