Giacomo Carissimi

FACT ARTICLES.

In Vitro Veritas: Gla.s.smaking After The Ring Of Fire

By Iver P. Cooper

In the early seventeenth century, there was already a vigorous international trade in gla.s.sware. The world center for gla.s.smaking was in Venice, and the Venetians were most famous for tableware and gla.s.s mirrors made of the colorless cristallo. Germany and Bohemia were known for large, decorated drinking gla.s.ses, 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 gla.s.s for windows, some of which was exported.

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

Up-Time Knowledge of Gla.s.smaking 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 gla.s.s technology from scratch. Most of the Grantville library books that are specifically about gla.s.s are really about collecting antique gla.s.s, appreciating art gla.s.s, and so forth, rather than about gla.s.s technology. It may be interesting to see how the Venetians react to photographs of the creations of Tiffany, Lalique and Chihuly, but art gla.s.s 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 gla.s.s 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 gla.s.s factory before the Ring of Fire ("The Wallenstein Gambit" Ring of Fire), and several retired gla.s.sworkers are listed in the "Up-timers Grid."

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

Two types of modern gla.s.s are already familiar to down-timers. About 90% of modern silicate gla.s.s production is of soda lime gla.s.s, 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 gla.s.ses ("Bohemian gla.s.s"), which feature potash (pota.s.sium oxide) instead of soda. These gla.s.ses were used, prior to 1632, to make stained gla.s.s windows.

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

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

Lead-alkali gla.s.ses 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 pota.s.sium) oxide. Collier's offers two recipes for lead-alkali gla.s.s; the simpler one, for optical use, being 44.6% silica, 0.5% sodium oxide, 8% pota.s.sium 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 gla.s.ses 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 gla.s.s. 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 princ.i.p.al 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.

Borosilicate gla.s.s contains, as you might expect, silica and boron oxide. It is used primarily in chemical gla.s.sware and in ovenware.

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

Seventeenth-century European gla.s.sworkers were extremely secretive about their craft. It is conceivable that prior to 1632, there was some European use of borax in gla.s.smaking. 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 gla.s.ses: for chemical gla.s.sware, 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 gla.s.s, "crown" gla.s.s: 68.9% silica, 10.1% boron oxide, 8.8% sodium oxide, 8.4% pota.s.sium oxide, 2.8% barium oxide, and 1.0% zinc oxide. Two more borosilicate gla.s.s 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 a.s.sayers. If we want to make more than just small quant.i.ties of borosilicate laboratory gla.s.sware, 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 Luneburg in Hanover, where it was first observed in 1787. . . . [A] ma.s.sive variety, known as sta.s.sfurt.i.te, occurs as nodules in the salt deposits at Sta.s.sfurt 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 gla.s.sworkers will need to adapt to the special properties of borosilicate gla.s.s. 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 gla.s.ses are typically about 750 and 1,000, respectively. (Just to complete the picture, the values for lead-alkali gla.s.s are 677 and 985, respectively. All these numbers are in the Encyclopedia Britannica.) * * *

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

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

Manufacturing Methods: Overview Gla.s.s is cast by pouring it, while liquid, into a mold. Because of its viscosity, gla.s.s does not fill a complicated mold shape without a.s.sistance. A gob of molten gla.s.s can be forced, by means of a plunger, to spread throughout the cavity. This is called pressing.

Gla.s.s may also be blown. A bubble of molten gla.s.s 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 gla.s.s and then raised. The gla.s.s adheres to the bait, and depending on the shape of the bait, a thread, rod or sheet of gla.s.s is drawn up. Gla.s.s may also be extruded through holes, as a result of centrifugal force, or a blast of air.

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

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 gla.s.shouse. 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.

Once you have an understanding of the subtleties of hand blowing, casting, drawing, pressing, rolling, grinding and polishing gla.s.s, you are ready to consider whether any of the steps can be automated. The World Book Encyclopedia sketches out methods of blowing gla.s.s bottles, pressing gla.s.s dishes, and drawing gla.s.s tubing (the Danner process). The 1911 Encyclopedia Britannica describes mechanical methods of pressing, blowing and drawing gla.s.s. It even discusses the famous 1904 Owens "suck-and-blow" bottling machine. The modern Encyclopedia Britannia divulges two more methods of making gla.s.s tubing (the downdraw and Vello processes), the workings of the 1926 "ribbon machine" (which makes 30 light bulb sh.e.l.ls per second), and the basics of blow-and-blow bottle making on the "Individual Section" machine. Both modern reference works describe the jewel of the modern gla.s.s industry crown, the Pilkington float process (discussed in a later section) for making plate gla.s.s without polishing steps.

