Medieval stained glass was made through a multi-step process that combined glassblowing, metal-oxide chemistry, hand painting, and lead assembly. From melting raw sand in wood-fired furnaces to fitting hundreds of colored pieces into lead frames, creating a single cathedral window could take months of skilled labor across several specialized crafts.
Making the Glass Itself
Medieval glassmakers started with three core ingredients: silica (from sand or crushed quartz), potash (from wood ash), and lime. The wood ash acted as a flux, lowering the melting point of the silica so it could be worked in a wood-fired furnace. These furnaces reached temperatures between 1,350 and 1,500°C in the melting chamber, with some spots spiking as high as 1,650°C. The result was a molten glass that could be shaped, colored, and blown into flat sheets.
Color was built into the glass at this stage. Glassmakers added specific metal oxides to the molten batch to produce different hues. Cobalt produced deep blue. Copper created shades ranging from turquoise to green. Manganese gave violet tones. Iron oxides were especially versatile: depending on the conditions inside the furnace, iron could yield either a light blue or a yellow. For opaque yellows, lead-based compounds were mixed in. Ruby red, one of the most prized and difficult colors, required precise control of copper or gold particles suspended in the glass. Because the color ran through the entire thickness of the material, medieval stained glass has a depth and saturation that surface-applied paints can’t replicate.
Blowing Flat Sheets From Molten Glass
Turning a blob of molten glass into a flat sheet was one of the trickiest parts of the process. Glassmakers used two main techniques, both rooted in glassblowing.
The crown glass method involved blowing a bubble of molten glass, then opening it and spinning it rapidly to form a flat disc. These discs were limited in size and had a characteristic thick “bullseye” at the center where the blowing rod had been attached. The surrounding glass was thinner and more uniform, and that was what got cut into pieces for windows.
The cylinder (or “muff”) method was more common for larger sheets. A glassblower inflated the molten glass into a long cylindrical shape instead of a globe. The top and bottom of the cylinder were cut off, and while the glass was still hot and pliable, it was split lengthways and flattened into a sheet. This sheet was then placed in an annealing oven, where it cooled slowly and evenly to prevent cracking from thermal stress. The cylinder method produced larger, more consistent pieces of flat glass, making it the preferred technique for ambitious window projects.
Designing the Window
Before any glass was cut, the window’s design had to be drawn at full scale. Artists created what was called a “cartoon,” a detailed drawing laid out on a large board or table that served as a visual guide for the entire project. In earlier centuries, these were drawn directly onto whitewashed wooden boards. Later, as paper became more available, cartoons were assembled from multiple sheets joined together. The cartoon mapped out every color, every lead line, and every painted detail, essentially functioning as the blueprint for the finished window.
Individual pieces of colored glass were then laid over the cartoon and cut to shape. Medieval glaziers scored the glass with a heated iron tool (the diamond-tipped cutter came later) and broke it along the scored line. Each piece was trimmed and fitted to match its corresponding section of the design. A complex window could contain hundreds or even thousands of individually shaped pieces.
Painting Details Onto the Glass
Colored glass alone couldn’t create faces, folds of fabric, architectural details, or fine lines. For that, painters applied a special vitreous paint directly onto the glass surface. The most common type was a dark paint called grisaille, made from a mixture of ground lead glass and iron or copper oxide pigments. This powder was mixed with a liquid binder like vinegar or water, along with a temporary adhesive such as gum arabic, to make it workable with a brush.
Painters used this mixture to add shading, outlines, facial features, and decorative patterns. Once the painting was complete, the glass was fired in a kiln at 650 to 700°C. At that temperature, the paint fused permanently to the glass surface, becoming part of the material rather than sitting on top of it. This is why painted details on medieval glass have survived centuries of weathering.
A major innovation arrived at the beginning of the 14th century with the introduction of silver stain. This technique involved applying a diluted silver compound to the back surface of a piece of clear or colored glass, then firing it at a slightly lower temperature, between 500 and 650°C. The silver ions penetrated the glass surface and produced a range of warm yellow to deep amber tones. For the first time, a single piece of glass could contain two colors without a lead line between them. A figure’s golden hair and pale face could be rendered on one piece of glass rather than two, giving artists far more flexibility and subtlety in their designs.
Assembling With Lead
Once all the pieces were cut, painted, and fired, the window was assembled on the original cartoon. Each piece of glass was fitted into H-shaped strips of lead called cames. The H profile gripped the glass on both sides, with the crossbar of the H sitting between adjacent pieces. Glaziers worked methodically across the panel, slotting each piece into place and bending the soft lead to follow the contours of the design.
Where cames met, they were soldered together to lock the panel into a rigid structure. The joints were then sealed with a putty or cement pressed into the gaps between the lead and glass, making the panel weatherproof. For large windows, multiple panels were assembled separately and then mounted into an iron armature, a framework of bars set into the stone window opening that bore the considerable weight of all that glass and lead.
Why Medieval Glass Looks Different
If you’ve ever noticed that medieval stained glass has a richer, more uneven quality than modern versions, that’s not just age. It’s a product of the process. Hand-blown sheets were never perfectly uniform. They had slight variations in thickness, tiny air bubbles, and subtle ripples that scatter and refract light in complex ways. The metal oxides were not always evenly distributed through the molten glass, creating streaks and gradations of color within a single piece. These “imperfections” are exactly what give medieval windows their luminous, almost liquid quality when sunlight passes through them.
The combination of colored glass, painted detail, silver stain, and lead lines meant that medieval glaziers were working with light itself as a medium. The lead wasn’t just structural; it functioned as the dark outlines of the composition, much like the black lines in a drawing. Every element of the process, from the chemistry of the furnace to the brushwork of the painter, contributed to a final effect that was as much engineering as art.