Faq on Glassmaking in 1632 by Iver P. Cooper

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FAQ on Glassmaking in 1632

by Iver P. Cooper

(Copyright Iver P. Cooper 2004. All Rights Reserved.)
This FAQ supplements my 1632 Slushpile article, "In Vitro Veritas". IVF concentrates on up-time techniques which might be feasible in 1632, while this FAQ is more concerned with what is already going on down-time. However, there is some overlap with respect to types of glass and uses of glass. If you find this FAQ of interest, be sure to read "In Vitro Veritas".


The USE's most important trading partner is the Venetian Republic, which, in 1632, was still the world leader in the glass industry. However, storm clouds are on the horizon. France and England had already made it plain, by recruiting master glass craftsmen from Venice, that they wanted to develop their own glass houses. This would allow them to curb the importation of Venetian glass into their own countries, and capture some of the Venetian's valuable export trade with other lands, too.

The Ring of Fire presents both danger and opportunity for the Venetian glass industry, and therefore for the Most Serene Republic. On the one hand, French and English spies will learn, half a century early, of the technological innovations that allowed them to encroach on the Venetian trade. On the other hand, the Venetians now have fair warning of their danger, and can take countermeasures. The most prudent of these would be to adopt these innovations themselves, and then take advantage of their established position in the industry to edge out the French and English competition before it can establish a beachhead.

In 1271, the fiolarii (makers of glass vessels) signed a formal agreement with the Venetian Republic. This Capitolare Fiiolariis provided, significantly, that no glass would be imported into Venice, and that no foreign masters would be allowed to make glass. Plainly, the Republic wanted to foster a homegrown glass industry and to avoid industrial espionage. In 1291, the Venetian government pressed the glassmakers’ guild to move to island of Murano, ostensibly to eliminate the risk that a fire from their furnaces would burn down Venice, but probably also to heighten security.

Two centuries later, the Venetian industrial policy paid off handsomely. The key experiment began on Feb. 21, 1457, when Angelo Barovoier (Beroviero?) and Niccolo Mozetto were given permission to work on "crystal glass" outside the normal production season (Polak, p. 65). In 1460, he began commercial production of a colorless soda lime glass, cristallo, named after rock crystal. It was used in many products, notably lenses and–once the problem of applying a metal coating was resolved–mirrors.

This cristallo was far superior in clarity to the older glasses, and it sold well. What was its secret?

One possible factor was the source of the "glass former," silica. Usually, glassmakers obtained it from either quarried sand or crushed siliceous stones. These sands and stones varied in composition, depending on where they were collected. Ancient Egyptian glass, for example, was made from a sand with a high iron content, giving its glass a greenish cast (Lambert, p. 109). One authority says that the key to the clarity of the Venetian cristallo was "crushed pebbles from a nearby river, the Ticino." (Origins). These may have been specially selected for their whiteness (Maloney, 59).

Another important glass ingredient is the flux. This was a salt that served to lower the melting point of the silica. The fluxes were usually salts of sodium, potassium or lead. The sodium salts were usually obtained from certain minerals (e.g., natron) or by burning seaweed (yielding "soda ash", sodium carbonate). The potassium salts were derived by combustion of hardwoods such as beech (yielding "potash", potassium carbonate)(Lambert, 104-5, 122, 124-6).

One also needed a stabilizer, such as a calcium, magnesium, barium or aluminum salt, to protect the glass from water. If this was not already present as an impurity in the silica (sand can be calcium-rich) or flux source (some plants are rich in magnesium), it had to be added. Roman glasses were "soda lime glasses", in which the silica was combined with sodium oxide (derived from soda ash) and calcium oxide (derived from lime).

After the fall of Rome, northern Europe switched to potash as the flux, while the Venetians continued to use soda ash. William S. Ellis says that "the breakthrough was achieved with the use of a vegetable ash rich in potassium oxide and magnesium. The sale di vetro, or glass salts, extracted from the ash were used as the fluxing agent in the melting process, mixed with the sand to produce a sodium-potassium-based crystal-like glass...." (Lambert, 104, 113). Other ingredients could also have been significant. Maloney says that the Venetian cristallo, like the Roman glasses, contained manganese as a decolorizer (p. 58). Melchior-Bonnet says that the Venetian masters knew that ashes of kali, an Egyptian herb, "acted as a bleaching agent because of its low phosphorous content and richness in manganese." (P. 19).

Thomaso Garzoni de Bangacavallo, in his Piazza universale, suggested that cristallo owed its advantages to the particular salinity of the Venetian seawater, the particular woods used in the firing process (which affected the nature of the flame), and the quantities of salt and soda.

Most likely, it was not any one ingredient, but rather an overall optimization, that was responsible for the sterling qualities of the new cristallo.

Tin-mercury amalgams were used by the Venetians in mirror-making as early as 1317. The process was called "foiling" because the mercury was used to bind tin leaf. In 1507, Andrea and Domenico de’Anzolo del Gallo 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. The critical advantage of this tin amalgam process was that it did not require heat, and therefore was unlikely to crack the glass through thermal shock. They petitioned the Venetian Council of Ten, claiming to be the possessors of “the secret of making mirrors of crystalline glass, a most valuable and singular thing ... unknown to the whole world, except for one house in Germany and one in Flanders, who sell their mirrors at excessive prices.” (Melchior-Bonnet, 18; Gros-Grovenor, 18, Goldberg, 140, Schiffer 6). They asked for 25 years exclusivity, but were awarded 20.