In general, the encyclopedias describe the basic operations that the modern machinery performs, but not the specific mechanisms which accomplish them. For example, the World Book Encyclopedia (WBE) describes a mechanical method of blowing gla.s.s bottles: drop a gob of gla.s.s into a bottle-shaped mold (neck end down); blow air in to force the gla.s.s into the neck; flip the mold; and then blow air in again to force the gla.s.s down to form the walls of the main compartment (WBE 214). But the neophyte mechanical engineers of Grantville will need to figure out the exact gadgetry that will ladle out or suck up that gob, manipulate the mold, and blow on command. They will also need to design the appropriate conveyors, guides, controls, safety rails, etc.

The same reference also explains the 1917 Danner process for mechanically drawing tubing: let a ribbon of gla.s.s entwine itself around a slowly rotating mandrel, and then pull it off. The mandrel itself is hollow, and air is blown through it so the walls don't collapse upon the hollow center. The tubing is fed by rollers to a cutting device (WBE 214). Someone still needs to figure out how the operating temperature, the rate at which the gla.s.s is supplied, the material of which the mandrel should be made, the rate of rotation and angle of inclination of the mandrel, and the rate of air flow through it.

Improved Manufacturing Methods: Plate Gla.s.s

To make good windows, you need to be able to make plate gla.s.s. Plate gla.s.s is not merely any old flat gla.s.s. It is gla.s.s which is clear, and has flat, parallel surfaces. Otherwise, it will afford only a distorted view. Some techniques produce plate gla.s.s directly, others produce a rough-surfaced sheets which must be ground and polished to convert them into plate gla.s.s.

There were two down-time methods of making of large panes of gla.s.s. In the Crown gla.s.s ("Normandy") method, a bubble of gla.s.s was blown, cut open, and spun about. The spinning resulted in a circular pane. The gla.s.s was frequently reheated during this process, giving it a high polish (fire polish). The gla.s.s was cooled, and the workers first cut out the center (bulls' eye), and then cut out straight pieces.

In the Broadsheet ("Lorrainer") method, the gla.s.s was blown, then swung to form a long cylinder, a "sausage." The craftsmen cut off the ends, opened the cylinder lengthwise ("m.u.f.fing"), placed it in a flattening oven, and polished it (Gros-Galliner, 32-33).

The first logical improvement would be the introduction, half a century earlier than in our time line, of the French method of making plate gla.s.s by grinding and polishing "table cast" gla.s.s. In 1687, Bernard Perrot "invented a method, hitherto unknown, of casting gla.s.s into panels, the way one does with metal . . ." He poured molten gla.s.s onto an iron plate covered with sand, and then rolled the gla.s.s flat. After it cooled, it was ground and polished with iron disks, abrasive sands, and felt disks. The Perrot method, as practiced in the great French manufactory of Saint Gobain, allowed production of plate gla.s.s (and flat gla.s.s mirrors) on a theretofore unheard-of scale. Saint Gobain produced the 306 mirror panes of Versailles' Hall of Mirrors, which effectively served as a permanent advertising kiosk for the French plate gla.s.s industry.

In 1903, John H. Lubbers partially automated the medieval Lorrainer method. A circular bait, at the end of a blowpipe, was dipped into a draw pot, and a cylinder of gla.s.s was drawn up. The cylinder still had to be manually cut and flattened. Nonetheless, the Lubbers technique allowed the fabrication of larger sheets of gla.s.s by less skilled workers. This method is mentioned in the 1911 EB, although that reference mysteriously remarks that during the drawing operation, the cylinder is "kept in shape by means of special devices."

By 1905, Emile Fourcault succeeded in vertically drawing a continuous sheet of gla.s.s directly from the gla.s.s furnace. The Fourcault process is also described in the 1911 EB. However, it naturally does not reveal the improved process (featuring a device called a debiteuse) which Fourcault developed in 1913, so the gla.s.s did not narrow at the base as the leading edge was drawn up (Douglas, 155). The drawn gla.s.s was still marred by rollers, and needed to be ground and polished to be suitable for optical use.

