A medieval recipe for the spectacularly colored and painted windows that decorated churches built between the 10th and 16th centuries.
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Most of what we know about how stained glass was made in medieval times comes from a 12th-century monk who, writing under the pseudonym Theophilus, left behind a sort of manual for medieval art, called De Divers Artu (“On various arts”). This handbook, which was rediscovered by a German librarian in 1744, contains invaluable information on the practice of medieval art, with its detailed instructions on how to painting, perform metal work, and create stained glass windows.
In his own words, here is Theophilus’ recipe for stained glass made from sand and ash (potash):
1. Prepare beech wood ash
If you have the intention of making glass, first cut many beech wood logs and dry them out. Then burn them all together in a clean place and carefully collect the ashes, taking care that you do not mix any earth or stones with them.
2. Mix sand with the ashes and place in a furnace
Mix them in a clean place, and when they have been long and well mixed together lift them up with the long-handled iron ladle and put them on the upper hearth in the smaller section of the furnace so that they may be fritted. When they begin to get hot, stir them at once with the same ladle to prevent them from melting from the heat of the fire and agglomerating. Continue doing this for a night and a day.
3. Melt the mixture (the frit) in ceramic pots
Meanwhile take some white pottery clay, dry it out, grind it carefully, pour water over it, knead it hard with a piece of wood, and make your pots. These should be wide at the top, narrowing at the bottom, and should have a small in-curving lip around their rims. When they are dry, pick them up with the tongs and set them in the red-hot furnace in the holes [in the hearth] made for this purpose.14 Pick up the fritted mixture of ashes and sand with the ladle and fill all the pots [with it] in the evening. Add dry wood all through the night so that the glass, formed by the fusion of the ashes and sand, may be fully melted.
4. Blow the melted frit into a cylinder.
At the first hour next morning, if you want to make sheets of glass, take the iron blowpipe, put its end in a pot full of glass, and when the glass sticks to it turn the pipe in your hand until as much glass as you want agglomerates around it. Take it out at once, put it to your mouth, and blow gently; then take it immediately out of your mouth and hold it near your cheek, lest by chance you should suck the flame into your mouth when you draw in a breath
5. …and flatten it into a sheet of glass.
Then, enfolding the rim in the middle [at the other end] with a wet stick, separate [the cylinder] from the pipe. Then give it to a boy who will carry it on a piece of wood inserted through one of its holes to the annealing furnace, which should be moderately hot.
6. To make colored glass, he explained:
If you see [the glass in] a pot changing to a saffron yellow color, heat it until the third hour and you will get a light saffron yellow. Work up as much of it in the same way above. And if you wish, let it heat until the sixth hour and you will get a reddish saffron yellow. Make from it what you choose. But [alternatively] if you see [the glass in] any pot happening to turn a tawny color, like flesh, use this glass for flesh-color, and taking out as much as you wish, heat the remainder for two hours, namely, from the first to the third hour and you will get a light purple. Heat it from the third to the sixth hour and it will be a reddish purple and exquisite.
7. To create the windows themselves:
If you want to assemble simple windows, first mark out the dimensions of their length and breadth on a wooden board, then draw scroll work or anything else that pleases you and select the colors that are to be put on it. Cut the glass and fit the pieces together with a grozing iron. Enclose them in lead [cames], putting in nails and solder on both sides. Surround with a wooden frame, strengthen with nails and set it up a place where you wish.