This was enough to provide a formidable jump-start to mirror-making in Venice, and the specchiai, or mirror-makers, became so important that, in 1569, they organized their own guild.

Venetian mirrors, besides being products in their own right, were incorporated into Venetian wall lights. By the late seventeenth century, these usually had a mirror glass back plate to enhance illumination (Mehlman, 169).

Glass mirrors commanded very high prices. One looking glass, two feet by four feet, and framed in hand-wrought silver, was valued in the late 1600s at three times the worth of a painting by Raphael (Schiffer, p. 7). In 1680, the cost of a 3 X 4 foot mirror was $40,000 in today's currency (Mirror Mirror). Hence, other nations began to take a great interest in acquiring the Venetian knowhow, producing Venetian-style glass and mirrors domestically, and competing with the Most Serene Republic for the export market.

Consequently, at least from the fifteenth century on, the Venetian Republic sought to keep glassworkers from taking their secrets elsewhere. The Venetians used both the carrot and the stick. On the one hand, the nobility and the glassmakers were allowed to intermarry without penalty. On the other hand, the penalty for seeking to leave the island without permission was death. (Zerwick, 45-6; Polak, 24-25). If such pressures did not succeed, assassins could be sent after fugitive glassworkers. (Zerwick, 45-6) "In Normandy, a Murano glassmaker, Paoli, was found by his daughter with a dagger in his heart and a note ‘traitor’ attached." (Gros-Galliner, p. 72)

Despite all blandishments, threats and punishments, glassmakers left Venice by the hundreds, spreading the knowledge of Venetian techniques to England, France and elsewhere (Gros-Galliner, 25; Wills, 42; Schiffer, 24). But the real threat to Venetian hegemony was not from renegade Muranese, but from innovators abroad. The British devised a new form of glass which was more sparkling, less brittle, and easier to cut. (See "Old and New Types of Glass", below.) And the French learned to make flat glass in a manner that made large windows and mirrors possible, even affordable.


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

In the Broadsheet ("Lorrainer") method, the glass was blown, then swung to form a long cylinder, a "sausage". The craftsmen cut off the ends, opened the cylinder lengthwise ("muffing"), placed it in a flattening oven, and polished it (Gros-Galliner, 32-33). The first written description of the process is in the treatise On Divers Arts (early 12th cent.). The window glass for the White House is still made from mouth-blown glass by the Lorrainer method. Each "sausage" of glass, when slit and ironed out, makes an 18 x 25 inch pane. They contain "air bubbles and wavy bands," but the White House treasures the antique look (Ellis, p. 23).

The manufacture of plate glass was revolutionized by Bernard Perrot, the first person, at least since Roman times, to form glass into panels by casting. This involved pouring molten glass onto an iron plate covered with sand. The glass was rolled flat. After it cooled, it was ground and polished with iron disks, abrasive sands, and felt disks. He communicated his idea to the French Academy of Sciences on April 2, 1687. The French authorities declared, "he has invented a method, hitherto unknown, of casting glass into panels, the way one does with metal...." (Polak, p. 127) In 1688, a royal factory (Manufacture Royale des Glaces des France) for production of cast glass was set up in Faubourg Saint-Germain in Paris. It moved to Saint-Gobain in 1693 (Polak, pp. 127-29).

Its technological headstart gave the French plate glass industry a continuing advantage; until the late eighteenth century, cast plate glass was only made in France (Pilkington). With its up-time knowledge, the USE is in a position to give that advantage to its Venetian collaborators. (Take that, Richelieu!)

Historically, the development of cast glass caused the cost of mirrors to fall dramatically, although the size of the mirror, the quality of the surface, and the frame would all affect the price. In the 1770s, Chippendale earned 160 pounds for the sale of a “fine” looking glass, 91 x 57 ½. At the other end of the scale, he sold servants’ dressing glasses for as little as four shillings apiece. (See generally Wills, 47-8, 148-9, 155, 157-8.) The improved economics naturally fostered the dissemination of mirrors. “It has been shown that between 1675 and 1725 ownership of mirrors (based on the inventory samples analyzed) rose from 58% to 80% in London, whilst in provincial towns the rise was from 36% to 74%.” (Edwards)


Since Grantsville was plopped down into the Germanies, it is prudent for the up-timers to know something about the local glass industry.

The German glass industry was responsible for a number of innovations. For example, in the twelfth or thirteenth centuries, Nuremburg craftsmen developed a new method of making mirrors. They introduced molten lead or tin into still hot blown glass bulbs, which were rotated until covered with a film of metal. When the bulbs cooled, they were cut to size for use as looking glasses (Schiffer, p. 6). These glass mirrors were a substantial improvement over the Roman metal ones, which could warp, tarnish, etc.

In 1632, the Germans were probably best known for large, elaborately decorated drinking goblets. These were gilded, enameled or engraved.

The German glassworkers were also known for a particular type of glass, Waldglas ("forest glass"). Waldglas is green in color, implying the use of a raw material containing iron compounds. It was so named because, beginning in the thirteenth century, the Germans had migratory forest glass houses. These used the nearby wood as fuel and, when it is exhausted, move on. There were Waldglashutten in the imperial forest near Nuremberg, the Fichtelgebirge, the Thuringian forest, Silesia and Solling in the fourteenth century, and in Bohemia, Hesse and Lorraine in the fifteenth. Gradually, the glassworkers settled down, the process becoming complete by the eighteenth century.

