Dear Readers,

As you may have seen elsewhere, in mid February my wife and I suffered the loss of our home in a fire, in the hills of central Massachusetts. The good news is that we got out safely and had no animals in our care at the time. The fire crews were able to contain the fire from spreading, in what turned into a 3-alarm, 5-hour-long ordeal in subzero temperatures; they did amazing work, and no one was injured. The bad news is that all of my physical historical materials and research of 30 years have gone up in smoke. As a result I have decided to suspend this blog for the time being. It will remain online as a resource for those interested in the history of glass and glassmaking in the seventeenth century and beyond. I do intend to resume writing when I can, but for now my time and energy are required in getting us back on our feet.

Friends are providing temporary shelter for us nearby and our intention is to rebuild as soon as possible. To those who have reached out with a steady hand, to those who have opened their wallets, and offered advice in our time of need, we thank you from the bottom of our hearts. In what are already difficult times for all of us, you have made a huge difference in our lives.

Paul Engle
6 March, 2021

Friday, November 30, 2018

Glass or Rock

Rock crystal ewer, Egypt (1000-1050)
V&A Museum, #7904-1862
Today, a sharp distinction is made between glass and rock, but in the early seventeenth century, differences in the two materials were not so well defined. One was the product of nature and the other of art, but after that the lines began to blur. Philosophers debated weather glass should be classified as artificial stone, and why not? It is a material that was actually made from crushed up rocks (quartz and calcined limestone) with the addition of plant salts. In a very accurate sense, it is a form of artificial rock. 

In his 1612 book L'Arte Vetraria, Florentine glassmaker Antonio Neri repeatedly illustrates the thinking that glass was an artificial form of rock. The very first part of the book concerns itself with cristallo glass, which was considered to be an imitation of natural rock crystal. He devotes a whole section to the imitation of the colorful stones variously known as chalcedony, jasper and oriental agate. About these he writes: 
It is often said, and it may well seem to be true, that art cannot match nature. However, experience in many things shows, and this is particularly true of colors in glass, that art not only challenges and matches nature, but by far exceeds and surpasses it. Why, if you did not see it for yourself, you would find it hard to believe the beauty and great variety of interplay seen in these particular chalcedonies.
Another entire section of the book is devoted to artificial gemstones: rubies, sapphires, emeralds, topaz, chrysolite; he even uses crushed natural gems to color his glass. He describes a "sky blue even more beautiful, from the garnets of Bohemia." 

An earlier 1540 book that Neri read closely was by Italian metallurgist Vannoccio Biringuccio, called De La Pirotechnia. This was the first book entirely devoted to mining and metal foundry practices. Biringuccio was from Siena, but was considered something of a folk hero in Neri’s nearby Florence because he oversaw the casting of iron cannons for the city's defense in the great siege of 1529–30. His book contains a short six-page chapter devoted to glass where he writes:
[I]t [glass] is one of the effects and real fruits of the art of fire, because every product found in the interior of the earth is either stone, metal or one of the semi-minerals. Glass is seen to resemble all of them, although in all respects it depends on art.
This is an observation that Neri echoes almost verbatim in the introduction of his own book, and in this light, it is not so difficult to see what connects glassmaking to alchemy. Neri was in the business of learning nature's secrets, and then using them to create new materials that were even better than the originals. These are aims quite familiar to any modern materials engineer. Neri and his contemporaries were successful up to a point. They were able to create artificial gems and other items that were impressive in color and clarity, yet they lacked some key properties of their natural counterparts, most notably hardness. 

Based on Neri's earliest known writing Treasure of the World, started in 1598, he was already familiar with mining practices before his glassmaking activities started and over a decade before he would write the book. He devotes this early manuscript to "all of alchemy, its furnaces, instruments and the mining of metals." Around 1600 he started work at Prince Don Antonio de' Medici's Casino di San Marco laboratory. Here, he may have had the opportunity to interact with characters such as Filippo Talducci (1543- c.1615), celebrated Florentine chemist and mining engineer, several of whose relatives worked at the Casino. In his 1613 manuscript Discorso, Neri strongly hints that he has personally been to more than one mine in connection with his alchemical activities. "I would not say this, had I myself not had the good fortune of being in such a mine from which, with much artifice, was extracted a small quantity of real gold liquor, which was the true golden seed. […] To this day I have never found another mine like it, and therefore suitable for this purpose."


Today, we hardly associate glass with the raw materials from which it is composed, just as we hardly ever think of metals in their unrefined state. Antonio Neri was more closely connected to the earth, by virtue of his profession, but also because daily life in the early seventeenth century was filled with such activity; refining raw materials into useful forms had a direct and immediate impact on quality of life. Although glass is now manufactured in highly automated facilities, far away from our daily lives, it is still essentially the same product that Neri made. The next time you come in contact with a piece of glass, to pour a drink or look through a window or read the text on a screen as you are probably doing right now, stop for a minute and think of it as Neri did four centuries ago, as artificial rock.

* This post first appeared here 17 Feb 2014.

Wednesday, November 28, 2018

Neri's Glass Furnace

From "De re metallica" 
Agricola (Georg Bauer) 1556.
In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the classic elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his 1612 book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.
The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.
Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.
Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.
While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

Monday, November 26, 2018

The Duke's Oil

Trajan's Column, Rome
Giovanni Battista Piranesi (1758)
In the seventeenth century, alchemy was a dangerous business. Yes, there were risks of sanctions by the authorities, which could be very harsh, but great dangers also lurked in the chemicals themselves. Some like lead and mercury accumulated in the tissues slowly, over a period of years, others could kill a man within a few minutes. Cardinal Francesco Maria del Monte had personal knowledge of just how deadly the products of alchemy could be.

In Rome, Del Monte was the unofficial ambassador to Florence and the Medici family. He regularly greeted dignitaries from around Europe and dazzled them in his sumptuous palace. He was an avid glass collector, a patron of the arts and more quietly a dedicated student of alchemy. He was a lifelong friend to Don Antonio de' Medici and visited the prince's laboratory in Florence several times. This is where Antonio Neri was making glass early in his career. Later, Neri worked at a secondary Medici glass furnace in Pisa, where the cardinal had fancy glass table service made for the Vatican.