What truly revolutionized the plate gla.s.s industry was Alastair Pilkington's float gla.s.s process (1952), in which the gla.s.s spreads out over a layer of molten tin. Because the surface of the molten tin is flat, the gla.s.s also becomes flat, settling to a thickness of six millimeters.

The Encyclopedia Britannica favors us with a schematic diagram of the Pilkington float process, and with a few process parameters. It is certainly worth trying to duplicate once we have mastered the earlier plate gla.s.s production methods. However, it is important to recognize that it took seven years, and seven million pounds, to reduce the idea to practice, using 1950s' technology. We do have the advantage of having a description of the perfected process, but there is no doubt in my mind that the explanation leaves out important details. For example, the operator needs to control the viscosity gradient by appropriate settings of the water coolers along the process line. And some of the details it does give, such as the need for a controlled hydrogen-nitrogen atmosphere to prevent oxidation of the tin, are daunting.

New Manufacturing Methods: Mirror The down-time state of the mirror-making art was the technique developed by Venetians Andrea and Domenico de'Anzolo del Gallo in 1507. They realized that the Venetian cristallo could be given a highly reflective surface by hammering tin into thin sheets, amalgamating it with mercury, and then laying the sheets of cristallo onto the amalgam.

We can greatly improve upon this century-old technique, dispensing with the poisonous mercury, and also obtaining a more uniform coating of controllable thickness. In 1835, Justus von Liebig discovered that silver could be deposited in a thin film on gla.s.s. There are many variations on the Liebig process, but in all of them, a solution of a silver salt is used as a source of silver ions. A reducing agent reduces the silver ions to neutral silver atoms, and the latter are deposited on the gla.s.s. In Liebig's original work, an ammoniacal solution of silver nitrate was heated with and reduced by an aldehyde (e.g., formaldehyde) to elemental silver. The process was commercialized in the 1840s, and true silvering replaced foiling.

Two related methods of reduction by solution have been developed. In the "hot" silvering method, the ammoniacal silver nitrate solution was boiled, and the condensing steam was reduced with tartaric acid (more precisely, with Roch.e.l.le salt, the sodium-pota.s.sium salt of tartaric acid.) This deposit method is slow; it may take an hour to form a thick film. For this reason, the "hot" or "Roch.e.l.le Salt" process is favored for making "one-way" mirrors, which have a partially reflective surface (Newman, 317, 322). In the "cold" method, the reducing agent is sugar (Gregory, 158). This is also called the Brashear process.

An improvement on the basic method is to sensitize the gla.s.s so it more readily accepts the metal. This is usually done by "tinning"; treating the gla.s.s with a dilute stannous chloride solution (Newman, 15, 314).

Originally, silvering solutions were poured onto the gla.s.s. However, they can be sprayed on, instead. Typically, two jets are used, one supplying the ammoniacal silver nitrate and the other a fast-acting reducing agent such as hydroxylamine sulfate (Schiffer, p. 7).

There are alternatives to silvering, such as aluminizing, but I don't expect them to be duplicated within the near term in the 1632 universe.

Miscellaneous Up-Time Manufacturing Innovations Optical gla.s.s must be h.o.m.ogeneous. Curiously, the importance of stirring the melt, so that the ingredients were efficiently mixed, was not recognized until 1790, when Pierre-Louise Guinand pioneered the use of a refractory ceramic stirring rod. (WBE, 218). This is one of those ideas which was long in coming, but was readily implemented.

It is also important to inhibit the formation of bubbles. This can be done by the addition of a fining (dega.s.sing) agent. Several are mentioned by the Encyclopedia Britannica (EB): a.r.s.enic oxide, sodium nitrate, sodium chloride, sodium sulfate and sodium nitrate (EB 300).

Early gla.s.s furnaces used wood as fuel. England needed the wood for shipbuilding, and, once the English figured out how to make a coal-fired furnace, they banned further use of timber (1610-1615). The new furnaces could achieve higher temperatures, which allowed for use of higher-melting gla.s.s compositions.

There is plenty of coal in the USE, but up-timers may find it advantageous to burn natural gas instead. It is readily available in the Grantville area, it is a more intense energy source, and it facilitates manufacturing. For automated feeding, the gla.s.s must have the correct viscosity, which in turn depends on temperature. It is easier to control the temperature of a gas-fired furnace. (Douglas, 42).