The principal glassmaking centers of Germany, prior to 1900, were in Cologne, Kassel, Frankfurt, and Nuremberg in western Germany, Augsburg, Munich, and Landshut in Bavaria, and Dresden, Berlin, Potsdam and Zechlin in Prussia. (Mehlman, 62) Unlike the more exalted glass workers of Venice and France, the Teutonic waldglas makers have a relatively low social status. They tended to intermarry, and hence there were certain families which made a name for themselves as glass craftsmen: "Gundelach and Kunkel in Hesse, Wenzel and Schurer in Bohemia, Preussler in Silesia, [and] Greiner in Thuringia...." ( Weiss, 127-28)

In the fifteenth and sixteenth centuries, the leading guild was the Hessicher Glasnerbund. It had the advantage of proximity to Almerode in the Kaufunger Wald, which was a source, not only of sand, but of a fire-resistant clay for making the pots. The protector of the guild was the Landgrave of Hesse. The glassworkers were allocated territories by the forest warden. The factories were set up beside streams, and the master and his two or three journeymen worked and lived there from Easter until St. Martin's Day (November 11). In 1557, there were two hundred glassmakers in Hesse, and their output was on the order of 2,500,000 beer glasses and 2,000 tons of window-glass.

Renegade Venetian glassmakers prospered in France, the Netherlands, Flanders, and England. In Germany, at least prior to 1632, the Venetian expatriates were not particularly successful. Giovanni Scarpoggiato established an operation in Munich in 1584, but it failed. Francesco Warisco attempted to produce cristallo in Cassel, beginning in 1583, but had abandoned this effort by 1586. In 1607-1609, there were Venetian glassmakers in Cologne, but their factory met an untimely end when it was burnt down by the natives.

There are probably Venetian glass workers in Thuringia at the time of the Ring of Fire. Duke Bernhard von Sachsen-Weimar (1604-39) at some point "had Venetian glass made at Tambach by Italian workers". (Weiss, 126-7)

Old and New Types of Glass

The soda lime glasses are perhaps 4,000 years old. 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).

About 90% of modern silicate glass production is of soda lime glass, which is used in bottles, windows, light bulbs, and table ware. The Encyclopedia Britannica gives representative ingredient proportions for each of these applications; in general, the soda lime glasses 70-75% silica, 14-18% sodium oxide, and 5-10% calcium oxide, with small, nonessential amounts of oxides of aluminum, magnesium, potassium and barium. However, there is no need for USE companies to rediscover the effective proportions if they hire knowledgeable down-time glassworkers.

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. As you might guess, it is possible for a glass to contain both potash and soda.

The soda lime and potash lime glasses are down-time technology, readily accessible to us if we are willing to hire knowledgeable down-timers. It is certainly possible to play around with their compositions to fine-tune them for particular uses.

So far as major new glasses are concerned, the major up-time contributions will be lead-alkali and borosilicate glasses. These are discussed in some detail in In Vitro Veritas. Here, I will just discuss the possible down-time foreknowledge of these developments.

Lead-alkali (flint) glass was supposedly "invented" in 1676 by George Ravenscroft (1632-1681), a glass merchant. Ravenscroft actually rediscovered an ancient invention; there are both Roman and Islamic glasses which are as much as 35% lead oxide (Lambert, 118).

Just how old is borosilicate glass? There is a story, perhaps apocryphal, that a Roman glassworker showed the Emperor Tiberius an "unbreakable" glass (vitrum flexile), expecting a reward. Instead, that angelic monarch had the glassworker put to death, and his workshop destroyed, so as not to reduce the value of the imperial glassware. Some scholars (Corning Glass) believe that the unfortunate craftsman had stumbled upon a form of borosilicate glass, most likely incorporating borax from the Tuscan or Turkish sources.

Whatever you think of the Roman story, there is evidence that in 1225, the Chinese were 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).

Down-time European glassworkers are 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 Kunkel (1630-1703) mentioned borax in a recipe for an artificial gem (Smith).

There are three more major types of glass. I don't think that any of them are realistic USE development targets, but I will review them briefly.

Aluminosilicate glass was developed in 1936. They have the chemical and thermal resistance which makes the borosilicates so popular, but can tolerate higher temperatures. That same characteristic also makes them more difficult to work.

In theory, aluminosilicates can replace borosilicates. However, the Encyclopedia Americana warns that aluminosilicate glasses "are more expensive and more difficult to manufacture".

Chemically speaking, the simplest glass is 100% silica. This silica glass is also known as fused silica or fused quartz. (Some scientists distinguish the two, based on the method of manufacture.) Its advantages included very high resistance to heat, very high corrosion resistance, and the ability to transmit ultraviolet radiation. Fused quartz is used in the most demanding applications, such as spacecraft windows. Unfortunately, it can be made only if you have access to an extremely pure source of silica, and it is difficult to work (its "working point" is greater than 2,000 deg. C).

The last major form of silicate glass is 96% silica, which is midway between borosilicate glass and fused quartz in its properties. Like 100% silica, it is the sort of material you use if you want to put the glass on top of a chunk of ice, and pour molten metal over it. Unfortunately, it is made from borosilicate glass by a proprietary (Corning Glass Works) process, developed in 1939,and hence is available to USE only if its technologists can reverse-engineer it.

The main ingredient of glass is silica, and a pure SiO2 glass ("fused silica") is colorless and transparent. Impurities, such as iron oxide, can cause it to become colored. In the sixth century B.C.E., the eastern Mediterranean civilizations discovered that antimony decolorized the glass. Later, possibly as early as the first century C.E., the Romans discovered that manganese had the same effect (Lambert, 111-13).