About a two mile walk from Saint Peter's Basilica, over the Tiber River, directly toward the Colosseum, is Trajan's Column. It commemorates Emperor Trajan's victory in the Dacian Wars at the beginning of the second century. It displays a scroll in base relief that winds all the way from the pedestal to the capital. The monument is large enough to contain an internal staircase leading to an observation platform at the top. In 1587, it was crowned by a bronze statue of Saint Peter that still stands today; the initial model was sculpted by artist Tommaso della Porta, who was under the patronage of Cardinal Del Monte. 

Giovanni Baglione picks up the story in his book Lives of Painters, Sculptors and Architects:
That man [della Porta], I think, suffered mentally and it showed at the end of his days. When he felt some kind of tingling in his abdomen, he went to the Cardinal del Monte his friend and master and asked for some of the "grand duke’s oil" that he hoped would relieve the tingling. The Cardinal indulged him; gave it to him and said that he should apply it only to the wrists and only a little, because the oil was potent and it could make him feel sick. He took it and went back to his house and after dinner he sent for the barber, to administer the medication, and while the messenger went on, Tommaso impatient and simpleminded, applied the oil himself and instead of touching the wrists, as the Cardinal had instructed, he lathered the arms, chest, body and entire abdomen, so that the powerful oil went to the heart and in fact killed him. The barber arrived to medicate him, found him dead and all attempts at revival were in vain. Tommaso della Porta, was buried at Santa Maria del Popolo.
The "grand duke's oil," was widely known, and widely cited in references throughout Europe well into the 19th century. Its other name was oil of tobacco – essentially a distillation of almost pure nicotine. In very small doses, it acts as a stimulant of the central nervous system, in slightly higher doses it is a narcotic, even greater, but still relatively small amounts act as a quick and lethal poison absorbed directly through the skin. Ingesting a single pill capsule of typical size full of pure liquid nicotine is more than enough to kill an adult in short order.

This story has one final twist. Antiques dealer Domenico Lupo was one of the men present at the reading of Della Porta's will on 7 March 1607. Twenty-five years later, an inventory of Lupo's assets listed a "small figure half old and half new that is said to be of Prior Ant. Neri," either the glassmaker or possibly his great uncle. 

* This post first appeared here 5 February 2014. 

Friday, November 23, 2018

Filigrana

Mezza Filigrana footed vase, circa 1950s,
by Dino Martens (for Aureliano Toso).
Filigrana is a classical glassmaking technique developed in the sixteenth century on the Venetian island of Murano. In the broadest sense, a piece of filigrana -- let's say a vessel -- is composed of transparent glass with very fine vertical threads of color running through it. Traditionally, these threads were opaque white lattimo (milk) glass, running through a colorless high quality product known as cristallo. Because of this, the technique was originally known as “latticino,” a term still in use, but now falling out of favor and being replaced by filigrana (filigree), a name that does not imply any particular color. 

Over the centuries, this and closely related techniques became a kind of trademark for the Murano glass industry. Parallel threads in a loose spiral winding around a vessel from top to bottom form what is perhaps the most basic application of the method. This is known as mezza filigrana (half filigree). The reason for the “half” becomes apparent when we consider its far more famous cousin reticello. With this technique, two sets of threads are used winding in opposite directions to form a fishnet pattern of  diamonds. The name recalls reticella, a traditional Venetian lace. When the work is done properly, tiny air bubbles are trapped inside the glass, one in the center of each diamond of the fishnet pattern.

Even more exotic variations have been developed, which we will discuss another time. First, let's explore how the glass artisan is able to achieve these fine threads in the glass, so perfectly spaced. I should hasten to say that I am not a glassblower and this description is not an instructional, but simply a window into some of the fabulous artistry that takes place in a glass shop. These techniques take hundreds or thousands of hours of practice to master. Even a shallow understanding of the steps that go into a piece of filigrana lead to a far richer appreciation than simply being able to identify it by name.

 “Cane” is a general term for long straight rods of glass. They have many uses in glass artistry and the method by which they are made can be surprising the first time you see it done. It is the same method as was practiced a thousand years ago. A gob of molten glass is removed from the furnace on the end of an iron rod. A second rod is attached by another artisan, with the lump of molten glass between the two rods. They start to pull in opposite directions, slowly at first. They swing and manipulate the hot glass as it cools, forming a mass of relatively uniform diameter. They continue to walk away from each other, the glass pulling thinner as they go. Practiced artisans can end up with a uniform pencil thin straight rod of glass that extends for many meters. It is laid on spaced wooden slats on the floor, allowed to cool and then snapped at regular intervals to form smaller rods.

In the case of filigrana cane, the artisan starts with a smaller gob of opaque glass; let us say lattimo (white). This gob is then dipped into clear glass, which encases it in a heavy transparent layer. When the cane is pulled, the result is a clear rod with a filament of opaque white glass running down the center. Short lengths of cane are laid side by side in a pan. The pan is heated so that adjacent rods start to fuse together into a mat. The glass artist will again take a gather of glass from the furnace around the end of an iron blowpipe and flatten it into a disk, leaving the blowhole unobstructed. The disk, known as a "collar"[2] is touched to the mat of canes at one end and rolled so that the canes wrap around and form a cylinder. The open end of the cylinder is then closed down, in effect forming a bubble on the end of the pipe. The glassblower can then treat this as if it were a bubble formed straight out of the furnace, but of course, this bubble has the threads of lattimo glass running through it. The bubble is then manipulated into a finished piece. [2]


Miniature flameworked vessels (aprox. 3cm tall)
in the style of filigrana, by Emilio Santini. 
Outside of the hot shop, there are methods that use only a torch to duplicate the appearance of filigrana and reticello on a smaller scale. This involves starting with glass tubing and "painting" the threads on using thin "stringers" of glass. It is a completely different technique which requires an entirely different set of skills. In the right hands, the results can be strikingly similar. Now that we have the basics down, we can discuss the more spectacular variations that have been developed, which we will talk about next time.


