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 six different languages; 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. 

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.

Enamels in the 17th Century

Enameled Pendant, Pope Gregory IX

Attr. Italian, mid 17th  century

In the sixth part of his book, L'Arte Vetraria, Antonio Neri presents his recipes for enamels. These are a form of glass meant to be applied in thin layers like paint, and fired quickly. In the early seventeenth century, enamels were used to decorate both glass and gold. Neri does not present any methods specifically addressing technique or application, but he does leave a number of recipes. He also praises the talent of a good friend, who worked on glass in 1601, at the Casino di San Marco in Florence.  "At that time, the task of scheduling furnace work fell to the outstanding Mr. Nicolò Landi, my close friend and a man of rare talent in enamel work at the oil lamp."

The recipes Neri presents in his book are geared towards metalwork, as he says, "In which I show the way to make all of the goldsmiths' enamels to fire over gold in various colors. Included are the rules, the colorant materials and the methods to make the fires for such enamels with exquisite diligence. I present the demonstrations as clearly as possible for this subject." He continues,

Now, this is not only a difficult art but also necessary. We see ornate enameled metals in many colors, and they make a pleasant and noble sight; they entice others to look and take notice. In addition, enameling is one of the main segments of the glass field, and quite necessary. It seems to me to cause universal gratitude and pleasure, so I will endeavor to describe many ways to make all sorts of enamels, which are special materials in the art of glassmaking. They form one of its noble domains, not common, but a particular niche, and because this work is not lacking in substance, and is pleasant, useful, and necessary, I have made this present sixth book, for the satisfaction and benefit of everyone.

Neri's interest in enamels seems to start with  Nicolò Landi. Within a couple of years, in 1603, the glassmaker’s friend Emmanuel Ximenes wrote from Antwerp to Neri in Pisa about the suitability of an enamel he had received from the priest, purposefully overloaded with color. "As far as the red glass, […] I am doing a test in enameling gold, because having such a thin layer of enamel, if it is not very full of pigment it would remain a pale color. […] If it will not grieve Your lordship, I would love to find out the composition."

The implication is that Ximenes was working with his brother in law Baron Simon Rodriguez d'Evora, a famous jeweler and diamond dealer. Soon, Neri would join them in Antwerp, however, the evidence points to a working relationship with goldsmiths before his visit.  "In Pisa, I made them [enamels] without [measuring] weights, but by rough estimate." In fact, Neri mentions goldsmiths in virtually every enamel recipe in the book, and unlike other chapters, he makes no mention of Antwerp, but only of Pisa.

As in all his endeavors, Neri pays close attention to ground truth; he is not as interested in presenting a tidy cut-and-dried recipe as he is in describing what is actually happening with the materials.

You should add the material in four doses, stirring [the melt] thoroughly each time, and leaving the glass to incorporate the powder. The way to check that the color pleases you is by proofing it, and watching to see if it is sufficiently loaded. Stop adding powder and proceed to make a goldsmith's proof [on metal]; always examine the colors to get to know them by eye, as I have always done, because in this matter, I cannot give specific doses. Sometimes the powder will tint more, other times less, therefore you must practice with your eyes to understand the colors.

* This post first appeared here 23 December 2013

Glassware in Alchemy

Antonio Neri (1598-1600),

"Libro intitulato Il tesoro del mondo" f. 38

In the introduction of L'Arte Vetraria, his 1612 book on glassmaking, Antonio Neri discusses the technical and scientific uses of glass. He rattles off an impressive list of items, many of which are still in everyday use in chemistry and medicine:

Beyond the ease and low cost with which it is made, and the fact that it can be made anywhere, glass is more delicate, clean, and attractive  than any material currently known to the world. It is very useful to the arts of distillation and spagyrics, not to mention indispensable to the preparation of medicines for man that would be nearly impossible to make without glass. Furthermore, many kinds of vessels and instruments are produced with it;   cucurbits,  alembics,  receivers,  pelicans,  lenses, retorts, antenitors,  condenser coils, vials, tiles, pouring-vessels (nasse),  ampules, philosophic eggs  and balls. Countless other types of glass vessels are invented every day to compose and produce elixirs, secret potions, quintessences, salts, sulfurs, vitriols, mercuries, tinctures, elemental separations, all metallic things, and many others that are discovered daily. Also, glass containers are made for aqua fortis and aqua regia, which are so essential for refiners (partitori) and masters of the prince’s mints to purify gold and silver and to bring them to perfection. So many benefits for the service of humanity come from glass, which seem nearly impossible to make without it.

