Smuggling Glassblowers

 

Berkshire Glass Works cane from 1878 –
Charles Flint collection. These were novelty items 
made by glassworkers after hours.

(hollow, filled with the fine quality sand of the area)

Since the early days of glassmaking, the technical aspects of formulating and manipulating hot glass have remained a closed and secretive business. Down through the ages, those with inside knowledge of this art have been highly valued and highly sought after. Even with the circulation of technical manuscripts and the ultimate publication of the technology in Antonio Neri’s 1612 book L’Arte Vetraria. The beating heart of the craft remained at the furnace among those with hands-on experience. [1] These artisans could and often did make the difference between success and failure in a business that was notorious for sending substantial fortunes up in smoke.  

In some places, strict laws forbade glass workers from leaving their employ and forbade outsiders from attempts to lure them away. Recruiting seasoned talent could be a delicate if not clandestine undertaking. This was as true in the ancient world as it was through the Renaissance and it also played a significant role in the 18th and 19th century for the nascent American glass industry.

Many have heard of the legendary prohibition forbidding Renaissance Venetian glass workers from travel abroad. The fear was that they might divulge secrets of their craft to outsiders, thereby compromising the virtual monopoly on high quality luxury glassware made on the island of Murano. Many also have heard that the penalty for violating these rules was death at the public gallows located between the columns at Saint Mark's square.  While there were laws on the books that listed possible penalties as severe as death, the records show that no glass worker was ever executed for leaving and many did leave and some even came back again. In fact, glass workers were used in highly prestigious state-brokered deals to exchange technology.

These Venetian laws have also led many to the incorrect conclusion that the glassmakers were held captive and treated like prisoners. Nothing could be farther from the truth; glass workers were treated like rock-stars of their time. The daughters of glassmakers were allowed to marry into the nobility, a privilege granted to no other Venetian guild. [2]

Early American efforts to recruit experienced glass workers provide some amusing stories. In 1894, William G. Harding, a principal in the Berkshire Glass Works in Massachusetts wrote a detailed history of glassmaking in the region.  Harding’s research was meticulous, drawn from factory records, official legal filings and personal correspondence. Two stories in particular relate the great lengths pursued in recruiting experienced European glass workers to New England.

The first concerns Robert Hew[e]s, who despite repeated failures in starting a glassworks in the late 1700s, finally partnered in Boston with one Charles F. Kupfer newly arrived from the Duchy of Brunswick in Germany. Together they formed Boston Crown Works on Essex street.  Kupfer immediately returned to Germany to recruit workers, as harding records,

“Mr. Kupfer upon his arrival at his old home, found it no easy matter to get his blowers. The works belonged to the Duke of Brunswick, and it was a penal offense either for the men to leave, or for him to entice them away. He was obliged to conceal his designs and operate in the dark, but succeeded in escaping in the night with a set of workmen and sailed from a German port before being overtaken. After a long voyage they landed in Boston and met with a Royal reception. So much interest in the new enterprise had been awakened in the citizens of Boston, that they turned out en masse…” [3]

The other story of clandestine recruiting related by Harding concerns a venture started in Sand Lake, about 10 miles east of Albany, New York around 1806.

“They had to import their skilled workmen. Mr. William Richmond, a Scotchman, was the Superintendent of their works. He went abroad to procure workmen. Disguised as a mendicant [monk/priest], with a patch upon one eye and playing upon a bag-pipe, he wandered through the glass district of Dumbarton in Scotland and engaged his blowers to cross the Sea. With great difficulty they secreted their tools on Ship-board, for it was a penal offense for glass-workers to leave Scotland as well as Germany.” [4]

[1] Antonio Neri, L’Arte vetraria, distinta in libri sette, del R.[everendo] P.[rete/ padre] Antonio Neri fiorentino. Ne quali si scoprono, effetti maravigliosi, & insegnano segreti bellissimi, del vetro nel fuoco & altre cose curiose. All’Illvst.mo et eccell.mo Sig., Il Sig, Don Antonio Medici (Florence: Giunti 1612).