Since decolorizing the glass is a nuisance, it is desirable to find a fine white sand deposit, low in iron oxide content, to use as the silica source. If the glass we make from our sand is green, we know that iron oxide is the culprit. Chances are that our down-time colleagues have their own favorite collecting sites.

While iron oxide is an undesirable impurity, other impurities are advantageous. For example, calcium oxide stabilizes glass. Calcium oxide was sometimes introduced into glass inadvertently. For example, broken shells, often found mixed in with sand, contain calcium carbonate (Macfarlane, 205).

Also, for a decorative glass, you may want to impart a particular color, by incorporating an appropriate metal salt: blue (cobalt), purple (manganese), green (chromium, iron), yellow (cadmium, uranium, antimony, silver), or red (gold). The famous ruby glass of Bohemia, which contained microscopic particles of gold chloride, was invented by Johann Kunckel in the 1670s.


Familiar and Novel Uses of Glass

Just like us, the down-timers are familiar with glass containers and windows. Some have seen spectacles and telescopes with glass lenses. Other up-time uses of glass will be alien to the down-timers.

Container Glass

In the modern world, at least one third of all glassware is container glass. Glass vessels are used to hold beverages, foods, drugs, cosmetics, and household and industrial chemicals. Most of the container glass is soda lime glass, although there is some use of lead glass at the upscale end of the market. Glass has the advantages of being inert (it doesn't react with the contents) and impermeable (so it protects the contents from outside oxygen, moisture, and microorganisms). Also, glass containers can be washed and reused.

Glass containers were common even in Roman times. And there aren't great differences in container glass composition between modern glass vessels and those made in Venice and other Renaissance glass centers. The principal up-time innovations in the realm of container glass have related to the hermetic sealing of the containers for long-term food storage, and the replacement of handcrafting with machinery.

The Emperor Napoleon offered a handsome prize (12,000 francs) to the first person to discover a method of preventing spoilage of military rations. Nicholas Appert met the challenge, developing a glass canning jar. The jars were corked and sealed with wax, and sent to sea for four months and ten days. When his jars were opened, all eighteen food samples were tasted, and found to still be fresh.

In response, the British developed tin cans, which had the advantage of not being breakable. For commercial canning, tin cans quickly displaced the glass jars. However, in the home canning market, the glass "mason jar", patented in 1858, is still king. Metal cans cannot be reused, and the chemical reaction of the foods with the metal led to off-tastes. One authority comments, "the mason jar is historically important because it freed farm families from reliance on inferior containers, and from using pickling, drying, and smoking food to prepare for the winter". Some form of glass home canning jar might be a real boon to seventeenth century Thuringian farm families.

The mason jar was hermetically sealed by the combination of a zinc lid and a rubber ring. The USE will need to either forego mason jars until it finds a rubber source, or solve the sealing problem some other way.

Glass Windows

Glass windows were known in 1632; the most extravagant examples would be the stained glass windows of the great cathedrals. They could also be found in the homes of the wealthy; it was not limited, by sumptuary laws, to the nobility.

The issue of how much glass windows cost, and who owned them, occasioned some spirited discussion in Baen's Bar. Virginia de Marcie urged, "The houses of anyone who could

afford it had glass panes, not just nobles...." By way of example, she points to Francken, "Supper at the House of Burgomaster Rockox" dated (1630-1635). "Upper bourgeois", she commented, "but definitely not nobility".

I have had some difficulty getting firm information on the cost and distribution of windows in the *early* 1600s.

In a chapter on "architectural glass in seventeenth century France", Polak 120-1 says that window glass was considered sufficiently valuable so that if the owner of the house moved, he took them with him, and window glass was bequeathed in wills.

On the other hand, Polak also mentions that Dutch painters, such as David Teniers the Younger (1610-1690) and Adriaen van Ostade (1610-1685) specializing in "low life" did show glass windows in peasant cottages and farmhouses.

I examined an online collection (Web Gallery of Art, of Tenier's paintings, and "Apes in the Kitchen" clearly has a three rectangular pane window, while "Village Scene" features a window composed of numerous small diamond panes. Alas, both are undated. However, I fortunately stumbled upon a work by Teniers the Elder (1582-1649), "Village Feast", which presents a similar window.

Virginia has called to my attention a painting by Adriaen Brouwer (1605-38), "The Operation", with another diamond-paned window. It shows a plainly dressed doctor treating a equally unfashionable patient. While it, too, is undated, it can't be later than 1638!

Diamond 113 says that by the *late* seventeenth century, most "well to do" people in England did have at least a few windows of glass. More helpfully, Mehlmann 205-6 says that after 1590, the domestic production of window glass in England made the windows affordable by those of "moderate means".

The earliest wholesale price for plate glass I can find is by back-calculation from a 1757 source. For a sheet 60 x 42.5", it was 37 British pounds for the raw plate glass, and 15 for grinding and polishing, so total is 52. Retail price was probably double. Note that this is after the price of plate glass went down thanks to St. Gobain, etc. (If one pound in 1675 is equivalent to $20 (1952), then that sheet of plate was equivalent to $2,000 in 1952.)