The glass book, as it was published by Neri, did not contain any illustrations. If we hunt around in the alchemical literature and in museums, we can find examples of the apparatus and vessels on his list, but still, we might feel disappointed at not seeing the specific pieces with which our glassmaker was referencing. As it happens, we actually can see a number of these pieces, exactly as Neri experienced them. Over a decade before writing  the glass book, when he had just completed Catholic seminary and become an ordained priest, Antonio Neri wrote a manuscript devoted to "all of alchemy" in which he shows us many of the same glass vessels. Here he lists and shows us (in the illustration, from left to right, top to bottom) a double vase, a urinal (yes, that kind of urinal), a pair of Florence flasks (the Italians now call this a pallone di Kjeldahl), a philosophic egg, another flask which Neri calls a "bozza longa," an alembic (or still-head), a retort, a bottle,  mouth-to-mouth urinals, a receiver (for a still or retort), a saucer, and assorted cups and ampules. Since many of these terms changed from place to place and over time, we can use this chart to get a much better idea of exactly what Neri was doing in his recipes. The use of urinals in his chemistry kit shows simple practicality; these were standard items made by glass factories. If a low-cost, readily available item could be used in the laboratory, so much the better.

Many of the items Neri lists were used in distillation, which was a basic technique of alchemists. A still could be set up in any number of variations, depending on the intended product, which could range from alcoholic spirits to powerful acids and other reagents. The "athanor" was a stove specially engineered to gently heat a large flask, called the "cucurbit," which contained whatever was to be distilled. The apparatus would include an "alembic"; a cap that fits on top of the cucurbit with a snout-like tube running downward from its top. The idea was that volatile ingredients would evaporate inside the cucurbit, rise up, condense in the alembic and run down its snout, to be collected in a "receiver" vessel. Sometimes, for convenience, all three pieces (cucurbit, alembic and receiver) are together referred to as the alembic. The process could be sped up significantly by adding a condenser coil, what Neri calls a "serpentine." As steam built up in the cucurbit, it was routed through its snout to a coiled tube that might be submerged in cold water. This way, the steam would condense more rapidly, sending more liquid to the receiver. Neri uses this method to produce acids in order to dissolve metal pigments for his glass, but the same basic technique is still applied today in producing industrial chemicals, medicines, perfumes and alcoholic drinks such as moonshine, brandy, vodka, rum and whisky. However, in the distillation of alcohol, metal (usually copper) containers are preferred. Neri was often producing chemicals that would react with metal, glass provided a very good solution to this problem but as he discusses at length, great pains must be taken to ensure that the glass vessels do not crack or break when heated or cooled too suddenly.

* This post first appeared here on 27 December 2015.

Glass: A poem by Henry Schoolcraft

Note: This is an abbreviated version of a piece appearing in the Autumn/Winter 2016 issue of the NAGC Bulletin. Many thanks for their permission to share it here. A copy of the complete article is available through inter-library loan from the numerous public and art museum libraries which subscribe to this journal, (including the Rakow Library at The Corning Museum of Glass). The Bulletin can also be obtained directly from its publisher, the National American Glass Club.

Glass: A poem by Henry Schoolcraft

In the final years of the 1700s, a third generation family that was living in New York’s Hudson River Valley welcomed a new son, Henry Rowe Schoolcraft (1793-1864). They welcomed him into a newly formed country, brimming with promise and expectation, the United States of America. His father was the superintendant of a new window glass factory near Albany. The family boasted a long line of military men, but perhaps more importantly, a long line of adaptable, self-motivated, life-long learners and young Henry was no exception.  

With the help of his father and like-minded family friends, Henry parlayed a rural education into private instruction in Latin, a premier collection of scientific books, and a museum quality mineral collection.  In 1809, at the age of 16, he started a hand-written literary magazine called “The Cricket” in which he published short pieces of prose and poetry among a circle of friends. His time at Union College was cut short by the opportunity to manage a glass factory further west on the shores of Seneca Lake, near Geneva, New York.

Along with his father, Henry went on to become one of the most sought after glass factory guru’s in New England. In 1814, he was running the Vermont Glassworks on the shore of Lake Dunmore. He had set up an experimental furnace with Prof. Frederick Hall from the college in nearby Middlebury.  Henry used the facility to research glass composition for a hefty book he was writing on the subject. Managing by day, experimenting by night, he still found time for his literary pursuits. It was here, in the autumn of 1814, that he composed a remarkable poem titled “Glass” in which he compares commonplace personalities of the early 19th century to the vitreous material of which he was so familiar.