[1] "Colletto"(Italian) "Coeto" (Venetian), means narrow neck or little neck.
[2] The following Youtube video shows American glass artist William Gudenrath, assisted by Harry Siemens pulling filigrana cane and executing a reticello vase at the Corning Museum of Glass. http://www.youtube.com/watch?v=xCrdewFgObc

Wednesday, November 21, 2018

Incalmo of Venice

Incalmo vessels by Tapio Wirkkala for Venini.
In this post, we will explore one of the classical techniques of glass art. Along with filigrana, latticino, reticello and pezzato, incalmo is a classical Venetian technique well established in the art, even if poorly understood by many outsiders.

‘Incalmo’, in Venetian dialect literally means “graft” as in joining two plants. That is a pretty good description of how this effect is achieved; think of a vase whose bottom-half is one color and top is another. The glass artist blows two separate bubbles of glass, opens them and joins them together to form a single bubble. It is a difficult operation because the two open lips must be exactly the same size to join properly. The process can be continued to add more colors; virtuoso pieces may include several sections, each a different color. In addition, the position of the iron rod that the artist uses to hold the bubble can be changed while the piece is under construction, leading to asymmetrical effects.
16th century incalmo plate,
unknown artist.
The above description is the classical way of achieving incalmo, but modern materials and equipment allow artists to achieve a similar effect with considerably less skill. For instance, precise diameter glass tubing is now available in a wide variety of colors. This can be cut into rings with a saw, then stacked in a kiln and fused together. From there, this “prefabricated incalmo tube can be worked by traditional methods. Whether or not this meets the definition of true incalmo depends entirely on whether one focuses on the method or on the end result.


9-10th century incalmo vase,
Syria or Iraq.
The name ‘incalmo’, was applied to glass in the first half of the twentieth century by the Venini factory on Murano, in Venice. [1] However, both the word and the method are much older. The Venini artisans revived the technique to great acclaim, but Venetian examples date from the sixteenth century and Islamic examples from ninth century Syria have also survived. It is not hard to imagine that this joining technique was experimented with shortly after glassblowing became common around the first century BCE. However, what is truly amazing is that any of these early examples survived to be sold to customers without breaking in the cooling process. The reason for this is a technical issue that we have not discussed yet.

All glass expands a little when it is heated and shrinks when it cools. Different formulations of glass generally expand by differing amounts. When a single piece incorporates more than one type of glass, and the thermal expansions differ significantly, the result is disaster. After the piece is finished it is placed in a kiln where it slowly cools back to room temperature. Because of the mismatch, one area wants to shrink more than the adjacent area and the glass cracks along the join. The expansion and contraction is microscopic, but it is enough to ruin hours and hours of work, leading to much gnashing of teeth the morning after, when the finished work is inspected. 

The Venini glass masters had the benefit of this knowledge, but for earlier artisans, trial and error must have played a big role in determining which formulas were compatible. Different colors mean different metallic additives and to match expansion other ingredients would need to be adjusted. Today, manufacturers produce glass in a series based on expansion; artists can be relatively sure that two different colors from the same series can be “grafted” and not self-destruct when cooled.

[1] I have not absolutely confirmed this, but authoritative secondary references credit Venini, and I can find no mention to "incalmo" as a glass technique prior to the twentieth century.

Monday, November 19, 2018

Botanical Gardens

Rudolf  II as "Vertumnus"(c. 1590)
Giuseppe Arcimboldo.
In 1543-44 new botanical gardens were founded in Pisa; L’Orto Botanico was its Italian name. It was the very first garden devoted to the research of plants. Literally within a year, similar gardens sprung up in Padua and Florence, and many other cities followed shortly thereafter. Exotic foreign species as well as important local plants were grown, studied, harvested distilled, and imbibed. These horticultural stations became centerpieces of medical programs throughout Italy, and then greater Europe. The concept of herbal (“simples”) gardens was centuries old. Almost every monastery, convent and hospital maintained a space to grow the plants they needed to transform into medicines for care of the infirmed. The grafting of fruit trees was actively practiced since before Roman times, but these new gardens were specifically planted as research spaces and run by universities. 

When Neri Neri, the father of glassmaker Antonio Neri, studied medicine at the Studio Fiorentino  in the mid 1550s, there can be no doubt he spent time at the gardens in Florence, and quite possibly at the ones in Pisa. (The Pisa gardens were moved twice before arriving at their current location in 1591). This was a period of vigorous expansion in the field of herbal medicine. Competition was fierce to obtain and study medicinal plants from around the globe. Cosimo I de’ Medici poured money into the medical school in Pisa, attracting students and faculty from around Europe. In 1554 famed botanist and physician Andrea Cesalpino took over the Pisa gardens  from his teacher, Luca Ghini, who first built them. 

In 1602, Neri was to be found working alongside Niccolò Sisti at the grand duke’s secondary glass furnace along the Arno River in Pisa. According to Neri’s own account, Pisa is where he worked on ferns as an alternative plant salt for glass and mentions many other plants with which he experimented: 
Set about making ash in the way previously described, however use the husks and stalks of broad beans after the farmhands have thrashed and shelled them. The same may be made from the ashes of cabbages, or a thorn bush that bears small fruit, called the blackberry, even from millet, rush, marsh reeds, and from many other plants that will relinquish their salt.
In a letter to Neri from his friend Emanuel Ximenes, the Antwerp based Portuguese banker expressed surprise that Neri was able to devise a fern based glass salt recipe so quickly. In all likelihood, Neri would have had access to the botanical gardens and the small adjacent laboratory located just a few blocks from the glass furnace. In the period of time the glassmaker spent there, the directorate of the gardens changed hands from Francesco Malocchi to Marco Cornacchini. Both of these men avidly pursued new botanical based cures, and corresponded internationally. 

In his Glassmaking book, L’Arte Vetraria, Neri devotes a number of recipes to making paint pigments from flower blossoms. While he could have easily obtained his stock material from any number of sources, the botanical gardens would have certainly provided a convenient cache of many different varieties.