A maid's annual income in 1675 was 12-15 pounds a year. So there is no way a servant could afford glass like this. Still, all windows are not created equal:

–a small window would cost less than a large one
–a small window salvaged from one or more fragments of a large one would probably cost less than a new one of the same size (Virginia mentions that "in rural churches, rounds of bottle glass, carefully saved from multitudes of broken bottles [were] leaded together" to make windows.)
–a window made of many small panes would cost less than a single large window of equivalent area
–a window with flaws, such as wavy lines, bubbles, rough spots, etc., would cost less than one which was of high quality.
–domestic glass would cost less than imported glass
So, even one of very modest means could manage to acquire some kind of window, just as servants could afford glass hand mirrors even when a psyche (a two meter tall dressing room mirror) would have been out of question.

We tend to take windows for granted, without giving much thought to how they benefit us. In essence, windows allow light to enter a structure, while blocking the movement of heat. They prevent heat transfer by three different mechanisms. First, as a physical barrier to the outward movement of the indoor air, they block loss of heat by convection. Secondly, because glass conducts heat poorly, the loss of heat by conduction is slow. Finally, the most common types of glass, while transparent to visible light, strongly absorb infrared (heat) radiation. Thus, glass inhibits the loss of heat by radiation from the contents of the room. Without glass windows, we either must live in totally enclosed rooms, lit only by artificial lighting (which has its own problems), or we must let light in through simple openings, which also allow hard-won heat to escape.

In 1632, glass windows were expensive enough so that only a chosen few could enjoy their advantages. The quality of the window glass is a further indication of the power or affluence of the owner. It is no accident that there is window glass "almost as clear as modern glass would have been" in the royal St. James Tower, while Admiral Simpson, in more provincial Magdeburg, has to content himself with glass which "wasn't very good, even by 17th-century standards" (1633 Chap. 6). Even so, it didn't just admit natural light, and allow him "a view of his domain", it was a status symbol (1633 Chap. 4).

The up-time contribution to the glass window technology will be in terms of techniques for making larger and clearer plates, and ultimately, for producing them relatively inexpensively.



A logical extension of the normal architectural use of the window is the greenhouse, which has glass walls and ceiling. Soon after the Ring of Fire, "medicinal and ornamental plants were [being] grown in the glass-roofed conservatory" of Grantville's hospital (Ewing, "An Invisible War", in GG#2).


Since the days of the Pharaohs, mirrors have been used for purposes of self-inspection. The first mirrors were made of metal, but there were a few glass mirrors even in Roman times. The earliest specimens were hand mirrors, but, as the art of mirror-making progressed, larger glass mirrors became feasible.

The extreme examples would be the mirrors which were large enough to be installed as building fixtures. The architectural advantages of opposed mirrors were pointed out in 1697: "by reflection, a room with two to four candles is brighter and gayer than another with twelve." (Polak, 129, Roche).

Two of Charles II’s rival mistresses, Nell Gwynne, and Louise de Keroualle (later made Duchess of Portsmouth), had mirror-walled rooms. No doubt once one had it, the other found it a necessity. The Duchess’ hall was visited by the Moorish ambassador, who commented that "he much wondered at the room of glass where he saw himself in a hundred places." (Roche, 16; Diamond, 139).***


Isaac Newton demonstrated a practical reflecting telescope in 1671; it used mirrors made of speculum metal. The first silvered glass reflecting telescope, just 4 inches in aperture (smaller than the one I owned as a high school student), was built by Steinheil in 1856. Silvering made large reflectors practical.

For military purposes, the refracting telescopes are more than adequate. Why, then, might the USE one day find a use for big reflectors? Bear in mind that it has an agenda that goes beyond winning the war. It is battling for the hearts and minds of seventeenth century Europeans. If savants from other countries were able to look at the skies through powerful telescopes, they would be able to confirm more of what we have told them about the Solar System. This, in turn, would help persuade them to accept the truth of our other teachings.


Solar Power

The use of glass lenses, or glass (or metal) mirrors, to utilize solar energy was known before the Ring of Fire, but it was a curiosity, not a feature of everyday life.

Giovanni Magini (1555-1617), an Italian astrologer, astronomer and mathematician used concave mirrors, which were probably less than two feet in diameter, to melt "lead, silver or gold in small quantities such as a coin held firmly with tongs." (Butti, 34) Villette, of Lyons, France, in the late 1600's, built a spherical mirror which was four feet in diameter and had a radius of curvature of 76 inches. A Villette mirror was said to be "able to make tin melt in three seconds, cast iron melt in 16." (Butti, 39, 257) Leonardo da Vinci asserted that "the sculptor Andrea del Verrochio employed a burning mirror to solder the sections of a copper ball lantern holder for the Santa Maria del Fiore Cathedral in Florence" (Butti, 33). This may well have been the first practical use of a burning mirror to melt a metal.

The Baron Ehrenfried Walther von Tschirnhausen (1651-1708) created the largest mirrors of his day by hammering copper into a thin sheet; his reflector was five and a half feet in diameter. In 1687, he reported that "a piece of tin or lead three inches thick, as soon as it is put into the focus, melts away in drops... A plate of iron or steel placed in the focus immediately is seen to be red hot on the back side, and soon after a hole is burnt through." (Butti, 38)

Burning mirrors could also be used to heat liquids. In 1515, Leonardo da Vinci conceived of a parabolic mirror concentrator for an industrial application (cloth dyeing). (Brief History) Had his conception been brought to fruition, the dying vats would be brought to a boil by solar energy. By 1561, alchemists knew that, to make perfume, one could place a clear vase, filled with water and flowers, at the focal point of a spherical mirror; and then heat the vase water with concentrated sunlight, causing the fragrances to be extracted from the flowers.

