How stained glass windows were made in medieval times: the glass, the colours, the lead, the paint and the firing, from Theophilus to Chartres

Architecture · 4 September 2026

A medieval stained glass window was made by melting sand and wood ash into glass in a wood-fired furnace, colouring the melt with metal oxides, blowing the glass into cylinders or discs and flattening it into sheets, cutting the sheets into pieces with a hot iron along a full-size drawing, painting the faces, hands and drapery folds onto the pieces with a brown paint of ground glass and iron and firing it on, and joining the pieces with strips of lead soldered at the crossings, the whole set into an iron frame in the window opening; the process is described in detail in the treatise "On Divers Arts" written by the monk Theophilus about 1120, and it is the process still used, with gas instead of wood and a diamond instead of a hot iron. The colour is in the glass itself, not on it, which is why the windows of Chartres, 800 years old, still burn; "stained" glass strictly means the yellow silver stain invented about 1300, and the rest is coloured or "pot metal" glass. Below: the manual, the glass, the colours, the sheets, the cartoon, the cutting, the painting, the leading and setting, the two inventions, the workshops and the cost, and why it lasts, with the windows in Chartres Cathedral facts and the buildings in Gothic architecture.

Theophilus and the manual

Almost everything known about how the windows were made comes from one book: "De diversis artibus", "On Divers Arts", written under the name Theophilus Presbyter about 1120, probably by Roger of Helmarshausen, a Benedictine monk and metalworker in Germany, whose second book, of 31 chapters, describes the making of glass and windows from building the furnace to setting the panels, with the confidence of a man who had done it; the manual survives in copies from the 12th century, was printed by Lessing in 1774, and was used by the 19th-century revivalists to rediscover what had been forgotten. The archaeology of glass furnaces in the forests of Lorraine, Normandy and the Weald, the analysis of the glass, and the windows themselves confirm him. Glass had been made since Egypt and windows of coloured glass set in lead since the 7th century, at Jarrow and Monkwearmouth in England and at Lorsch in Germany, but the art as Theophilus describes it is that of the Romanesque and Gothic church, from Augsburg's prophets of about 1100, the oldest complete windows, to the 13th century's height. Stained glass.

Making the glass

• Ingredients. Two parts of beech or fern ash, which supplied potash as the flux, to one of river sand washed clean, Theophilus says; the "forest glass" of northern Europe, greenish from iron in the sand, made in the woods where the fuel was, as against the soda glass of the Mediterranean made with plant ash from the coast, which was clearer and lasted better. The 12th-century glass of Chartres and Saint-Denis is a soda-lime glass, which is why its blue is still bright, and the 13th-century potash glass has corroded and darkened.
• Furnace. A clay-and-stone furnace with a fire chamber below and a working chamber above holding clay pots of the batch, fired with wood for a day and a night to about 1,100 degrees until the batch melted and cleared; a second furnace annealed the finished sheets slowly so they would not crack, and a third dried the wood and pre-heated the batch.
• Colour. Added to the pot as metal oxides: cobalt for blue, from Bohemia and Saxony, the rarest and most prized, whose secret Chartres kept; copper for red and, in a different state of oxidation, green; manganese for purple and pink, and to clear the green tinge from white glass; iron for green and yellow; antimony and lead for yellow. Each pot gave one colour, "pot metal", and the glassmaker could not control it exactly, so that a window has a dozen blues.
• Red. Copper red was so dense that a sheet of it was nearly black, so from the 12th century red was "flashed": a gather of clear glass dipped in red and blown, giving a thin red skin on a clear sheet, which could later be abraded to make patterns of two colours in one piece.

Blowing the sheets

Window glass was blown, not cast: the glassmaker gathered a blob of molten glass on the end of a blowpipe, blew it into a bubble, swung it into a long cylinder, cut off the ends, slit the cylinder lengthwise with a hot iron and reheated it in the annealing oven until it opened out flat, giving a sheet of about 30 by 45 cm, the "muff" or cylinder method that Theophilus describes; or blew a bubble, transferred it to a rod, and spun it until it opened into a flat disc up to 60 cm across with a thick "bull's eye" in the middle, the crown method of Normandy, which gave clearer glass with circular ripples. The sheets were uneven, 3 to 6 mm thick, full of bubbles, seeds and streaks, which is what makes old glass glow, each piece bending the light differently; the glass was sold to the window-maker by the sheet, and the colours were bought in from different furnaces, blue from one, red from another.

The cartoon and the cutting

• The table. The designer drew the window at full size on a large wooden table whitewashed with chalk, in red and black lines, showing every piece, the lead lines between the pieces, the colour of each, and the painted details; paper was not yet cheap enough, and the table was scraped and reused, so almost no cartoons survive before the 14th century, when the Girona cathedral tables of about 1330, the only ones known, were found.
• Cutting. The sheet was laid on the table over the drawing, and a hot iron, a rod heated in the fire, was drawn along the line so that the glass cracked after it, the crack steered by wetting; the edges were then nibbled to the exact shape with a grozing iron, a hooked tool, leaving the chewed edge that old glass shows under the lead. The diamond cutter arrived in the 16th century.
• Fitting. Each piece was laid on the table in its place, and the pieces of a face, a hand or a robe were chosen for their colour and their streaks, the whole window being a mosaic of a few hundred to a few thousand pieces, each about the size of a hand, with the leads following the outlines of the drawing.