[2] A daugher of a glassmaker could marry into the nobility, which was a great honour for a family, but strategically, the son of a glassmaker had no such privilege, meaning that there were no inheritance rights for the family.

[3] Collections of the Berkshire Historical and Scientific Society, (Pittsfield: Sun Printing, 1894),  p. 37-39
Note that Robert Hews is also known as 'Hewes' in other sources.

[4] op cit. p. 39-40.

Benjamin Franklin and Glass

 

Note: This is a shorter version of a piece appearing in the Spring/Summer 2016 issue of the NAGC Bulletin. Many thanks for their permission to share it here. A copy of the complete article is available through interlibrary 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.

Benjamin Franklin and His Gathering of Glassmakers

Benjamin Franklin by Joseph Duplessis, 1778

Benjamin Franklin (1706-1790) became famous in his own lifetime as a printer, author, inventor, statesman, diplomat and scientific investigator. The man gracing the hundred-dollar bill has been celebrated for his work in a formidable range of fields, so perhaps we should not be surprised to learn of one that has largely escaped notice. In fact, there is an extraordinary further chapter to be told. Franklin fostered a lifelong fascination with glass and spent considerable energy in efforts to attract talent from Europe to work in America.

Glass and glassmaking garnered not only Franklin’s own enthusiasm, but also that of his family and friends. He worked closely with artisans on two continents and applied his considerable knowledge of glass to areas ranging from music to optics to electrical experimentation. On his European diplomatic missions, he tirelessly encouraged foreign glass workers to set up shop in the colonies. In the decades before America gained independence, he recognized and promoted the vital importance of glassmaking. Unbeknownst to many, Benjamin Franklin played a sustained and influential role in the formation of the American glassmaking industry. 

His connections to the field were wide ranging; his older, favorite brother, John Franklin, became co-founder of a glass factory in Braintree Massachusetts from the late 1740s to early 1750s. [1] In Philadelphia, Benjamin befriended Thomas Godfrey (1704–1749), a glazier, optician, glass and instrument maker who rented space from Franklin to work on new inventions. Lambert Emerson was another glass related acquaintance and fellow Freemason; an émigré from Dublin who advertised in Franklin’s newspaper The Pennsylvania Gazette as a “looking glass maker at the Sign of the looking Glass in Front Street, Philadelphia.” [2] Franklin’s neighbor, Caspar Wistar, was a German glassmaker from Cologne. He owned the nearby glassworks at Alloway Creek in Salem County, New Jersey, just twenty miles south of Philadelphia. Besides making windows and bottles, Wistar manufactured special “philosophical” glassware for Franklin, used in his electrical investigations. 

But Franklin had much more than a passing familiarity with glassmaking. In 1746, he advised Connecticut businessman Thomas Darling on the particulars of running a glass foundry, referring to Wistar’s New Jersey operation. In his correspondence, Franklin consistently referred to his neighbor’s ‘Wistarburgh Glass Manufactory’ as “our glasshouse,” strongly implying a business relationship with Wistar. He is credited with several of his own glass inventions. Two popular items that particularly employed glass were bifocal spectacles and his musical armonica. 

Over his lifetime, Franklin traveled to Europe four times between 1725-75. In London, Paris and on side excursions throughout Europe, he was in frequent contact with glassmakers and he spent considerable effort encouraging them to immigrate to America. 
He understood the strong potential of a glass industry as an economic driver and as a window to groundbreaking scientific discoveries. Critically, he also understood the value of inviting foreign workers to participate in the American dream. In London, he frequented the Royal Society (of scientific investigators) and was correspondent and houseguest of such luminaries as Joseph Priestly, David Hume and Erasmus Darwin. Glass was a hot topic of discussion in these circles due to the recent development of electrostatic generators and Leyden jars, which all had critical components made of glass. 