Until now the poem has never been published beyond the first few stanzas.  “Glass” runs for 268 lines of rhyming couplets in a lose meter. The handwritten manuscript is archived at the Library of Congress. It begins with an assessment of mankind: [1]

1 Mankind resemble glass; they are, like it, For use or fashion, show or service fit; Some bright and fair, some dull and more obscure, These prized as good, those, estimed poor; 5 To grace a kitchen, or a parlour made, As use is most consulted, or parade; But all as various; and eke they are, As frail, as brittle, and as keen a ware. Their bases differ, as our chemists say, 10 This made of sand, that fashioned out of clay Yet shall we, in both compositions find, Similitude in beauty, use and kind. To man, tis true some small objections lie In point of texture and transparency, 15 But though we grant him, in material blind, Yet lacks he not, transparency of mind And we no surer faults in each detect By rays of light, than rays of intellect. So nice the processes, the art requires, 20 So pure th’ ingredients, so intense the fires, Where tumours grow, where phthysic’s fitful breath, Forbodes the public faith, a sudden death. Felons, freckles, frightful fire warts, Are all disclosed as clear as limpid quartz. 25 His voice and pen are graced with equal skill, To lash, report, or advocate a bill. Speak without nostrums, clear his throat when lost, But ever loudest, when they shuffle most. Alike to him, the subject, time or stage, 30 Fierce to discuss, and ready to engage If finance—there Blaberius is at home If raising troops, he votes with general glum. In peace he’s noisy, but if wars involve’ He blasts the foe by one august “Resolve.” 35 Prate, prate, prate, prate! the error of the land, His voice, by every vulgar breeze is fanned Nor learn from Witherspoon his course to run, The simple cause, “to stop when he has done.” (Read full poem)

NOTES  

(Line numbers are referenced in parenthesis) 

The exact manuscript title as written by Schoolcraft is: GLASS, | A Satire Poem. | Lake Dumnore, 1814.

(4) Estimed: Vernacular loanword from the French, estime; valued.

(7) Eke: also.

(10) “Feet of clay” is an expression referring to a weakness or character flaw. The phrase derives from the interpretation of the dream of Nebuchadnezzar, King of Babylon, by the prophet Daniel as recounted in the “Book of Daniel.” (Daniel 2:31–33, 2:41–43).

(21) Phthisic: a wasting illness of the lungs, such as asthma or tuberculosis; phthisis. Any wasting disease. A person suffering from phthisis.

(23) Felon: a carbuncle or other localized infection of the skin. Fire warts: “The hand that reached farthest down on the shovel is burned forever with calloused fire warts.” William E. Bain, Frisco Folks: Stories and Pictures of the Great Steam Days of the Frisco Road (St. Louis-San Francisco Railway Company) (Denver: Sage Books, 1961), p. 96.

(25) In the ms, “Graced” is struck out and replaced by a word that is smeared and illegible to me.

(27) Nostrum: a medicine, especially one that is not considered effective, prepared by an unqualified person.

(31) Blaberius: possible Latinization for ‘one who blabbers’. Also blaberus: a genus of giant Central American cockroach.

(35) Prate: to talk foolishly or tediously about something.

(37) Witherspoon: John Knox Witherspoon (1723–1794).

Footnotes:

[1] Paul Engle, “Glass, A Poem by Henry Schoolcraft” in Glass Club Bulletin, of The National American Glass Club, No. 230, Autumn/Winter 2016, pp. 5-14.

Torricelli on Glass

Evangelista Torricelli

by Lorenzo Lippi, circa 1647

Evangelista Torricelli (1608–1647) is remembered as the inventor of the mercury barometer. Lesser known are a number of significant contributions he made to mathematics, astronomy and physics. There is no direct connection to the Florentine alchemist and glassmaker Antonio Neri—Torricelli was only a boy of six when Neri died—yet there are unmistakable echoes left by Neri that are amplified when we examine Torricelli’s time in Florence.   

In 1632, Torricelli wrote a letter to Galileo, which began a friendship that lasted until the famous astronomer died a decade later. In fact, Galileo invited Torricelli to stay at his house where they spent the last three months of Galileo’s life working together. If Torricelli had not heard of Neri before, perhaps he became acquainted through the copy of L’Arte Vetraria that Galileo had on his bookshelf. Afterward, while preparing to return to Rome, Torricelli was intercepted by the Grand Duke of Tuscany, Ferdinando II de' Medici, who asked him to succeed Galileo as the chair of mathematics at Pisa. He was given a good salary and quarters at the fabulous palace in the center of Florence, that is now called the Medici-Riccardi.  