In the winter of 1603-4 Neri traveled From Pisa to visit his friend in Antwerp. If he followed Ximenes suggested route, he would have passed back through his native Florence, then on to Venice where he would meet up with a caravan of merchants on their way to the Frankfurt spring fair, and then on to Antwerp by river. Upon his return to Italy, seven years later, he wrote his glassmaking book, but then devoted himself fully to alchemy and medicine. In January of 1614, in what might be the very last manuscript he worked on before his death, he wrote about some recipes “copied from an old book here in Pisa.” At that time, the director of the botanical gardens was Domenico Vigna, who continued to direct the gardens on and off until 1634.

It would be interesting to know how Neri the alchemist thought about his raw materials. Did he see all the possibilities of what could be made with them? For instance, how did he approach a towering pile of May ferns, large enough to produce a hundred pounds of ash, or a giant sack of rose petals? Did he ever lean forward and breathe in the delicious musty aroma? Did he ever dig in with his hands and bury his face in an arm-load of soft, pure color? How could he not?

*This post first appeared here 22 Jan 2014.

Friday, November 16, 2018

A Recipe for Transmutation

The recovery of copper from vitriolated waters,
from De Re Metallica, 1556, by Agricola (Georg Bauer).
In Discorso, one of the last manuscripts written by Antonio Neri just before his death, he reveals several transmutation recipes. One describes turning iron into copper; it is instructive because it uses common materials that we can identify and because the chemistry is now well understood.

"Take some iron sheets and lay them in vitriol water, being immersed in that, they will rust. Scrape off this rust, which will be a red powder, melt it in a crucible, and you will have perfect copper. The same effect can be had from various waters that are naturally vitriolated, because they flow through mines of vitriol, such as those of a source some distance from Leiden, and another below the fortress of Smolnik, [now in Slovakia].

Vitriol is an acidic sulfate dissolved in water, it could be made in the laboratory, but it also occurred naturally around mining operations where sulfurous minerals were present. Alchemists knew this solution as "oil of vitriol" and "spirit of vitriol." The mine that Neri references in Smolnik became famous for transmutation. As late as the eighteenth century, scientists and experimenters from around Europe made the pilgrimage to see the effect for themselves and tried to figure out what was happening. It may be a surprise to some readers, but following the above instructions will, in fact, produce copper just as Neri claimed. There is no deception or sleight of hand involved; the explanation is straightforward, but first, Neri treats us to a rare glimpse of his own reasoning on the subject:

"Some estimate and not without reason, that this experiment, being used to prove the transmutation of metals, is not suitable for this purpose. They say that the vitriolated waters become such because they are already heavy with the corrosive spirits of sulfur, having passed through the copper or iron mine, these waters corrode copper in the same way aqua fortis corrodes silver. So that really the substance of the copper remains in the water, which attacks the surface of the iron, which always remains iron. However, if that were true then the iron would not get consumed, or if it were consumed it would mix with the substance of the corroded copper in the water, and if it were fused, it would remain a mixture of iron and copper. And yet in this experiment, all the iron is consumed; it is reduced by the vitriolated water into powder, […] which in the fusion is still pure copper, so there should remain no doubt that this is a true transmutation.[1]

Given the state of chemistry at the time, Neri's reasoning is clear and rational. The iron disappears and a copper coating materializes in its place. What better evidence of transmutation could one ask for?

The key to what was actually happening is in the criticism leveled by skeptics. It turns out that they were on the right track, but neither they nor Neri had the full picture. Today, we understand it as a simple ion exchange reaction; blue vitriol water is a transparent saturated solution of copper sulfate (CuSO4), in the presence of solid iron, the liquid dissolves the iron; copper from the vitriol is deposited in its place. The two metals, copper and iron, change places: the iron dissolves, forming green vitriol (FeSO4) and copper is expelled from the solution. The result is a reduction in the amount of the iron, which is replaced by a proportional deposit of pure copper.

On a physical level, this chemical reaction is no different today than it was in the seventeenth century. What has changed is our interpretation of the experiment. What Neri viewed as a transformation of iron into copper, we now see as an exchange. There is, however, a deeper lesson in all this. As an alchemist, Antonio Neri was not being delusional or dishonest; he was careful, observant and applied his knowledge as best he could. This is no different from the way science works today. Both then and now, to be successful in unraveling nature’s secrets, one must become accustomed to a very uncomfortable situation: In the past, careful reasoning by brilliant thinkers has led to utterly wrong conclusions. The fact that much of our world is a mystery is unsettling; that the very process we use to understand it can be so flawed is harder to accept. Even more difficult is that the faculty we all rely on for survival—our own wits—can lead us so far astray.[2, 3]

[1] For more, see Discorso sopra la Chimica: The Paracelsian Philosophy of Antonio Neri”, M.G. Grazzini / Nuncius 27 (2012).
[2] This post first appeared here on 31 January 2014.
[3] For further reading,  see Pavol Rybár, Mário Molokáč, Ladislav Hvizdák, Jana Hvizdáková "Utilization of simple presentation methods for comparative studies in the historical mining" Geotourism 1–2 (40–41) 2015: 49–54.  http://journals.bg.agh.edu.pl/GEOTURYSTYKA/2015.40-41/geotour.2015.40-41.49.pdf

Wednesday, November 14, 2018

Fabergé and Purpurine

Fabergé c.1900. Purpurine cherries,
nephrite leaves, gold stalk, rock crystal pot.
Peter Carl Fabergé is known the world over for producing elaborate jeweled fantasy eggs for the Russian royal family in the late nineteenth and early twentieth centuries. [1] The artisans of his firm made use of a wide variety of exotic and precious materials in the execution of their commissions and later in items made available to the general public. Among the most exotic and sought after were objects made with an opaque bright red stone-like material known as ‘purpurine’. This was a glass based concoction whose composition was kept a tightly guarded secret. In fact, it was so tightly guarded that the formula was subsequently lost. Purpurine was typically cast into blocks which were then sawed and carved using traditional lapidary and gem carving techniques. The final appearance was of an unknown exotic mineral.


