Inside a Renaissance Glasshouse

During the Renaissance, the government of Venice took extreme measures to maintain the city’s monopoly on supplying the finest luxury glass throughout Europe. In 1292, for example, the workers were moved to the island of Murano. To help prevent the spreading of coveted trade secrets, they were forbidden to travel abroad. In such an environment, it would be surprising to find any detailed written descriptions of glassworking processes. To my knowledge, there is only one such document from that period. It is a wonderfully precise narrative that explains how to create enameled decoration on glass vessels. This late 15th-century document is preserved in the library of S. Salvatore in Bologna, and it reports:

To paint glass, that is to say, cups or any other works in glass with smalti [enamels] of any colour you please.—Take the smalti you wish to use, and let them be soft and fusible, and pound them upon marble or porphyry in the same way that the goldsmiths do. Then wash the powder and apply it upon your glass as you please and let the colour dry thoroughly; then put the glass upon the rim of the chamber in which glasses are cooled, on the side from which the glasses are taken out cold, and gradually introduce it into the chamber towards the fire which comes out of the furnace, and take care you do not push too fast lest the heat should split it, and when you see that it is thoroughly heated, take it up with the “pontello” [punty, or pontil] and fix it to the “pontello” and put it in the mouth of the furnace, heating it and introducing it gradually. When you see that the smalti shine and that they have flowed well, take the glass out and put it [back] in the chamber to cool, and it is done.19

To see this process in action, view the latter half of the videos associated with the Enameled Goblet, Enameled Bowl, and Enameled Tazza.

This document is a genuine revelation in that it answers a fundamental technical question: How did the artisans fire high-temperature enamels (low-temperature enamels would not be invented for centuries) on glass vessels without having the vessels collapse under the force of gravity? In fact, the objects themselves reveal the answer: seven “symptoms” of the pontil-firing process have been identified.20 However, even when some or all of these symptoms are detected on an original object, they may not fully illuminate the process. The Bologna document is as detailed a period description of a complicated glassworking process as one could hope for. Would that there were more such examples!

Fortunately, we have a number of illustrations of 16th-century Venetian (or Venetian-style, at least) glasshouses in operation. Most of them show the architecture of the furnace and a variety of tools, without conveying any specifics about the actual glassworking practices taking place (Figs. 37–39). However, one example is so detailed that it warrants close scrutiny and analysis.

FIG. 37. Glass furnace, showing the upper annealing chamber. Vannoccio Biringuccio (Italian, 1480–1539). In De la pirotechnia, [Venice], 1540. Rakow Research Library, The Corning Museum of Glass (93699). Photo: The Corning Museum of Glass.
FIG. 37

Glass furnace, showing the upper annealing chamber. Vannoccio Biringuccio (Italian, 1480–1539). In De la pirotechnia, [Venice], 1540. Rakow Research Library, The Corning Museum of Glass (93699). Photo: The Corning Museum of Glass.

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FIG. 38. Glass furnace, with annealing furnace attached. Georg Agricola (German, 1494–1555). In De re metallica, Basel, 1556. Rakow Research Library, The Corning Museum of Glass (93693). Photo: The Corning Museum of Glass (24020).
FIG. 38

Glass furnace, with annealing furnace attached. Georg Agricola (German, 1494–1555). In De re metallica, Basel, 1556. Rakow Research Library, The Corning Museum of Glass (93693). Photo: The Corning Museum of Glass (24020).

FIG. 39. Glass furnace, with workers. Georg Agricola (German, 1494–1555). In De re metallica [Berckwerck Buch, Frankfurt-am-Main, 1580, p. cccxc]. Rakow Research Library, The Corning Museum of Glass (66820). Photo: The Corning Museum of Glass.
FIG. 39

Glass furnace, with workers. Georg Agricola (German, 1494–1555). In De re metallica [Berckwerck Buch, Frankfurt-am-Main, 1580, p. cccxc]. Rakow Research Library, The Corning Museum of Glass (66820). Photo: The Corning Museum of Glass.

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By special arrangement with the Venetian government, the grand duke of Tuscany, Francesco I de’ Medici (r. 1574–1587), was allowed to have a Muranese-style glasshouse in the gardens of the Pitti Palace in Florence. This was a rare privilege. The only other such “legal” Venetian-style glasshouse was located in Innsbruck, Austria.

Between 1570 and 1575, the artist Giovanni Maria Butteri (1540–about 1606) depicted in paintings a number of the Medicis’ workshops, including those for making objects of bronze, jewelry, and Venetian-style glass (Fig. 40). Vasari’s glassmaking painting is as dynamic as the somewhat earlier woodcuts are static. Even the affect of some of the workers is vividly and unambiguously portrayed. For example, the seated glassblower on the left is carrying out the most uninteresting part of the process: reheating. He looks relaxed and, frankly, a bit bored. A reheating of the glass might take only 30 seconds. During that time, aside from placing the work in the correct position and turning the blowpipe (or punty), all the glassblower can do is wait.