Augustin Mouchot (1825-1911) was a prolific inventor of solar-powered devices. In his solar cooker, food went inside a blackened copper cylinder. This was placed inside a glass cylinder; with a one inch airspace separating the two concentric cylinders. A trough-like mirror, made of silver-plated wood, reflected sunlight onto the glass cylinder. This trough, possibly parabolic in cross-section, was positioned to face the sun. (Butti, 66) Used by the French Foreign Legion, it cooked a pound of beef in 20 minutes. His solar still was of similar design. Wine was heated to a vapor in the copper vessel, and the vapor was collected in another receptacle, yielding brandy. In his solar pump, the air inside the copper cauldron expanded when heated by the concentrated sunlight, pressing down on water in a tank below. Ultimately the pressure was sufficient to cause a jet of water to shoot out of the other end of the tank. Finally, in his solar steam engine, the parabolic trough reflector concentrated sunlight onto a one inch diameter copper tube, boiling the water within. (Butti, 66-68)

Another solar energy pioneer was Abel Pifre, Mouchot’s assistant. In 1880, he used a dish-shaped solar collector to drive a steam engine, which in turn powered a printing press. (Butti, 74)

Recently, the solar furnace has drawn attention as an inexpensive replacement for wood-burning fires in equatorial countries. Physicist Steven Jones designed a solar cooker featuring a cardboard "half-funnel" covered with aluminum foil. Inside the funnel is a black pot or jar, and it is surrounded by a clear plastic bag to help hold in the heat. (The solar cooker also doubles as a cooler, when used at night.)

Some modern glass is specialty glass; laboratory glassware falls into this category.

I will discuss this utility in more detail here than I could in In Vitro Veritas. In 1632, Greg Ferrara commented, "Sulfuric acid is about as basic for modern industry as steel" (Chap. 40). That's definite, in fact, you can deduce the relative industrial strength of a country by comparing its sulfuric acid production with that of other nations.

In 1736, Joshua "Spot" Ward began making sulfuric acid (oil of vitriol) from saltpeter and sulfur. Because of the corrosive effect of the sulfuric acid on the available metals, he manufactured it in fifty gallon glass jars.

Glass vessels aren't an absolute requirement for processes which make or use sulfuric acid. "Spot" was upstaged by John Roebuck, who put sulfuric acid into lead foil-lined wooden reaction chambers. Lead resists sulfuric acid of concentrations less than 70%. If you need to work with more highly concentrated liquors, you switch to stainless steel. (Which, in turn, is corroded by the dilute formulations that lead shrugs off.)

In designing a sulfuric acid plant, chemical engineers can use steel or lead, as appropriate, in reaction chambers, ducts, storage tanks and shipping drums. However, when the same apparatus can encounter both dilute and concentrated sulfuric acid, glass is superior. Glass is resistant to corrosion by sulfuric acid at all of the concentrations and temperatures at which the latter is in the liquid state. The only metals with this wide a gamut of resistance to it are gold and platinum.

For work with nitric acid, stainless steel is the material of choice for large-scale production. But glass has excellent resistance, too.

Hydrochloric acid (HCl) is used in our time line in the manufacture of ethylene dichloride, vinyl chloride, methyl chloride, ethyl chloride, calcium chloride and chlorine. It is also used directly, to descale metals, to acidize oil wells, to recover semiprecious metal catalysts, and in food processing. Unfortunately, stainless steel is not resistant to HCl. If you want to use a metal vessel, it had better be either a molybdenum-rich alloy, or tantalum. As you can imagine, these are not going to be readily available in the 1632 universe. Glasses (of the right composition) can be used at any HCl concentration or temperature. Glass, or glass-lined steel, is commonly used in up-time plants which handle HCl.

For laboratory scale chemistry, glass is clearly superior. 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. Glass is used extensively in the bottles, graduated cylinders, beakers, flasks, pipettes, condensers, test tubes, watch glasses, burets, funnels, crucibles, and retorts of modern chemical laboratories.

Borosilicate glass, such as that sold under the trademark PYREX, is preferred, because it is especially resistant to chemicals, to high heat and also to thermal shock (the last being the result of its low coefficient of expansion).

Borosilicate glass wasn't in common use until the twentieth century; until then, chemists made do with soda lime or flint glass. Glassware made using 1632 glass formulations is fine for making many useful chemicals, notably, pure alcohol. In 1634: The Galileo Affair, Stoner explained to his comrades, "when I drew a Liebig condenser for them, there were a few guys slapping foreheads, and a couple of the glassware shops did a roaring trade in the things for a couple of weeks."

Even in the twentieth century, some laboratory apparatus is still made from soda lime glass. Since its corrosion resistance is only fair, it should be used to handle corrosive liquids only for short durations; making a pipette out of soda lime glass is fine. It also should not be subjected to high temperatures, or to rapid changes in temperature.

The bottom line is that some of our needs for laboratory glass ware can be met by down-time glassblowers using their traditional soda lime recipes, but that if we want to work with the more corrosive chemicals, or with highly exothermic (heat producing) reactions, we are going to need to move them up to borosilicate glass.

Glass Insulators and Capacitors

Some modern specialty glassware takes advantage of another of glass' properties: its low electrical conductivity. The standard USE diplomatic mission antenna includes a glass insulator. (Boatright, "Radio in the 1632 Universe", GG#1). Glass insulators are also used in telephone installations (Boatright, "So You Want to Do Telecommunications in 1633?").

