In Vitro Veritas: Glassmaking After The Ring Of Fire

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In the early seventeenth century, there was already a vigorous international trade in glassware. The world center for glassmaking was in Venice, and the Venetians were most famous for tableware and glass mirrors made of the colorless cristallo. Germany and Bohemia were known for large, decorated drinking glasses, especially those of the green shade which came to be known as waldglas. The French craftsmen of Lorraine and Normandy made both clear and stained glass for windows, some of which was exported.

What, then, do the up-timers of Grantville have to offer experienced Renaissance glass workers? New types of glass (notably borosilicate and lead-alkali glass) will make possible much improved laboratory glassware and optical instruments. New manufacturing methods will allow the production of glass products at a greater rate and at a lower price than what the down-timers would have thought possible. And there are some new glass products for them to consider.

Up-Time Knowledge of Glassmaking

It is fortunate that the up-timers will be attempting merely to inject new ideas into an already vigorous and innovative down-time industry, not recreating glass technology from scratch. Most of the Grantville library books that are specifically about glass are really about collecting antique glass, appreciating art glass, and so forth, rather than about glass technology. It may be interesting to see how the Venetians react to photographs of the creations of Tiffany, Lalique and Chihuly, but art glass books are not going make it easier to operate a chemical laboratory or manufacture modern optics.

Fortunately, at least four different encyclopedias were transported to 1632 by the Ring of Fire. The public library has the Encyclopedia Americana, and both the modern and the 1911 editions of the Encyclopedia Britannica. The high school has the World Book Encyclopedia, and the junior high, the Collier's. Collectively, they provide sample glass compositions and at least outlines of several important manufacturing processes.

There may be more information available from Grantville residents. Edith Wild (1949-16??) was employed in a glass factory before the Ring of Fire ("The Wallenstein Gambit" Ring of Fire), and several retired glassworkers are listed in the "Up-timers Grid."

Types of Glass

About 95% of modern glass production is of "silicate system" glass, in which the glass-forming material is silicon dioxide (silica). The properties of a silicate glass can be altered by adding to it a variety of substances. Fluxes reduce the temperature at which the glass softens, making it easier to work. Stabilizers improve the chemical and mechanical properties of the finished glass. Colorants and decolorants change its optical properties.

Two types of modern glass are already familiar to down-timers. About 90% of modern silicate glass production is of soda lime glass, which is used in bottles, windows, light bulbs, and tableware. The silica is combined with sodium oxide flux and calcium oxide stabilizer. Usually, the silica is from sand (or quartz pebbles), the sodium oxide is formed from sodium carbonate (soda ash), and the calcium oxide is derived from calcium carbonate (limestone). There are also potash lime glasses ("Bohemian glass"), which feature potash (potassium oxide) instead of soda. These glasses were used, prior to 1632, to make stained glass windows.

So far as major new glasses are concerned, the major up-time contributions will be lead-alkali and borosilicate glasses.

Lead-alkali (flint) glass was supposedly "invented" in 1676 by George Ravenscroft (1632-1681), a glass merchant. The "new" glass, besides being more sparkling (because of its higher refractive index than soda lime glass), was also softer and therefore easier to cut. Within twenty years, over one hundred English glass houses were producing lead glass.

Lead-alkali glasses are used in our time line for prisms and lenses, for the more demanding electrical insulation applications, and in higher-end tableware. They contain silica, lead oxide, and at least one alkali (sodium or potassium) oxide. Collier's offers two recipes for lead-alkali glass; the simpler one, for optical use, being 44.6% silica, 0.5% sodium oxide, 8% potassium oxide, and 46.9% lead oxide. The one for electrical use has only 21% lead oxide. In the Encyclopedia Britannica formulation, the lead oxide content is 25%.

Ravenscroft actually rediscovered an ancient invention; there are both Roman and Islamic glasses which are as much as 35% lead oxide (Lambert, 118). Given this history, and the availability of suitable lead ores, I am not expecting that the USE will have great difficulty in duplicating lead-alkali glass. And this, in turn, will give it the ability to produce attractive cut crystal (earning some coin on the export markets) and, more importantly, high quality optical equipment.

Where can we find lead oxide? As it turns out, lead oxide has, for thousands of years, been a byproduct of the "cupellation process" of producing silver. There is a small amount of silver in galena, the principal lead ore. The main component of galena is lead sulfide, and it is readily oxidized, when roasted in a wood fire, to form lead oxide. The lead oxide ("litharge") can then be separated out by absorbing it with bone ash.

Encyclopedia Americana lists Germany as a leading lead-mining country, although well behind the United States and Australia. It also says that there are major deposits of galena in Germany. Down-time miners should be well aware of it, as it has a very distinctive appearance.

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Borosilicate glass contains, as you might expect, silica and boron oxide. It is used primarily in chemical glassware and in ovenware.

Modern borosilicate glass was developed in 1912. Curiously, by 1225, the Chinese were already aware of the use of borax in Arab glassmaking. Zhao Rukuo noted that "borax is added so that the glass endures the most severe thermal extremes and will not crack" (Smith).

Seventeenth-century European glassworkers were extremely secretive about their craft. It is conceivable that prior to 1632, there was some European use of borax in glassmaking. However, the first definite reference dates back only to 1679, when Johann Kunckel (1630-1703) mentioned borax in a recipe for an artificial gem (Smith).