Historian Mario Gliozzi writes: “Torricelli remained in Florence until his death; these years, the happiest of his life, were filled with the greatest scientific activity. Esteemed for his polished, brilliant, and witty conversation, he soon formed friendships with the outstanding representatives of Florentine culture.” [1]  The ancient palace itself was largely empty in this period, inhabited by a handful of relatives, officials, intellectuals and artists connected with the Grand Ducal court. [2]

Among Torricelli’s companions at the palace were the three sons of Don Antonio de’ Medici, Antonio Neri’s long time benefactor. The boys, Paolo (1616-1656), Giulio (1617-1670) and Antonfrancesco (1618-1659) moved there in 1646. None of the brothers had personally met Neri, as they were all born shortly after his death, but they must have heard plenty about him growing up. As children, they had the run of the Casino di San Marco, the palace where Neri had made glass and pursued the secrets of alchemy. After Neri’s death, their father, Don Antonio spent significant time trying to hunt down Neri’s secret recipe for transmutation. Years later, when Giulio died in 1670, among his possessions were found a box of elixirs and “a booklet, entitled: Material of all the compounds of Priest Antonio Neri; there is a red dustcover, which says ‘experiments.’” [3] The materials were handed over to Jacinto Talducci, the Grand Duke’s chief chemist, and master of the new glassworks established in the Boboli Gardens, a man whom Torricelli depended on for glass. Talducci was also a veteran of the Casino di San Marco Laboratory; according to legend, as a boy he personally witnessed Neri’s transmutation of gold. Curiously, at Giulio’s death he was listed as living on Borgo Pinti in Florence, the same street on which Antonio Neri grew up.

While in Florence, Torricelli took a great interest in optics. Again quoting Gliozzi:

[T]here is very good evidence of his technical ability in working telescope lenses, a skill almost certainly acquired during his stay in Florence. By the autumn of 1642 he was already capable of making lenses that were in no way mediocre, although they did not attain the excellence of those made by Francesco Fontana, at that time the most renowned Italian telescope maker. Torricelli had set out to emulate and surpass Fontana. By 1643 he was already able to obtain lenses equal to Fontana’s or perhaps even better, but above all he had come to understand that what is really important for the efficiency of a lens is the perfectly spherical machining of the surface, which he carried out with refined techniques. The efficiency of Torricelli’s lenses was recognized by the grand duke, who in 1644 presented Torricelli with a gold necklace bearing a medal with the motto “Virtutis praemia.” 

The fame of Torricelli’s excellent lenses quickly became widespread and he received many requests, which he fulfilled at a good profit. He attributed the efficiency of telescopes fitted with his lenses to a machining process that was kept secret at the time but was described in certain papers passed at Torricelli’s death to the grand duke, who gave them to Viviani, after which they were lost.

Gliozzi continues to describe that in 1924 one of Torricelli’s lenses was examined optically using the diffraction grating. “It was found to be of exquisite workmanship, so much so that one face was seen to have been machined better than the mirror taken as reference surface, and was constructed with the most advanced technique of the period.”

In addition to precision glass for lenses, Torricelli depended on Talducci and the grand duke’s furnace for scientific glassware; his experiments that demonstrated the measurement of air pressure required glass tubes, sealed at one end, two ‘cubits’ long (about four feet). They needed to be strong enough to be filled with mercury (which is very heavy) without breaking. It took his colleague Mersenne a couple of years (until 1646) to match the Florentines and obtain an acceptable tube from the French glassworks. 

Torricelli worked with former employees of the Casino di San Marco laboratory who knew Neri, he lived with Don Antonio’s three sons and he took a keen interest in glass; it seems impossible for him to be unaware of Neri and the echoes of his work in Florence.

[1] Mario Gliozzi "Torricelli, Evangelista" in Complete Dictionary of Scientific Biography. 2008. Encyclopedia.com.  http://www.encyclopedia.com/doc/1G2-2830904345.html

[2] 1609-1659 - The last inhabitants of Palazzo Medici http://www.palazzo-medici.it/mediateca/en/Scheda_1609-1659_-_Ultimi_abitanti_di_Palazzo_Medici_

[3] Covoni 1892, p. 193.