Glass can also be used to construct capacitors. A capacitor is a device for storing electric charge; the charges collect on conductive metal plates which are separated by a dielectric (an insulator). The first capacitor, the "Leyden Jar", was invented independently by Ewald Georg von Kleist in 1745 and Pieter van Musschenbroek in 1746. It was a glass jar with metal coatings on the inside and the outside.


Further in the post-ROF future, fiberglass products will appear. The third largest twentieth century market for glass is in the manufacture of fiberglass. Coarse glass fibers were made and used in pre-Roman times, but merely for decoration of tableware. The down-timers don't even have glass wools or glass cloths, let alone fiber optics or fiberglass composites.

Fiberglass composites (glass fiber-reinforced plastics, "FRPs") have become a major structural material, because of their high strength-to-weight ratio. That is particularly useful in a small water craft. The hull of Eddie's warship, the speedboat Outlaw, was made of a fiberglass composite.

In his fine article on telecommunications, Rick Boatright comments, "I hope that I do not need to go into why Grantville will not be making Teflon coated wire or fiberglass for several years."

One-Way Mirrors

So far as I am aware, the "one-way" mirror principle had not been used before the Ring of Fire, unless, perhaps, it was in one of the small optical illusions practiced by della Porta.

"One-way" mirrors are typically used in malls, casinos, department stores, and police interrogation rooms (so a witness can see a suspect but not vice versa). Can, and should, the USE try providing hostile foreign embassies with offices or conference rooms featuring one-way mirrors?

A "one-way" mirror is really a "two-way" (partially transparent) mirror that is lit on only one side. Modern one-way mirrors are glass mirrors with a very thin coating of a reflective metal. Someone on the lit side would just see a reflection, as the reflected light would be much stronger than the light transmitted from the dark room. In contrast, on the dark side, the light transmitted from the lit side would be much stronger than the light reflected off the mirror surface, so the darkness-shrouded spy can perceive everything happening in the lit room.

In the early seventeenth century, the "one-way" mirror would have been a simple sheet of plate glass, just as in the Pepper's Ghost stage illusions of the nineteenth century. The Renaissance glassmakers would not have been able to improve the security of the effect by metallizing the glass, as they could not have controlled the thickness of the coating.


What Is Safety Glass and Why Can't Grantville Make It?

What is now called "safety glass" was invented in 1903 by Edouard Benedictus; he is said to have accidentally knocked down a glass flask containing cellulose nitrate and discovered that, even though the flask shattered, the pieces were held together by the polymer. Safety glass was first used commercially during World War I, in the lenses of gas masks. The equivalent modern safety glass is a laminate in which the plastic polyvinyl butyral (PVB) is sandwiched between two sheets of glass. Obviously, this form of safety glass cannot be duplicated until USE has a plastics industry.

What Is Bulletproof Glass And Why Can't Grantville Make It?

Given that we are immersed in the Thirty Years' War, it seems appropriate to comment on the nature of "bulletproof glass". It is a laminate of alternating layers of glass and polycarbonate. The glass faces the "threat" side. The glass absorbs some of the kinetic energy of the bullet by breaking, while the flexible polycarbonate depletes it some more by deforming. Sounds like a nice material to have, but until we have polycarbonate or its equivalent, we will need to manage without it.


Where Did Edith Learn About Glass?

In "The Wallenstein Gambit", we learn that Edith Wild (1949-16??) had been employed in a glass factory in Clarksburg. But the Up-timer grid says that the factory was in Fairmont. Is one of these sources in error? Or did she work at two different factories? And can we identify her employer(s)?

There are at least two prospects in Clarksburg. The mammoth Hazel-Atlas Glass Company, once the largest glass company in the United States, operated a Clarksburg plant until 1972. That year, the factory, which produced "Hazel-Ware", was purchased by Brockway Glass. The Anchor Hocking web site says that it acquired this factory on April 11, 1979 and closed it in 1987. Edith would have been 38 at that time.

Another possible Clarksburg employer for Edith Wild was Eagle Convex Glass (now called Eagle Glass Specialties). It buys glass from others, and fabricates it. Over the years, it has produced panels for cathode ray tubes, "bent glass" for the furniture industry, and decorated glass and advertising specialties for the appliance industry. Prior to the ROF, it had "water jet" glass cutting equipment, and a horizontal tempering furnace.

If Edith worked in Fairmont, there are again at least two possibilities: the plant of the former Monongah Glass Company, also owned by Hazel-Atlas, and plant #3 of Owens-Illinois Glass. The latter factory was shut down in 1981.


Glass, Generally

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"Industrial Glass", "Ravenscroft, George" in the modern Encyclopedia Britannica

"Glass", in the 1911 Encyclopedia Britannica

"Glass", in the Encyclopedia Americana

Gros-Galliner, Gabriella, Glass: A Guide for Collectors (Stein & Day: 1970)

"Basics of Design Engineering–Engineering Materials–Additional Materials–Glass"


Glasstopia, "Glass is..."
(various pages)

Glass Online, "A Brief History of Glass,"

Lambert, Joseph B.,Tracing the Past: Unraveling the Secrets of Archaeology Through Chemistry (Perseus Books: 1997).

Ellis, William S., Glass: From the First Mirror to Fiber Optics, The Story of the Substance That Changed the World (Avon Books: 1998)

Polak, Ada, Glass: Its Tradition and Its Makers (G.P. Putnam’s Sons: 1975)

Diamond, Freda, The Story of Glass 139 (Harcourt, Brace: 1953).