Important energy savings result from the use of a regenerator. Essentially, the flue (waste) gases are used to heat a brick "checker work" (shown in Fig. 4 of the Encyclopedia Britannica "Industrial Gla.s.s" article, and also discussed by the 1911 EB), which in turn is used to preheat the combustion gases. Heat regeneration was first used in the 1860s and reduced fuel consumption by about 90%.

A major continuing expense for a pretwentieth-century gla.s.s factory was the pot used to hold the molten gla.s.s. In early nineteenth-century America, this pot cost about one hundred dollars, and took eight months to build from clay. It was able to resist the tremendous heat, but its life span was only eight weeks, and so the pots had to be replaced over and over again (Polak). Modern gla.s.s furnaces use highly refractory ceramics. (Different gla.s.s melts may necessitate different ceramics.) The Encyclopedia Britannica has two helpful comments on this point. First of all, it teaches that "clays of a high alumina-to-silica ratio, with minimal impurities," are more resistant. Secondly, it singles out the "electric-arc fusion-cast" ZAC refractory (35% zirconia, 53% alumina, 12% silica), developed in 1942.

Alumina is aluminum oxide. One major alumina ore, bauxite, is available in France and in Ireland. Please note that we aren't proposing to extract aluminum, but rather to use the bauxite, a claylike mineral, directly. A possible alternative to bauxite is kaolinite (aluminum silicate). According to the 1911 EB article on "kaolin," there is kaolinite "near Schneeberg in Saxony."

Modern gla.s.smaking operations employ a tank furnace. The raw materials are fed in at the loading end and the molten gla.s.s is removed from the working end. These tanks can be operated continuously, while pots process gla.s.s one batch at a time, which is less efficient. The first continuous furnace was constructed by Friedrich Siemens in 1867.

While the Grantville Library provides critical information concerning both lead-alkali and borosilicate gla.s.s, the fact remains that quality control is going to be an ongoing problem. The mineral content of sands, ashes, and so forth are going to vary from source to source and even from lot to lot.

In the short term, USE gla.s.smakers will keep careful records as to which raw materials were mixed together, in what proportions, to obtain a particular batch of gla.s.s, and the physical and chemical properties evidenced by that gla.s.s. If a particular batch does not pa.s.s muster for its intended purpose, it can be used for some less demanding task, or remelted. (A significant portion of the input to modern gla.s.s furnaces is rejected gla.s.s, called "cullet.") The high school chemistry laboratory also should be capable of performing qualitative tests for some metals, using the standard flame and bead tests. I would expect the high school science teacher, Greg Ferrara, to know about these a.s.says.

Grantville gla.s.s companies can make gla.s.s production more predictable by purifying the silica and the gla.s.s modifiers, so that the gla.s.smakers know just how much of each ingredient they are adding to the melt.

Also, as the USE develops capabilities for production-scale inorganic chemical synthesis, it will be able to use alternative starting materials which are cheaper or more readily available. By way of precedent, the late eighteenth-century French government wanted to eliminate its dependence on Spain as a source of soda. (A Spanish seaweed, when burnt, provided an ash that was 20% soda.) The French offered a 2,400 livre prize, which was won by LeBlanc in 1787. LeBlanc synthesized a purified soda (sodium carbonate) from sea salt (mostly sodium chloride).

Improved Gla.s.s Products: Laboratory Gla.s.sware Laura Runkle has pointed out that "in order to make pharmaceuticals, the people of Grantville need stainless steel, or gla.s.s-lined vessels." ("Mente et Malleo: Practical Mineralogy and Minerals Exploration in 1632," Grantville Gazette Vol. 2).

In 1632, Greg Ferrara commented, "Sulfuric acid is about as basic for modern industry as steel" (Chap. 40). Where, exactly, do you put sulfuric acid? Clearly, you need a corrosion-resistant vessel, whether that be gla.s.s, lead, or steel. If you want to play with hydrochloric acid, you need gla.s.s, a molybdenum-rich alloy, or tantalum.