Red Glass Beads, 1st cent. BCE, Tissamaharama, Sri Lanka
The origin story of purpurine begins much earlier than Fabregé, in fact, not hundreds but thousands of years earlier. “The art of making this type of glass seems to have originated in India; glass beads of a similar material have been found in the Indus Valley and were dated to the late 2nd millennium BCE.” [2] In southern Sri Lanka deep red opaque glass beads have been found dating to the first millennium BCE; these turn out to be closely related to purpurine, through a long glassmaking tradition. [3] A version of the bright red glass was made in the Egyptian- Roman era. The first century CE historian Pliny the Elder noted that in Greek it was called ‘haematinon’ or blood-red ware. [4] He implied that this specialty glass, was routinely produced in Rome and indeed archaeologists have recovered numerous examples. The glass was used in a wide range of applications from dinner plates to pieces of elaborate mosaics. Eventually, though, the method of making haematinon was forgotten and remained so for several hundred years.


A small (1cm) Medusa's head in
opaque red glass c.1st. cent. CE.

The Renaissance era had been marked by a strong motivation to recover lost knowledge of the ancient world, but many challenges were beyond the technology of the time. However, attempts were made that eventually led to success.  In the second half of the 16th century, Pope Gregory XIII instituted the Vatican mosaic studio to decorate the new Saint Peter’s Basilica, begun by Pope Julius II in 1506. As an aside, this workshop continues today, repairing and conserving the ceilings of St. Peter’s. [5] Having quickly exhausted local talent in Rome, Gregory brought in Venetian masters to teach the art. With the mandate to make the mosaics appear as if painted, the studio developed many new formulations for the glass tesserae – the individual tiles used to form mosaics. It was in this environment that the deep red purpurine [Ital: porporino] was eventually rediscovered.


C. 1st cent. BCE/CE Roman bowl (patella cup) in
red opaque glass (haematinon).

It is still an open question whether the secret was discovered in Rome or brought there. There is evidence that the fabled red glass was being produced in Venice in the eighteenth century and possibly earlier.[6] One (Swedish) source credits Vatican studio employee Alessio Matteoli, in the 1700s, when he oversaw the development of many new colors. In the early 1800s, interest rekindled in the ancient material and by then, analytical methods were up to the task of finding their composition. German chemist, Martin Heinrich Klaproth, analyzed haematinon from the Villa Jovis, a first century palace built by Roman emperor Tiberius on the island of Capri in southern Italy. [7] He correctly found copper, but incorrectly assumed the material was glassy slag, a byproduct of the smelting process. Later, in 1844, Schubarth did further work indicating haematinon was, in fact, a true glass. [8] King Ludwig I of Bavaria intended to build a reconstruction of a Pompeian villa for educational purposes. He assigned Max Joseph von Pettenkofer to the task of rediscovering the method of manufacturing the antique “blood glass,” and the young chemist reported success in 1853. [9] His process fused standard alkali-lead glass with copper oxide and magnetite in the presence of small amounts of magnesium oxide and carbon, followed by very slow cooling of the resultant mass, which would then take on a deep red color.


Roman Mosaicist
Michelangelo Barberi, 1809.
Other sources name one of two students of the famous Roman mosaicist Michelangelo Barberi (1787–1867). Barberi had a long standing relationship with the Russian royal family and accepted Russian pupils at his studio in Rome, he even set up a mosaic shop in St. Petersburg at the request of Tsar Nicholas I. [10] In 1846, these two pupils of Barberi, brothers Giustiniano and Leopold Bonafede were called to St. Petersburg by the tsar to work for the royal court. Giustiniano (1825-66) had served as head chemist at the Vatican studio and both would attain that post for the tsar at the Russian Imperial Glassworks. It is Leopold (1833-78) who is now most often credited with the invention of purpurine as a recreation of the fabled haematinon. His formulation was based on a standard potash lead crystal.


Purpurine taza made at the Russian
Imperial Glassworks, c.1867.
(Shown at Paris Exposition)
The first documented uses in objects of art were five entries of the Imperial Glassworks at the Paris Exposition Universelle in 1867, for which the glassworks was awarded gold medal status. [11] “After Bonafede's death in 1878, purpurine continued to be made at the factory under the direction of the chief chemist, S. P. Petuchov.” [12]

In 1882, after considerable training and apprentice work, which began when he was a teenager, a 46-year-old Peter Carl Fabergé fully assumed control of his father’s small jewelry shop in St. Petersburg. Within a short time, he was supplying the royal family with his exquisite eggs and many other items made by a growing assemblage of master craftsmen. The first use of purpurine by the Fabergé shop occurs early in Carl’s tenure, perhaps as early as 1880. Initially, they use material supplied by Petuchov at the Imperial Glassworks. Over a period of years, though, the Fabergé shop developed its own recipe based on soda lead glass, more similar in composition to the ancient samples of haematinon.[13]  Other isolated examples of purpurine are known to exist made by competitive jewelers of the time, but no documented recipe has been found. [14] Apparently, Petuchov took the Imperial Glassworks formula for purpurine to his grave. As fame grew for Fabergé, their version is the one that became familiar to a growing clientele in Great Britain and in the United States. When the February Revolution of 1917 brought an end to the Romanov dynasty in Russia, Carl Fabergé fled the country, his company disbanded. In the west, the Fabergé name only multiplied in prestige among the elite and wealthy and items made with purpurine continue to command stratospheric prices.

Significant analytical work has been done on the ancient haematinon as well as purpurines of the Imperial Glassworks and of Fabergé. [3,5] The technical differences could easily be the subject of a separate treatment; suffice it to say that knowing the composition of a glass is not the same as knowing the recipe. (Just as knowing the composition of a cake does not mean that one can bake it.) The exact method for making the glass must have involved a long period in which snowflake-like crystals of cuprous oxide (Cu2O) would be encouraged to form, grow and spread throughout the glass forming a tightly interlocking network in the glass. One interesting point is that unlike many other opaque glasses, the ingredients of purpurine do not include a discrete opacifier; it is a clear glass base, which is so loaded with deep red cuprous oxide crystals that light does not pass through even small or thin pieces of the material. Another point is that this glass was not suitable for blowing on a blowpipe and therefore did not take forms typically expected for glass. Perhaps because of this, it has been largely overlooked.

The history of purpurine is a reminder of the fragility of human knowledge; it was discovered in ancient times, lost, rediscovered and lost again in modern times.