Thomas Hobbes on Glass

Thomas Hobbes

by John Michael Wright , (National Portrait Gallery).

Last week we started a tour through Europe that followed the introduction of an item called a "glass drop,"  "Prince Rupert's drop" or "Dutch tear." [1] Today we will explore the attempt of an enlightenment philosopher to explain the phenomenon. First, here is a brief refresher.

In the late seventeenth century, the demonstration of glass drops was sweeping through the parlors of Europe. Consisting of nothing more than palm sized piece of glass with a bulbous nose and a tail that tapered to a point, this little item became a topic of fascination for intellectuals and experimenters alike. [2] Formed by simply letting a gob of glass drip into a bucket of cold water, the fat end could endure strong blows with a hammer, yet snap off the slender tail and the whole piece would erupt into a hail of glass dust and fragments.

Robert Hooke conducted a series of experiments, and observed the results carefully under a microscope. He arrived at conclusions that were largely correct even though the very nature of matter was still under debate. The molten glass on the surface of the drop cooled rapidly, shrunk, and compressed the interior. The result was a highly stressed surface that resisted the hammer. Snapping the tail caused a shock wave of cracks to propagate through the drop, releasing the tension, shattering the entire object into fragments no larger than a grain of sand.

Fig. of glass drop,
Thomas Hobbes, Problematica Physica, 1662 

Enlightenment philosopher Thomas Hobbes was less successful in his attempt to explain the phenomenon, yet not entirely off the mark. [3] Hobbes asks us to consider all matter as possessing "circular internal motion" even when the object as a whole is at rest. He reasoned the heat generated in a glass furnace increased this internal motion both in its "compass" or extent and in speed. When the drop was quenched, the glass that hit the water first has its internal motion changed:

Because the main drop A comes first to the water, it is therefore first quenched, and consequently the motion of the parts of that drop, which by the fire were made to be moved in a larger compass, is by the water made to shrink into lesser circles towards the other end B, but with the same or not much less swiftness.

He further reasons that this action causes the glass to form a structure like threads that run the length of the drop:

Seeing also this motion in every small part of the glass, is not only circular, but proceeds also all along the glass from A to B, the whole motion compounded will be such as the motion of spinning any soft matter into thread, and will dispose the whole body of the glass in threads, which in other hard bodies are called the grain.

Finally, he concludes that all these "threads" are bundled together tightly at the tail end of the drop. When the tail is snapped, 

By the breaking of the glass at C, [the threads] be all at once set at liberty; and then all at once being suddenly unbent, like so many brittle and overbent bows, their strings breaking, be shivered in pieces.

A somewhat tortured connection could be made between Hobbes' theory of internal motion and molecular kinetics, but it is a stretch. The truth of the matter is that his explanation of the glass drop demonstration is elegant and unfortunately, it is also wrong. How did Robert Hooke arrive at a correct conclusion while Hobbes and many others failed? It is a good question because it gets to heart of how successful science is done. Part of the answer lay in Hooke's careful experimentation and detailed observations; he tests his theories wherever he can, and extends his senses with instruments like the microscope. Hobbes develops a series of analogies, but he never devises experiments to test them. To be fair, both men end with conclusions that go beyond what could be observed or measured at the time, and in that realm either one of them could have stumbled. In the end, a correct theory can only be one that does not contradict what actually happens in nature.

In science, obsessive testing and measuring is certainly necessary to arrive at correct conclusions, however it is not sufficient. In other words, there is no guaranteed path to ensure a correct explanation. For this reason alone it is essential to understand that the universe does not follow the laws of physics, it is the other way around; the universe does what it does whether we have a rule for it or not. The 'laws' are our best guess at how nature operates; calling them by that name is a bit pretentious. However, when these laws have been tested repeatedly by our brightest minds, when we observe they are consistent with nature whether peering at the infinitesimal through a microscope or across the galaxy through a telescope, that is what makes science worthy of celebration. [4]

[1] Paul Engle, Conciatore Blog, 2 January 2017; http://www.conciatore.org/2017/01/hookes-tears.html
[2] Niccolò Angelo Tinassi , ed., Il Giornale de Letterati: per tutto l'anno 1672 (Rome : Nicolò Angelo Tinassi, 1672), p. 95.
[3] Thomas Hobbes, Problematica Physica, 1662 (translated in English in 1682 as Seven Philosophical Problems) pp. 36-39, 146-148.
[4] For an interesting rhyme about glass drops recited by Benjamin Franklin see Engle, "Benjamin Franklin and his Gathering of Glassmakers" in Bulletin of the National American Glass Club, Spring/Summer 2016.
* This is an updated version of a post that first appeared on Conciatore Blog, on 7 January 2015.

