Chapter 13. Theophilus (Glass)


If you have the intention of making glass, first cut many beechwood 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. After this build a furnace of stones and clay, fifteen feet long and ten feet wide,…

When you have arranged all this, take beechwood logs completely dried out in smoke, and light large fires in both sides of the bigger furnace. Then take two parts of the ashes of which we have spoken before, and a third part of sand, collected out of water, and carefully cleaned of earth and stones. 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 at once with the same iron ladle to prevent them from melting from the heat of the fire and agglomerating. Continue doing this for a night and a day.

Meanwhile take some white pottery clay, dry it out, grind it carefully, pour water on 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 tongs and set them in the red-hot furnace in the holes made [in the hearth] for this purpose. 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.

 — Theophilus, On Divers Arts, ca. 1100 AD [1]


Have you been observant, my brothers and sisters? Have you chosen to see things for yourself or are you afraid to touch what might burn your lily-white fingers, to taste what might upset your sensitive stomach, to smell what might clear your stuffy sinuses, to hear what might offend your timid ears? Did you stop to examine the lining of your crucible in your haste to retrieve your nugget of bronze, or did you notice that the slag had given the interior of the crucible an amazing glossy finish? If so, you have seen what went unappreciated by generations of potters and smelters until I attached significance to it.

It was in the early days of smelting technology, 4527 BC, if meme-ory serves, a millennium before the Bronze Age began in earnest. I had been raised to smelting from early childhood, sorting ores, making crucibles and gathering firewood for the furnace. I pestered my father and brothers constantly with questions and unsolicited advice. "Why is the inside of the crucible black while the outside is white?" "What happens if you leave out the charcoal?" "What happens if you leave out the soda ash?" When I was older I began to answer these questions for myself. A waste of good material, my father said. I always suspected he was talking about me as well as the ore I used. I discovered that the inside of the crucible is blackened by the charcoal. But if you leave out the charcoal, the inside of the crucible is blue, not white, as if the malachite ore had been melted like wax. If you leave out both the charcoal and the soda ash, that is, if you simply heat malachite ore in a crucible, it turns black but does not melt. And soda ash alone melts right into the clay. It seemed to me that the soda ash made it easier to melt malachite, and melted malachite is a beautiful thing.

By 4000 BC I had learned to glaze quartz beads with a combination of soda ash and lime colored with malachite. At the same time I began applying the glaze ingredients to the surfaces of clay tiles, but because soda-lime glazes and clays shrink at different rates as they cool, they are not suitable for curved pottery. This limitation was overcome 2000 years later with the introduction of galena (lead sulfide), rather than soda ash, as the flux. From that point glass and glaze followed separate paths. Glaze developed as I wended my way down through generations of potters using lead in the form of galena and cerrusite (lead carbonate) to flux sand and clay. Glass emerged from the smelting tradition some 500 years later, with soda ash and potash used to flux lime and sand.

Having grown out of the metal arts, glass was cast in molds much as metals are. But beginning in about 30 BC glass began to be worked in its molten state, blown on the ends of iron pipes. Today glass is cast, blown, molded and drawn into almost any shape that can be conceived. Without it churches and skyscrapers would be pitch-black caverns, the color of fine wines would remain unappreciated in opaque clay bottles and cups. The microscope, telescope and electric light would be impossibilities. And the greatest contribution of all, that which has literally returned eyesight to billions, goes by the name, glasses. In short, had I seen slag as merely the refuse of the smelter, much of the world would have remained impenetrably opaque and many of its inhabitants hopelessly blind.



Reference [27], Book II, Ch. 4-5.