[1] Peter Carl Fabergé =Карл Густавович Фаберже. For more, see Abraham Kenneth Snowman, The Art of Carl Fabergé, Faber & Faber, 1974.(original ed 1953). Also see
 https://en.wikipedia.org/wiki/Peter_Carl_Faberg%C3%A9
[2] Gowlett, J.A.J.: High Definition Archaeology: Threads Through the Past, Routledge, 1997, pp. 276–277. Quoted in https://en.wikipedia.org/wiki/Purpurin_(glass)
[3] Rösch, Cordelia; Hock, Rainer; Schüssler, Ulrich; Yule, Paul; Hannibal, Anne. “Electron Microprobe Analysis and X-ray Diffraction Methods in Archaeometry: Investigations on Pre-Islamic Beads from the Sultanate of Oman” in: European Journal of Mineralogy, 9 (1997), 763–783. (Specifically, beads found at Tissamaharama, pp. 771,772). http://archiv.ub.uni-heidelberg.de/propylaeumdok/volltexte/2009/305
[4] Natural History, xxxvi, LXVII, 198.
[5] For more, see http://www.30giorni.it/articoli_id_10283_l3.htm
[6] RR Harding, S Hornytzkyj, A. R. Date. “The composition of an opaque red glass used by Fabergé”in the Journal of Gemmology, 1989. No.5, pp. 275-287.
[7] Klaproth M.H., Beiträge zur chemischen Kenntnis der Mineralkörper Vol. VI (1815), p. 136
[8] Schubarth. "Einige Notizen über rothes und blaues Glas." Journal für Praktische Chemie Vol. 3 (1844), pp. 300-316
[9] Pettenkofer, M. "Ueber einen antiken rothen Glasfluss (Haematinon) und über Aventurin-Glas." Abhandlungen der naturw.-techn. Commission der k. b. Akad. der Wissensch. I. Bd. München, literar.-artist. Anstalt, 1856. Also see https://en.wikipedia.org/wiki/Purpurin_(glass)
[10] Alessio Matteoli https://nononsensejewellery.wordpress.com/tag/purpurin-faberge/ , for more on Matteoli see http://www.aiellomosaics.com/about-mosaics/techniques-and-materials/roman-or-byzantineglass-or-marble-tilesmicromosaic-or-glass-enamels/ .  On Michelangelo Barberi, see Renata Battaglini Di Stasio, “Michelangelo Barberi” in Dizionario Biografico degli Italiani – v. 6 (1964)  http://www.treccani.it/enciclopedia/michelangelo-barberi_(Dizionario_Biografico)/
[11] Catalogue Special de la Section Russe a l'Exposition Universelle de Paris en 1867, p. 44, Classe 16, no.111.
[12] See http://art.thewalters.org/detail/77444/pair-of-tazzas/
[13] Op cit. RR Harding, S Hornytzkyj, A. R. Date, 1989.
[14] For more on competitive jeweler’s purpurine, see: Géza von Hapsburg: “Some of Fabergé’s Other Russian Competitors” in Fabergé, Imperial Craftsman and His World, London: Booth-Clibborn, 2000, pp. 323-325.

Monday, November 12, 2018

What Goes Around Comes Around

The German city of Ulm in the 16th century
Georg Braun, Franz Hogenberg 1570-78
(Click image to enlarge.)
In the spring and summer of 1525, peasants and farmers throughout German speaking Europe staged a popular revolt now called the Deutscher Bauernkrieg. [1] At the heart of the matter was an oppressive system of taxation run by the Roman Catholic Church, in which little or none of the revenue was used to improve life locally. Often, action was lead by Protestant clergy, but to little effect against the mercenary armies hired by the aristocracy. In the end, up to 100,000 of the poorly armed and organized peasants were slaughtered.

Along the banks of the Danube River in southern Germany lies the ancient city of Ulm. Besides being the birthplace of Albert Einstein, Ulm was, in the sixteenth century, near the center of the Peasant’s Revolt, which brings us to a curious story which traces the migration of a technical recipe from Ulm over the alps to Venice, then to Sienna and finally Florence. The recipe is for the metal alloy to make mirrors, and it is told from one friend to another while chatting amiably in Venice.
Among other things, he said that he had made one [concave mirror] almost half a braccio across [about 13 inches], which extended the clear rays of its brightness more than a quarter of a German league when he caught the sun with it. One day, when for amusement he was standing in a window to watch a review of armed men in the city of Ulm, he bore with the sphere of his mirror for a quarter of an hour on the back of the shoulder armor of one of those soldiers. This not only caused so much heat that it became almost unbearable to the soldier, but it inflamed so that it kindled his jacket  underneath and burned it for him, cooking his flesh to his very great torment. Since he did not understand who caused this, he said that God had miraculously sent that fire on him for his great sins. [2]
The story was told to Vannoccio Biringuccio, who recalls it in 1540 in his Pirotechnia, the first printed book devoted to metallurgy. Specifically, the recipe is a variant of what today we would call white bronze, which as Biringuccio states is similar to the metal used to cast bells. He recites ancient formulations that used three parts copper and one part tin. To this was added 1/18th part of antimony and optionally 1/24th part of fine silver to give it a neutral color. Indeed, other ancient formulations for what was known as speculum metal specify a 3:1 ratio of copper to tin. He continues,
But nowadays most of the masters who make them take three parts of tin and one of copper, and melt these together. When they are melted, for every pound of this material, they throw in one ounce of tartar and half an ounce of powdered arsenic, and let them fume and melt and incorporate well. 
Biringuccio’s version reverses the copper and tin ratio from the classical composition. Compare it with Antonio Neri’s prescription which appears half a century later, it is almost identical:
Have 3 lbs of well-purified tin, and 1 lb of copper also purified. Melt these two metals, first the copper, then the tin. When they fuse thoroughly, throw onto them 6 oz of just singed red wine tartar, and 1½ oz of saltpeter, then ¼ oz of alum, and 2 oz of arsenic. Leave these all to vaporize, and then cast [the metal] into the form of a sphere. You will have good material, which when you burnish and polish, will look most fine. This mixture is called acciaio and is used to make spherical mirrors.
To be clear, the tartar, saltpeter and alum act as a surface flux - they form a layer that floats on the liquid metal, preventing oxides from forming, which can foul the melt.  their addition does not change the base alloy composition.