Hooke's Tears

Glass drops or tears coated in glue,

after detonation, (cross section is left)

from Robert Hooke's

Micrographia 1664, between p. 10, 11.

In 1661, an Italian reprint of Antonio Neri’s book of glassmaking recipes appeared. One year later, an English translation was published in London by physician Christopher Merrett. As an appendix, Merrett included an account of “glass drops” or tears as demonstrated to the Royal Society. These were molten gathers of glass that were allowed to drip into a bucket of cold water and cool. They formed a round, bulbous front end and a tail that trailed off to a thin filament. What made them so fascinating was that the bulbous end can easily endure strong blows with a hammer, but when the thin filament tail is snapped off, the whole piece explodes into a hail of tiny fragments “and in the dark, sparks [flash] at every break of their surface.” [1]

These glass drops became a novelty at royal courts throughout Europe, given a glass furnace they were easy to make, easy to demonstrate, and never failed to amaze observers who had not seen them before. They sparked animated discussion in the many scientific societies that had sprung up; what forces of nature were involved that a piece of glass could resist a hammer yet explode into dust at the loss of its slender tail? 

In the late seventeenth century a Roman publisher by the name of Tinassi [2] regularly issued compilation of noteworthy letters. In his journal’s edition for the year 1672, he published two letters by Geminiano Montanari, a mathematics professor at the university in Bologna, both on the subject of glass drops. In the introduction, he suggests that these curiosities were

Believed to be introduced in Sweden, Holland, then in England, France and Italy; In Paris in the year 1656, many experiments were made at the Academia which met at the home of Mr. Montmor. [3] Many have written of this, among others Monconys [4] in his "Journey to England,” [5] Thomas Hobbes in his Problematica Physica, [6][…] and Christopher Merrett, which in the Latin translation of L’Arte Vetraria of Antonio Neri, [7] is inserted the experiences of the Royal [Society] of England, [and] Mr. Robert Hooke [8] in his Micrographia. [9]

At a time when the concepts of atoms and molecules were still being debated, the glass drops became a nucleus around which a new science developed of mechanical tension and compression. A simple drip of glass caused sharp minds to puzzle and to take a closer look. 

In  Micrographia, Robert Hooke wrote about how he “ground away neer two thirds of the ball, yet would it not fly to pieces, but now and then some small rings of it would snap and fly off, not without a brisk noise and quick motion, leaving the Surface of the drop whence it flew very prettily branched or creased, which was easily discoverable by the Microscope. This drop, after I had thus ground it, without at all impairing the remnant that was not ground away, I caused to fly immediately all into sand upon the nipping off of the very tip of its slender end.”

Hooke continues to describe coating drops in fish glue (isinglass) which was tough enough to hold the piece together when the tail was snapped. “The drop gave a crack like the rest, and gave my hand a pretty brisk impulse: but yet the skin and leather was so strong as to keep the parts from flying out of their former posture and, the skin being transparent, I found that the drop retained exactly its former figure and polish, but was grown perfectly opacious and all over flaw’d, all those flaws lying in the manner of rings, from bottom or blunt end, to the very top or small point.” (See illustration above.)

He discovers that heating the glass drop and then allowing it to cool slowly neutralizes the explosive effect. Finally, he puts it all together: rapid cooling of the surface causes the interior to be compressed like a spring. Snipping the tail, where the skin is thinnest releases all the pent-up energy at once and the piece explodes.


[1] Tinassi 1672, p. 95.
[2] Niccolò Angelo Tinassi, active 1654-1690.
[3] Henri-Louis Habert de Montmor (c. 1600–1679), founded the Montmor Academy, which met at his house in Paris from 1657 until its dissolution in 1664.
[4] Balthasar de Monconys (1611–1665). 
[5] Monconys 1677,  for glass drops see pp. 32, 42 (fig. 4). https://books.google.com/books?id=L0b687oZRVQC
[6] 1662. Problematica Physica (translated in English in 1682 as Seven Philosophical Problems)
[7] This reference is not to Merrett’s 1662 translation of Neri (1612), but to the 1668 or 1669 edition by Frisius in Amsterdam which includes Merret’s annotations.
[8] Robert Hooke (1635–1703). 
[9] Hooke 1664, pp. 33–44.

*This post first appeared here 2 January 2015