Painting and firing

The colour is in the glass; the drawing is painted on. The painter used one paint only, a brown or black "grisaille" of powdered glass, iron or copper oxide and a flux, ground with wine or urine and gum, and applied it with brushes in three ways: as trace lines, the outlines of eyes, noses, fingers and folds, drawn with a fine brush; as washes, thin layers stippled or smeared over a piece to shade it and then partly removed with a stiff brush or a stick to make highlights, the "matt"; and as back-painting on the outside of the glass for extra shadow. The painted pieces were laid on an iron tray in a bed of lime or ash and fired in a kiln to about 600 degrees, at which the paint's glass fused to the surface and became part of it, permanent as long as the firing was right; underfired paint has flaked off many 13th-century windows, leaving faceless saints. The style of the painting, the stylised folds and staring eyes of the 12th century, the more natural figures of the 13th, and the fine modelling of the 15th, is how windows are dated.

Leading, soldering and setting

• Cames. Strips of lead of H-section, cast in a mould in the 12th century and later drawn through a mill, with grooves on both sides that gripped the edges of the glass; the pieces were laid on the table and the cames bent around them one by one, following the drawing, and the joints soldered with tin-lead solder and a hot iron on both faces, the lead lines being part of the design, the black outlines of the figures.
• Cementing. A putty of chalk and linseed oil was brushed into the gaps between glass and lead to make the panel stiff and weatherproof, and the surface cleaned with sawdust.
• Panels. A window was made in panels of about 60 by 60 cm, small enough to carry and to fit the iron; each panel was one scene of the story, a medallion of the saint's life, which is why the tall lancets of Chartres read as columns of pictures.
• The armature. An iron frame set into the stone of the window, of bars in the pattern of the panels, circles, squares and lozenges in the 12th and 13th centuries, and straight saddle bars later, to which the panels were tied with lead or copper wire around the bars; the iron carried the wind load, and the pattern of the armature was designed with the window.
• Setting. The panels were carried up the scaffold and fixed from inside, and the window was done: a great rose of 13 m took a workshop a year.

The two inventions

• Flashed glass, 12th century. The thin skin of red or other colour on clear glass, which made red usable and, when the skin was ground away in places with a stone, gave two colours in one piece, for heraldry.
• Silver stain, about 1300. A compound of silver, silver chloride or sulphide, painted on the outside of the glass and fired, turned the glass yellow, from pale lemon to orange, in the painted place only, so that a piece could be white and yellow, hair and face, crown and head, without a lead line between; it is the only true "stain", the source of the name, and it arrived in Normandy and England about 1300 and made the 14th-century window paler and more like painting. Enamels of other colours, fired on, came in the 16th century and, by making the window a painting on glass, ended the medieval art.

The workshops, the cost and the survival

A window was made by a workshop of a master, journeymen and apprentices, some monastic in the 12th century and lay by the 13th, travelling from church to church, the same hands appearing at Chartres, Bourges and Sens; the master designed and painted the heads, the journeymen cut and leaded, and the glass came from furnaces in the forest, often owned by the abbey. The donors paid: kings, bishops and nobles for the great windows, and at Chartres the guilds, whose bakers, furriers and masons are painted at the bottom of the windows they gave; a large window cost as much as a house, and the 176 windows of Chartres were given in forty years. The windows survived when they were not smashed: the Reformation in England and the Netherlands, the Revolution in France, and the wars took most of the medieval glass of Europe, and what is left, at Chartres, Bourges, Canterbury, York, Augsburg, Strasbourg and Cologne, is a small fraction; it survives because the colour is in the glass and the paint is fired into it, and it fails by corrosion of the potash glass, by the flaking of underfired paint, and by the lead, which lasts a century and has been replaced on every old window several times, the glass being re-leaded on the original lines. Protective glazing, a clear sheet outside the medieval one, is now the standard way to keep it.

Versions and theories

• The recipes are lost. Theophilus wrote them down, and every colour can be made; what cannot be reproduced is the exact glass of 1150, its impurities and 800 years of weathering, which is why replacement pieces look wrong.
• Chartres blue is a secret. Cobalt in a soda-lime glass, from a source the Chartres makers had; the "secret" is the durability of that glass beside the potash glass that came later.
• Stained glass is painted glass. The colour is in the glass; only the drawing is painted, in brown, and fired; the yellow silver stain is the only stain. Renaissance enamelled windows are painted glass, and that is why they faded.
• Medieval glass was made with urine. Urine was used to grind the paint and as a mordant, and sometimes in the batch; the glass is sand and ash.
• The windows taught the illiterate. They told the stories, in medallions read from the bottom up, and the clergy explained them; whether the peasant 30 m below could read a 20 cm figure is doubted, and the windows were as much for God and the donor as for the people.
• The lead lines are a flaw. They are the drawing: the medieval designer used the black lines of the lead as the outlines of the figures, and the 19th century, which tried to hide them, made worse windows.

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