FIG. 40. The Medici Glass Workshop (detail). Giovanni Maria Butteri (Italian, 1540–about 1606), 1570. Palazzo Vecchio, Studiolo of Francesco I. Photo: Scala / Art Resource, NY.
FIG. 40

The Medici Glass Workshop (detail). Giovanni Maria Butteri (Italian, 1540–about 1606), 1570. Palazzo Vecchio, Studiolo of Francesco I. Photo: Scala / Art Resource, NY.

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By sharp contrast, the worker third from the right is probably gathering glass on the end of a blowpipe (the diameter of the tool appears to be too large for a punty). Gathering required speed and dexterity, and it is at least somewhat uncomfortable because of the unavoidable proximity of the hands to the working hole (bocca, “mouth”) of the furnace. The brightness in this area of the painting very effectively conveys how close the worker’s left arm is to the flames; the body language indicates a catlike readiness to spring away from the heat the instant that the hot task is completed.

In general, the scene immediately around the furnace is one of intense purposefulness. The gracefulness and the fluidity of movement are depicted in a late Mannerist style reminiscent (to my eyes) of the painter Tintoretto (Venetian, 1518–1594). There can be no doubt that Vasari, or whichever of his disciples carried out the design or layout of the painting, observed and carefully studied the actions of the Medici glasshouse workers in preparation for this painting.

Hundreds of drawings (both published and unpublished) made by leading Medici court artists as designs intended for the glassblowers of the Medici glasshouse have survived. In the center of the painting’s foreground, a manager appears to be speaking to a worker, or perhaps an artist is explaining a design. In any case, a point is being made somewhat forcefully: the artist has moved backward the arm of the figure at the left in order to reveal the pointed hand gesture of the other man. Curiously, the figure who is perhaps being admonished towers over the more adamant one with body language that conveys imminent aggression. The outcome of the discussion is by no means certain.

At the left of the foreground, two workers are likely engaged in the preparation of raw materials for glassmaking: crushing and sieving appear to be the actions taking place. Perhaps quartz river pebbles are being pulverized and sorted (by sieving) to attain low-iron-content silica (SiO2). Alternatively, they may be preparing the flux/stabilizer by crushing plant ash from plants that have been collected along the Sicilian and Spanish coasts.21

Moving to the right of the foreground, we see a rather burly figure—probably the furnace stoker—carrying wood to the furnace. Just out of view, beyond the stoking hole, is a downward incline into the slightly subterranean fire box (combustion chamber) at the base of the furnace.

In my experience over the last 10 or so years of working and demonstrating glassblowing at wood-fired furnaces, the importance of the stoker’s work cannot be overstated. For example, about every 10 to 15 minutes, the wood must be adjusted. Furthermore, two or three times a day, glassblowing must cease while the stoker scrapes out all of the accumulated ash in the fire box.

Both of these procedures require the good judgment that comes from long experience. If too much wood is added too quickly, workers are immediately blinded by clouds of smoke. On the other hand, if wood is added only slightly too slowly, the resulting gradual decline in temperature may cause finished work to crack in the annealing chamber, the uppermost part of the furnace. If the ash level is not carefully monitored and rises too high, the furnace can “suffocate” from a lack of space in the fire box.

In my experience over the last 10 or so years of working and demonstrating glassblowing at wood-fired furnaces, the importance of the stoker’s work cannot be overstated.

What is not shown in the painting is the glassmaking phase of the work. This was traditionally carried out overnight by a separate team of workers who charged (loaded) the ceramic crucibles in the furnace full of raw materials. Throughout this work, the stokers had to carefully manage both the temperature and the atmosphere of the furnace. If too much fuel was provided at one time, the rich, oxygen-starved gases could easily turn the glass green, brown, or gray. Perhaps needless to say, if the furnace is not sufficiently hot, the chemical reactions that create the glass will not occur. This again underscores the importance of the stoker’s work.

The scene immediately surrounding the furnace is truly a narrative. It probably shows much of the process in the making of a vessel similar to the one that is perched on a ledge halfway up the furnace, somewhat to the right of center. The object appears to be a narrow-necked vase, half full of water.

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Vasari’s choice was prophetic. Today, among glass historians and Studio Glass movement glassblowers, the form is well known, perhaps even iconic. It is widely called the “Veronese vase,” after a similar object depicted in a painting by the Venetian artist Paolo Veronese (1528–1588). The Annunciation (Fig. 41), which dates from 1578, is housed in the Accademia Museum in Venice. Many variations of this form can be seen in other paintings by Veronese and some of his approximate contemporaries working in Venice. Among them are Titian (about 1488–1576) and Tintoretto. In the 20th century, Paolo Venini, founder of what is arguably the most famous Venetian glasshouse of recent times, co-opted the design for his company. It remains in production, many decades later. For contemporary studio glass artists worldwide, the Veronese vase is a common-currency chef-d’oeuvre that is often used to demonstrate a glassblower’s skill and good taste.