The similarity of the two recipes alone is not enough to draw any conclusions. Biringuccio himself reports that the contemporary artisans favored the tin rich formulation. However, there are other details to consider. The Sienese born Biringuccio was something of a hero in Florence where Antonio Neri was raised. The famous metallurgist helped cast cannons, mortars and guns for the Florentines to defend themselves in the late 1520s, when the city was under siege, just a few years after Biringuccio’s conversation in Venice with his German Friend from Ulm.

Neri was definitely familiar with Biringuccio’s book Pirotechnia. In fact, the introduction to Neri’s own book L’Arte Vetraria is patterned after the metallurgist’s survey of glassmaking.  In his chapter 14, book 2 Biringuccio wrote:
… it [glass] is one of the effects and real fruits of the art of fire, because every product found in the interior of the earth is either stone, metal, or one of the semi-minerals.  Glass is seen to resemble all of them, although in all respects it depends on art. [3]
And here is the opening to Neri’s introduction a half century later in 1612,
Without a doubt, glass is a true fruit of the art of fire, as it can so closely resemble all kinds of rocks and minerals, yet it is a compound, and made by art. [4]
Both passages go on to cover much of the same ground, albeit with a change in focus reflective of new thinking about chemistry and nature. In one sense, Neri is paying homage to his distinguished predecessor, and there can be little doubt that he read Biringuccio’s book and its technical recipes closely. 

Lastly, the story of the burning mirror itself mimics a widely known story about the Greek polymath Archimedes. About 200 BCE during the siege of Syracuse, he is said to have set invading Roman ships on fire with a concave mirror, which focused the radiation of the sun.

In fact a depiction of this scene was painted in Florence on the walls of the Uffizi Palace in 1600, when Neri was at the height of his employment for the ruling Medici family. This particular rendering would have been all but impossible for him to miss. 


Uffizi Gallery, Florence, Italy, Wall painting
showing the Greek mathematician Archimedes' mirror 
being used to burn Roman military ships. 
Painted in 1600 by Giulio Parigi.
In the years leading to the publication of Neri’s book, he left his home in Florence and traveled to visit a friend in Antwerp. If he had read the book on metallurgy early, perhaps as part of his education, then he was already familiar with the mirror alloy recipe. If he followed the route suggested by his friend, he would have taken the recipe back to Venice, and then over the Alps and likely through Ulm on his way north to the Low Countries, where he would spend the next seven years before returning to Italy. [5]

[1] For more on the German peasant wars of 1524-25 see https://en.wikipedia.org/wiki/German_Peasants%27_War
[2] Vannoccio Biringuccio, Pirotechnia. Ed., Tr. Cyril Stanley Smith, Martha Teach Gnudi (New York: Basic Books, 1959), pp. 385-390. (Original Italian published in 1540.)
[3] Antonio Neri, L’Arte Vetraria (Florence: Giunti, 1612). p. iv.
[4] Ibid,  p.126 (in original, ff.41r-44v).
[5] Special thanks to Jamie Hall (@PrimitiveMethod) for inspiring this post.

Friday, November 9, 2018

A Reluctant Glassmaker

The Sun, Robert Fludd
from Utriusque Cosmi (1617),v. 2, p. 19.
(alchemical symbol for gold)
Today, Antonio Neri is best known for his 1612 book, L'Arte Vetraria, in which he exposes the secrets of the art of making glass. In publishing his volume, he helped to fuel new discoveries in chemistry and medicine simply by making glass apparatus more available to experimenters. A 1662 translation of his book into English was one of the first acts undertaken by the newly formed Royal Society in London, at the behest of Robert Boyle. Neri himself lived for only two years after his book went to press, in his native Florence, and never saw the seeds of his labor come to fruition. If he had lived, he might well be surprised that his legacy is in glassmaking, and not in the subjects that he himself held most dear.

In his death at the age of thirty-eight, Neri missed a rapid advancement in our basic understanding of nature. In the space of only a few decades the face of science and medicine would start to change irrevocably. Soon, experimenters were finding new chemical elements and began to map out the periodic table, often with apparatus made of glass. For centuries, the ancient Aristotelian concept of air, earth, water and fire as the basic building blocks of the universe had endured. By the end of the seventeenth century, the inadequacies of the old model were becoming clear. 

But Neri was not privy to any of this. In his time, any cracks in the Aristotelian model were minor. Like his sponsor, Medici prince Don Antonio, Neri was an adherent of new doctrines of the physician Paracelsus, who rebelled against the old system, but was still very much a product of it. First and foremost, Neri thought of himself as an alchemist. While history has not generally been kind to his ilk, a true understanding of early modern science rests on the methods and reasoning developed by alchemists like Neri. 

Although alchemy covered a wide range of activities, it will forever be associated most closely with the mistaken notion that base metals such as lead or iron could be transmuted into gold. Once science had established this idea as specious, the race was on to separate "new science" from the old. It became fashionable to cast alchemists into the mold of charlatans, tricksters and self-deceived fools. While many such characters did exist, Neri was not one of them; his work was based on careful reasoning and experimentation. The final irony is that through the kind of advancements that he himself helped to pioneer, the majority of his life's work has fallen by the wayside. What has endured the test of modern science is his treatise on glassmaking.

As early as the age of twenty, Neri was demonstrating transmutation to expert gold refiners. As late as the year before his death he was writing authoritatively and coherently on the subject. To understand how this is possible – to be rational and methodical, and at the same time completely wrong – is to get a sense of the true difficulties involved in science. Based on what he was taught, what he read, and his own experimentation, Neri thought metals and other materials "matured" over time. He thought that more "imperfect" metals like lead and iron were part of a continuum that ended with the "perfect" metal gold. Furthermore, he thought that primordial "seeds of gold" left over from the creation of the earth could be mined and isolated. Like wheat and other plants, given the correct nurturing, and conditions, this seed material could be encouraged to mature into vast quantities of gold. Writing in his 1613 manuscript Discorso, he says,
The response is that the chemical art lets the gold proceed from that present and immediate cause, because this is the seed of gold, which acts naturally when art cooperates. The chemist does nothing but extract the seed from gold and apply it to suitable bodies, with which it is united to render the fruit multiplied in the same way that the farmer does. He does not produce the fruit, but provides and prepares the earth and the seed, uniting them in such a way so that they bear fruit.*
Neri thought that ultimately, for gold transmutation to be successful one needed the blessing of the Creator. He documented his process in a heavily coded (and incomprehensible) recipe he called "Donum Dei" (the most precious gift of God). This name traces to alchemical writings from as early as the fifteenth century. He maintained that those who might harm society with this knowledge or wished to profit personally or swindle others would be denied the blessing and therefore be unsuccessful. 