For laboratory scale chemistry, gla.s.s is clearly superior to stainless steel and various exotic metals. Not only is it corrosion-resistant, it can be made transparent, so you can observe the chemical processes as they take place. Or it can be amber-tinted, to protect photosensitive chemicals. Gla.s.s is used extensively in the bottles, graduated cylinders, beakers, flasks, pipettes, condensers, test tubes, watch gla.s.ses, burets, funnels, crucibles, and retorts of modern chemical laboratories.

Borosilicate gla.s.s, such as that sold under the trademark Pyrex, is preferred, because it is especially resistant to acids, high temperatures, and sudden changes in temperature (thermal shock).

Improved Gla.s.s Products: Optical Instruments Another form of specialty gla.s.s is optical gla.s.s. Dutch Admiral Maarten Tromp, awaiting the approach of the Spanish fleet, enviously remembers his brief experience with up-time optics: "The stunning visual clarity, featherlight weight, and exquisite craftsmanship of the binoculars had been convincing evidence of the marvels of which American artisans were capable." (1633, Chap. 19). King Gustav II Adolf of Sweden was equally impressed with Julie MacKay's spotting scope (1632, Chap. 48). And the nearsighted cavalryman Andrew Lennox appreciated his new American-made spectacles (1632, Chap. 16).

Like window gla.s.s, optical gla.s.s must be transparent. However, the real power of optical gla.s.s is realized when the gla.s.sware has a curved surface, creating a diverging or converging lens. Optical gla.s.s makes possible not only better spectacles and telescopes, but also microscopes. The latter is extremely important if medicine is to advance.

The preferred optical gla.s.s is a lead-alkali gla.s.s, which has a higher refractive index (a measure of the ability of the gla.s.s to alter the path of light which strikes its surface obliquely) than soda-lime gla.s.s.

Incandescent Light Bulbs and Fluorescent Light Tubes Letting light escape, while keeping air from entering, is the function of the gla.s.s bulb of an incandescent light. The down-time master gla.s.sblower Hensin Hirsch is making light bulbs by hand, evacuating them using a vacuum pump scavenged from a refrigerator. (Gorg Huff, "Other People's Money," Grantville Gazette, Vol. 3). If the up-timers can duplicate the ribbon machines of our time line, then they can ma.s.s-produce light bulb sh.e.l.ls.

In a fluorescent tube, the gla.s.s enclosure confines the mercury vapor. Electricity causes the latter to emit ultraviolet light, and this in turn stimulates a phosphor coating on the gla.s.s to absorb the light energy and re-radiate it as visible light.

Down-time gla.s.sblowers can certainly duplicate the tube itself, and mercury was available in 1632. The issues are how to inject the mercury safely, and how to obtain and apply the phosphor. I don't consider fluorescent lamps to be a practical development target for the USE, at least in the short-term.

Greenhouses A logical extension of the normal architectural use of the window is the greenhouse, which has gla.s.s walls and ceiling. Greenhouses would allow the USE to grow plants that can only thrive under tropical conditions, or to obtain additional crops of plants that die back or become dormant in the northern European winter. Soon after the Ring of Fire, "medicinal and ornamental plants were [being] grown in the gla.s.s-roofed conservatory" of Grantville's hospital (Ewing, "An Invisible War," Grantville Gazette, Vol 2). If USE explorers venture into Latin America, they can bring back seeds of the Hevea brasiliensis rubber tree for greenhouse cultivation, and ultimate transplantation to a tropical country friendly to USE.

The concept of the greenhouse is not entirely foreign to seventeenth-century Europeans. De Serre protected individual plants by covering them with gla.s.s "bells" in 1600. There are also reports that orangeries with gla.s.s windows were established in Pisa (1591) and Leiden (1600) (Muijzenberg, 45). The greenhouse is quite practical if we can produce the necessary plate gla.s.s; "seconds" from the window gla.s.s factories would be probably be good enough.

Protective Gla.s.ses Gla.s.s can be used in the windows of military vehicles and structures, but then we need to worry about the effect of enemy fire. A relatively low-tech way of reinforcing the gla.s.s is to use wire gla.s.s, which is sheet gla.s.s b.u.t.tressed with a wire netting. Wire gla.s.s is made by lowering a wire mesh into a stream of molten gla.s.s. (Or by laying down a ribbon of gla.s.s, then the wire mesh, then another ribbon, and finally rolling them together.) Wire gla.s.s won't keep out a cannonball, but it will give some protection against, say, flying debris.