The Encyclopedia Britannica gives us a starting point for formulating these glasses: for chemical glassware, use 81% silica, 12% boron oxide, 4.5% (sic) sodium oxide, and 2% aluminum oxide. The same table also gives a formula for an optical borosilicate glass, "crown" glass: 68.9% silica, 10.1% boron oxide, 8.8% sodium oxide, 8.4% potassium oxide, 2.8% barium oxide, and 1.0% zinc oxide. Two more borosilicate glass formulas can be gleaned from Collier's; these leave out the barium and zinc oxide, but do contain a little aluminum oxide. And a fifth recipe appears in the World Book Encyclopedia. So take your pick. Collectively, the indicated range in boron oxide content is roughly 10 to 25%.

Boron oxide is readily obtained from borax (hydrated sodium borate), other borate salts, or boric acid. In 1632, borax was imported to Europe from Tibet under the name of "tincal." We shouldn't have any difficult getting our hands on tincal from our Venetian trading partners. The real question is price. Borax from Tibet was a luxury item in Renaissance Europe, used primarily by goldsmiths and assayers. If we want to make more than just small quantities of borosilicate laboratory glassware, we will want to exploit nearer sources. Fortunately, the encyclopedias provide some clues.

Boric acid can be obtained from the lagoons in the "Maremma" of Tuscany (this source was not known in our time line until 1777). The 1911 EB entry for "boric acid" describes in some detail how the boric acid is recovered.

The encyclopedias also reveal that "pandermite," a hydrous calcium borate, can be obtained from Panderma (Panormus) on the Sea of Marmora: "it occurs as large nodules, up to a ton in weight, beneath a thick bed of gypsum." Panderma (Panormus) also is said to have a trade in "boracite."

Boracite, a mineral containing magnesium borate, can even be found in Germany. "Small crystals bounded on all sides by sharply defined faces are found in considerable numbers embedded in gypsum and anhydrite in the salt deposits at Lüneburg in Hanover, where it was first observed in 1787 . . . . [A] massive variety, known as stassfurtite, occurs as nodules in the salt deposits at Stassfurt in Prussia." (1911 EB, "boracite"). One of my field guides to minerals actually has a photograph of a boracite crystal found in Bernburg, in Thuringia, Germany. (Hochleitner, 206).

Down-time glassworkers will need to adapt to the special properties of borosilicate glass. It has a softening point of 820 deg. C and a working point of 1,245 deg. C. In contrast, the values for the familiar soda lime glasses are typically about 750 and 1,000, respectively. (Just to complete the picture, the values for lead-alkali glass are 677 and 985, respectively. All these numbers are in the Encyclopedia Britannica.)

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There are four other major types of modern glass: aluminosilicate; fused quartz; fused silica; and 96% silica. These glasses are all more resistant to high temperature, heat shock, and corrosive agents than borosilicate glass, but also more difficult to make and work. The other USE industries have not yet advanced to the point where they are needed.

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Down-time glass makers already have many colorants and decolorants. However, they don't yet know how to make the famous ruby glass of Bohemia, because its inventor, Johann Kunckel (1630-1703), is still in diapers. The secret to reproducing this glass is the use of microscopic particles of gold chloride.

Manufacturing Methods: Overview

Glass is cast by pouring it, while liquid, into a mold. Because of its viscosity, glass does not fill a complicated mold shape without assistance. A gob of molten glass can be forced, by means of a plunger, to spread throughout the cavity. This is called pressing.

Glass may also be blown. A bubble of molten glass is placed inside a mold and more air is forced into it, causing it to expand into contact with the mold.

In drawing, a tool called a bait is lowered into the molten glass and then raised. The glass adheres to the bait, and depending on the shape of the bait, a thread, rod or sheet of glass is drawn up. Glass may also be extruded through holes, as a result of centrifugal force, or a blast of air.

Molten glass can also be squeezed between rollers to produce a flat glass. Rolled glass may subsequently be floated on a bath of molten metal, so it smooths out.

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Almost all of the processes mentioned above were initially carried out by hand, and later by machine. As a nation of "master mechanics," Grantville will certainly attempt to find ways of automating the seventeenth-century glasshouse. However, it would be a grave mistake for them to attempt to duplicate, with machinery, a process which they have not carried out manually. All sorts of little things go wrong when you try to automate a complex process, and, if you don't have a deep understanding of the handicraft, then you aren't sure whether the problem is with the machinery, the raw materials, or whatever.

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About Iver P. Cooper

Iver P. Cooper, an intellectual property law attorney, lives in Arlington, Virginia with his wife and two children. Two cats and a chinchilla rule the household with iron paws. Iver has received legal writing awards from the American Patent Law Association, the U.S. Trademark Association, and the American Society of Composers, Authors and Publishers, and is the sole author of Biotechnology and the Law, now in its twenty-something edition. He has frequently contributed both fiction and nonfiction to The Grantville Gazette.


When not writing (or trying to get an “orange blob” off his chair so he can start writing), he has been known to teach swing dancing and folk dancing, or to compete in local photo club competitions. Iver adds, “I can’t get my wife to read my fiction, but she has no trouble cashing the checks.”

Iver’s story “The Chase” is in Ring of Fire II