Polak, Bottles: Identification and Price Guide (19__)

Mehlman, Felice, Phaidon Guide to Glass (Prentice Hall: 1983)

PPG Glass, "History of Glass,"

Mirrors and Mirror-Making

Wills, English Looking Glasses

Schiffer, The Mirror Book

Melchior-Bonnet, Sabine, The Mirror: A History 18 (Routledge: 2001)

Goldberg, The Mirror and Man (U. Virginia Press: 1985).

Newman, The Mirror Book

Roche, Serge, Mirrors (London 1957)

Child, World Mirrors

Mirror Mirror, "History of Mirrors,"
Edwards, Clive, "Eighteenth Century Mirrors",

quoting Hans Seeger, Militaerische Fernglaeser und Fernrohre, 2.6, page185, Scissor Telescopes. Hans Seeger, Hamburg and Alfred Koenig, Herborn.
"Submarine Periscope Manual", Chap. 1, p. 1, reprinted at
"The Invention of the Submarine,"

"Submarine Centennial," http://www.chinfo.navy.mil/navpalib/ships/submarines/centennial/subhistory.html


"Personal View of C.F. von Hermann," in A National Weather Service Publication in Support of the Celebration of the American Weather Services...Past, Present and Future,

Ciolek, "Global Networking: A Timeline, 1000-1799"

citing Coe,

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Plum, William Rattle, The military telegraph during the Civil War in the United States with an exposition of ancient and modern means of communication, and of the federal and Confederate cipher systems; also a running account of the war between the states. (Chicago : Jansen, McClurg & Co.,1882.)

Holzmann, Gerald J., "MEMS the Word," Inc magazine (Nov. 15, 2000),

Rolak, Bruno J. "The Heliograph in the Geronimo Campaign of 1886." Military History of the Spanish-American Southwest: A Seminar. Ft Huachuca, AZ, 1976.

pp. 167-79. F786M5, available online at
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Harris, J.D., "WIRE AT WAR- Signals communication in the South African War 1899-1902", Military History Journal - Vol 11 No 1 (South African Military History Society)

Miles, Nelson A., Personal recollections and observations of General Nelson A. Miles, etc. (1992 reprint)

Wrixon, Fred B., Codes, ciphers & other cryptic & clandestine communication: making and breaking secret messages from hieroglyphs to the Internet (Black Dog & Leventhal Publishers : 1998).

Ruffell, W.L., "The Gun: sights and laying - rangefinding" (1996);
Solar Furnaces

Butti and Perlin, A Golden Thread (Van Nostrand Reinhold: 1980)

"A Brief History of Solar Energy" http://www.uccs.edu/~energy/courses/160lectures/solhist.htm

"Energy-Conserving Devices: Solar Cooker,"

Jones, Steven E., "The Solar Funnel Cooker: How to Make and Use The BYU Solar Cooker/Cooler,"

and related pages.

Jones, Stephen E., "Basic Principles of Solar Cooking, and Introducing the Foil-ware Solar Cooker," (July 25, 2001)

Medieval Glass

Hawthorne, John G. and Smith, Cyril Stanley, transl., On Divers Arts: The Treatise of Theophilus, Chapters 6 ("How to Make Sheets of Glass"), pp. 54-5, and 9 ("Spreading Out the Glass Sheets"), p. 57 (U. Chicago Press: 1963). "Theophilus" is believed to be a pseudonym for Roger of Helmarsshausen, a Benedictine monk and metalworker. Id., xvi.

Kunzig, Robert, "The Physics of Glass," DISCOVER Vol. 20 No. 10 (October 1999),
Venetian Glass

Mehlman, Felice, Phaidon Guide to Glass (Prentice Hall Inc.: 1982).

Walker, Della, "The History of Venetian Glass"

"Origins of Glass",

I. De Raedt, B. Vekemans, K. Janssens, F. Adams, "Synchrotron light through ancient glass," Europhysics News (2000) Vol. 31 No. 6 ,
Plate Glass and Float Glass

"The Inventor of Float Glass",

Pilkington company history,
BBCi, "Historic Figures: Sir Alastair Pilkington,"http://www.bbc.co.uk/history/historic_figures/pilkington_alastair.shtml

"Float Glass"

Silvering Mirrors

Kay Reat and Gerry Munley , "Justus von Liebig: An Educational Paradox," http://step.sdsc.edu/projects95/chem.in.history/essays/liebig.html
Lead Glass

Winder, "The History of Lead"

Borosilicate glass

Corning Museum of Glass, "The Unbreakable Glass of Ancient Rome"
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Smith, "Borax in Glass–Ancient or Modern?", Pioneer Magazine (January 1997), online
Hochleitner, Minerals: Identifying, Classifying, and Collecting Them
Clarksburg/Fairmont Glass Factories

Eagle Glass Specialties, Inc. website, accessed through
"Brockway Glass"
"Hazel Atlas Glass Company"
"About Anchor Hocking"
"Hazel Atlas: A Home-Grown Corporation"

Glass and Corrosion

Chap. 18, "Spot and Roebuck (Acid)", in Caveman Chemistry
Fontana, Corrosion Engineering (1986)

Barbour, Glassblowing for Laboratory Technicians (1978)

Wilson, John Harold, Nell Gwyn, Royal Mistress 159-61 (Pelligrini & Cudahy 1952).

"History of Hydroponics"
"How Does Bulletproof Glass Work",
Hecht, Jeff, City of Light: The Story of Fiber Optics

"History of Canning" and "Canning Jars" at
"History of Canning"
"Can Central History"
Hinson, "A Primer on Fruit Jars"

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