Franklin continued his efforts to woo glassmakers to America, but plainly, it was not always easy. In a correspondence of 1771, we see a rare pessimism rear its head. In Philadelphia, Joseph Leacock, a cousin of his wife, partnered with a local tanner to start a glass factory on what is now Richmond Street. They wrote to Franklin in London hoping to find workers, but his reply was not encouraging, “It is always a Difficulty here to meet with good Workmen and sober that are willing to go abroad. I heartily wish you Success in your laudable Undertaking to supply your Country with so useful a Manufacture…” [3] He had already seen a deal fall apart a couple of months earlier. Dutch glassworker Jacob Schaub borrowed money from Franklin to book passage across the Atlantic bound for the Stiegel Glassworks in Lancaster, Pennsylvania. Unfortunately, Schaub then failed to appear for work. [4]

After nearly four decades championing an American glass industry, word seems to have gotten around. Although his responses are not available, glassmakers wrote to Franklin from around Europe. In 1778, he received a letter from French master glassmaker Müller de la Piolotte, who explained his family’s long history in the art, starting with his ancestors in the Black Forest of Germany. The 43-year-old bachelor had worked in Champagny, Burgundy and in the Alsace-Lorrain regions of France. 

By 1783, the war was over; Britain conceded and signed an armistice with the United States. Franklin received official permission to entertain applications from those willing to immigrate to the former colonies to work; the floodgates opened and finally he received the interest among glassmakers that he had sought all along. Among the letters from many diverse tradesmen around Europe, in July he received one from Bremen, Germany. The family of Herman Heyman thanked Franklin for letters of introduction. They asked for the ambassador’s consideration of “a Plan which we lately received from one of our principal Glass Manufacturers in upper Germany, who intend to establish a Glass Manufactory in Nord [sic] America.” [5]

In October, he received a letter from a Paris glassmaker who worked for the famed Brossard family of Normandy. This man, Sutter, had heard rumors that Franklin was looking for master glassmakers to work in Philadelphia. He offered his services and suggested that he could convince several other glass workers to accompany him. [6]

In January of 1784, Franklin again heard from the Heyman family, in Germany, who wrote,

… to inform you that three other Gentlemen with me Considered most Earnestly … to Erect a Glass-Manufactory in some part of the United states, and we Chused Maryland to be the properest Country for it. … One of my three Friends Mr. John Fried Amelong who had the Mánage of a Glass Manufactory here in Germany will go himself in the spring by the first Vessell [sic] over to Baltimore and take the Direction of the intended Establishing Glas[s] Manufactory, he Carries besides him 80 more Families all Experiented to our Purpose in the Vessell for Baltimore [sic]. [7]

In February of 1784, Caquery de Mezancy wrote to Franklin, on behalf of five other French glassworkers. He had come from a well-known glassmaking family and a month earlier, he discussed with Franklin his desire to establish a glassworks in America. Franklin assured him that once there, they would have no trouble finding a partner who would furnish all necessary funds; he also indicated that they would receive free passage on an American ship. [8]

In 1785, Franklin himself boarded one of those ships and returned to America for good. Even into his eighties, as a senior statesman-scientist he was engaged and still thinking about glass. In 1787 his good friend, astronomer and mathematician David Rittenhouse wrote him this note, perhaps about a sample of copper ruby or gold ruby glass, in which the color develops upon reheating, a so-called 'striking glass':

I broke a little bit off the colourless end of the Glass Tube and placed it in the focus of the Burning Glass leaving it there several minutes, but no change was produced in its Colour. After examining it I exposed it again to the collected rays of the Sun without observing the least change in its colour, but touching it with the end of a small splinter of Cedar wood the wood took fire and the Glass immediately became a fire red. [9]

Franklin and Rittenhouse enjoyed a regular Wednesday appointment, when they met with others and talked about their interests. Thomas Jefferson once commented that he would happily trade a week in Paris for a single evening shooting the breeze with Franklin and Rittenhouse at one of their gatherings. [10]

Franklin’s America was driven by the promise of fresh talent flooding in from abroad. In the realm of glassmaking, he spent a lifetime courting workers from foreign lands to settle here and become his trusted friends and neighbors. Perhaps we should end with a light-hearted but poignant piece of advice written in his Poor Richard’s Almanac, in August of 1736 when he was just thirty-years-old, “Don't throw stones at your neighbors, if your own windows are glass.” [11]