FIG. 41. The Annunciation of the Virgin, oil on canvas. Paolo Veronese (Italian, 1528–1588), 1578. H. 279 cm, W. 551 cm. Gallerie dell’Accademia, Venice. Photo: Alinari / Art Resource, NY.
FIG. 41

The Annunciation of the Virgin, oil on canvas. Paolo Veronese (Italian, 1528–1588), 1578. H. 279 cm, W. 551 cm. Gallerie dell’Accademia, Venice. Photo: Alinari / Art Resource, NY.

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In the Vasari painting, the action both begins and ends on the far right side. The shadowy uppermost figure may be managing the backmost—and coolest—part of the annealing chamber in the “attic” of the furnace. His job is to slowly pull the completed work out of the heat and remove it from the oven. Perhaps he also begins the process of packing. His tasks are closely coordinated with those of the leftmost worker, who also occupies himself with matters pertaining to the annealing oven. In recent times, this worker’s title is forcellante, or capo. In the painting, he is probably shown in the act of placing a finished piece in the annealing oven, using a long, forklike tool. His crucial job is to then manage the gradual cooling of finished objects by pushing them (in small increments) from the hottest to the coolest part of the chamber—that is, near his colleague on the opposite side of the furnace.

The process of manufacturing the vase begins with the worker at the far right side gathering glass on the end of a blowpipe. As was mentioned above, this work is particularly hot and uncomfortable. In front of the next workstation, the figure who directly faces us is inflating a gather of glass. At the next station, the body of the vase has been created and the seated worker (yes, female and nude—the studiolo paintings are allegorical in nature) is delivering to herself either a bit of glass to be used to create a tiny stemlike element or perhaps a bubble of glass from which to fashion a small knop. This is particularly interesting in that, while the painting clearly shows teamwork in action, it also shows—unexpectedly—solo working at a rather difficult moment in the process.

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Next, in the shade of one of the thick fire-clay heat shields, a worker is probably preparing a blown foot for the seated worker to his right. A gather of glass has been inflated, and a narrow constriction is being formed with the two-bladed tool called the jacks.

The leftmost glassblower is undoubtedly the maestro of the shop. It is he who completes the object and oversees all of the previously described activities. When the vase is completed, he breaks it free of the punty (or pontil) and passes it on to the forcellante, who was noted above.

A few observations should be made about the overall working setup. The furnace is the three-level, beehive-shaped type that we see illustrated in the drawings by Biringuccio and Agricola (see Figures 37 and 38). In the painting, the stoking hole has probably been moved forward; in actuality, it was more likely situated at the back of the furnace, well out of the way of the glassblowers. There are four working holes and three workstations. By observing the leftmost station, we can see the hooks that were used as supports (yokes) for the blowpipes and punties during reheating. Interestingly, before about 30 years ago, this same system (with two hooks) was still employed at every glassworking furnace I encountered on Murano.

Vasari’s painting predates the nearly ubiquitous glassblower’s bench (seen in the videos) by at least 100 years. We can see that the two glassblowers on the left have, visible under their garments, a narrow board strapped to the right thigh. It was against this that they would roll the blowpipe or punty back and forth while tooling the soft glass. Perhaps the seated maestro rested the other end of his blowpipe or punty on the ledge of the furnace. His colleague who is making the blown foot appears to have a special shoe-shaped platform on which to place his right foot, thus raising the right thigh to a more convenient working level.

From my own experience in working at wood-fired furnaces, I would say that, although the ceiling/roof depicted in the painting is plausible, this would probably have been an outdoor scene with little in the way of walls. This would be consistent with our knowledge that the Medici glasshouse was located in the garden of the palace.

Not excluding the excellent drawings of glassblowing processes in Diderot’s mid-18th- century Encyclopédie (Fig. 42), Vasari’s painting may be the most instructive depiction of glasshouse activities before the advent of photography. It is certainly our richest available source of information on Venetian-style glassworking practices during the Renaissance.

FIG. 42. “Glassworking at a Wood-Fired Furnace.” Denis Diderot (French, 1713–1784). In Encyclopédie [Selections], 1751–1772. Corning, New York: Corning Glass Center, n.d. Rakow Research Library, The Corning Museum of Glass (67353). Photo: The Corning Muse
FIG. 42

“Glassworking at a Wood-Fired Furnace.” Denis Diderot (French, 1713–1784). In Encyclopédie [Selections], 1751–1772. Corning, New York: Corning Glass Center, n.d. Rakow Research Library, The Corning Museum of Glass (67353). Photo: The Corning Museum of Glass.

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