The remarkable thing here is that Neri's understanding of chemistry was supported at every turn by experimentation. His recorded methods, for transforming lesser metals into one-another, were repeatable and stood the test of scrutiny by contemporary experts. In the light of modern chemistry, these transformations depended on subtle physical processes and chemical reactions that would not be understood for another century or more. By performing these experiments under controlled conditions, he was taking the first steps in what would become modern science. Eventually, it would be understood that while chemical compounds can be created and destroyed by various manipulations, individual elements cannot. Today we know that iron, lead and gold were formed in the cores of ancient stars, not too different from our sun. Lighter elements are successively transformed into heavier ones under a star's "nurturing" conditions. While he lived in a period in which he had no chance of getting the particular details correct, in a poetic sense Neri was not far from the truth.

* See Maria Grazia Grazzini, “Discorso sopra la Chimica: The Paracelsian Philosophy of Antonio Neri” Nuncius 27 (2012) 311–367.

Wednesday, November 7, 2018

Reflections on the Mirror

Jan van Eyck
The Arnolfini Portrait (1434)
L’Arte Vetraria, Antonio Neri's 1612 book, would eventually become the glassmakers' bible throughout Europe. By 1900 it had been translated into five different languages besides the original Italian; English, Latin, German, French, and Spanish (and in this century Japanese). Because of its seminal importance in the spread of glass technology, often overlooked are a few recipes at the back of the book, which have only a tenuous connection to the main subject.

Among these is a metallurgical formula for making convex mirrors. Neri gives instructions for producing what we would now call a "white bronze" that may be cast into a rounded form and polished to take on a highly reflective surface finish. This "spherical" form of mirror was popular throughout the Renaissance. It reflected a wide-angle view of the space in which it was hung, but at the cost of distorting the image. Nevertheless, upon looking into such a mirror, objects are still quite recognizable. 

Here is Neri's prescription:
A Mixture to Make [Mirror] Spheres:
Have 3 lbs of well-purified tin, and 1 lb of copper also purified. Melt these two metals, first the copper, then the tin. When they fuse thoroughly, throw onto them 6 oz of just singed red wine tartar, and 1½ oz of saltpeter, then ¼ oz of alum, and 2 oz of arsenic. Leave these all to vaporize, and then cast [the metal] into the form of a sphere. You will have good material, which when you burnish and polish, will look most fine. This mixture is called acciaio and is used to make spherical mirrors.
Of note is the fact that the word Neri uses for this alloy, acciaio, translates to "steel." Over the intervening four centuries, the meaning of this term has been refined so that today it denotes not simply a hard white metal, but a specific range of alloys containing iron and carbon. 

This recipe and a few others in the book show the breadth of Neri's experience in arts other than glassmaking. It is a conclusion greatly amplified by a perusal of his other manuscripts on alchemy and medicine. There is good evidence that our priest was a voracious reader, however he was also quite cautious about repeating techniques only after he had verified them personally. Besides, artisans never wrote down much of this knowledge – only passed in confidence between trusted parties – since, in a very concrete way, superior knowledge represented a competitive advantage over ones rivals. Even if Neri was in the business of divulging secrets, it is safe to assume that many of the artisans and craftsmen he interacted with were decidedly not. Apparently, Neri was not familiar with the process of mirroring glass directly with mercury/tin amalgam; a process for which Venetian glassmakers had already become famous for perfecting. It is an interesting omission from his book, since he almost certainly would have seen examples in Florence and in Antwerp.

Two centuries before Neri, the beginning of the fifteenth century saw the invention of moveable type printing in Germany, but also the mastery of perspective illustration in Italy. The contribution of printing to early modern science is well documented, less obvious is the role playerd by artists and perspective illustration. Moveable type made possible the mass production of books; what did get committed to paper now stood a much better chance of survival and transmission. Perspective illustration played a more nuanced role, one that ultimately brings the convex mirror back into the discussion.

In Venice and especially in Florence (Neri's hometown), perspective drawing became the rage among artists, largely due to the Italian translation of a book entitled Deli Aspecti, or "Alhazen's Book of Optics." Suddenly, paintings were made to look three-dimensional, with a realistic sense of depth to them. The new techniques were largely kept in Italy, but interest spread across Europe. Patrons placed great value on work depicting scenes in correct perspective, and in excruciatingly accurate detail. 

Jan van Eyck
The Arnolfini Portrait (detail).

In Flanders, in 1434, Jan van Eyck produced "The Arnolfini Portrait," (above). Behind the main subjects, hanging on the wall is a convex mirror. The reflection in the mirror shows the backs of the two subjects, but also two other figures further back, one of which is thought to be the artist himself, and beyond him a strong light source. The image in the mirror is distorted exactly as one would experience in real life. 

There is growing speculation that among the secrets of "realist" (or naturalist) painters was a growing arsenal of optical tools and lenses used to map out and understand the attributes of perspective. The mirror, in the Arnolfini Portrait was a sort of boast of the artist's proficiency in recreating reality on the canvas.

The point is that here is a case where art led science into new realms. Painters started to take great pains in reproducing reality "as it is" on canvas. Soon minor experimenters like Neri and major luminaries like Galileo were taking great pains to do the same. They strove to observe nature "as it is," not as was prescribed in ancient texts, or dictated by authority. Once that process started, awareness of the world grew and there was no turning back.

Finally, it is amusing to note that in his many manuscript illustrations, Antonio Neri himself never quite mastered perspective drawing, although he did try.

* This post first appeared here 17 January 2014.