ENDNOTES:
 [1] The glass factory in Braintree Massachusetts was founded by Joseph Crell, John Franklin and Peter Etter in the late 1740s. In 1752, others assumed management. See Carla J.Mulford, Benjamin Franklin and the Ends of Empire, Oxford: Oxford Univ. Press, 2015, p. 156. Thanks to Gail Bardhan of the Rakow Research Library of the Corning Museum of Glass for her assistance on this point and many others throughout the article.
[2] The Knight of Glin, James Peill, John Rogers, Paul Mellon Centre for Studies in British Art., et al (2007), Irish Furniture, New Haven: Yale Univ. Press, p. 37. For the Freemason reference, see The Pennsylvania Gazette, June 25, 1741. Franklin was the chapter’s grand master by 1734.
[3] Letter to Joseph Leacock and Robert Towers from BF, London dated 22 Aug 1772. See The Papers of Benjamin Franklin (1959–) New Haven: Yale Univ. Press, v. 19, p. 282. Also, see http://founders.archives.gov/documents/Franklin/01-19-02-0184. 
[4] Henry William Stiegel (1729-83) owned three glasshouses, in Lancaster County Pa. See letter from Richard Bache to Franklin, dated 16 May 1772 (Bache was Franklin’s son-in-law). http://founders.archives.gov/documents/Franklin/01-19-02-0100.
[5] Letter from sons of Herman Heyman, Bremen to BF, Passay, dated 31 July 1783. Op. cit. Papers of BF, v. 40, p. 143.
[6] Letter from Mr. Sutter to BF, Passay, dated 29 October 1783. Op. cit., Papers of BF, v. 41, p. 548.
[7] Letter from Herman Heyman Jr., Bremen to BF, Passay, dated 19 January 1784. Op. cit., Papers of BF, v. 41, p. 489–90.
[8] Op. Cit., Papers of BF, v. 41, p. 552.
[9] Note to Dr. Franklin, from David Rittenhouse, Monday noon [c. 1787]. Op. cit. Papers of BF, (forthcoming).
[10] Kevin Keim, Peter Keim, (2007). A Grand Old Flag, a History of the United States through Its Flags. New York, New York: Dorling Kindersley Ltd. p. 43.

[11] BF (1736), Poor Richard’s Almanack, Philadelphia, August 1736, (v. 2, p. 141), see https://en.wikiquote.org/wiki/Poor_Richard%27s_Almanack.

Mrs. Johnston, 18th Century Glass Artist

 

Woman flameworking glass
(Attribution Unknown, late 19th cent.)

In 1743, Britain was ruled by George II, although the Jacobites in Scotland were plotting to install Bonnie Prince Charlie to the throne. That year, Samuel Johnson was a 33 year old struggling writer and his still-to-be famed biographer James Boswell was just a toddler in Edinburgh. Also in Edinburgh, in 1743, exhibiting for a short time only, was Mrs. Johnston, an itinerant fancy glassblower.

‘Fancy' glassblowing refers to the process of working, not at a furnace, but at a table over an oil lamp to artfully melt rods and tubes of glass. A skillful artist could form the glass into small objects; rigged ships, animals, flowers, religious icons, beads and other ornaments. Glass spinning was a related process in which the heat of the lamp flame was used to draw an extremely fine continuous filament of glass that was collected on a large spinning wheel. The result was a mass of almost silk-like floss that was soft and flexible; nothing like the brittle glass of a cup or a window pane. Spinning demonstrations never failed to fascinate audiences and were a staple of fancy glass blowing acts well into the twentieth century.

Artists would often take suggestions from spectators on what to make and then form the piece on the spot. A common technique was to repeatedly touch a thin rod of glass, called a stringer, along the piece under construction forming a series of little loops in the flame. Rows of loops build up a surface that resembles knitting and a skilled artist could form finished pieces quickly. Eventually, both spinning and the knitting techniques became known generically as ‘spun glass’.

Although not well chronicled, this type of demonstration was performed at fairs and other shows as far back as the fifteenth century, and probably earlier. Because of their popularity with women and children, female fancy glass workers were not only well accepted, but commanded a premium at these events.

Below is a lovely correspondence appearing in the local Edinburgh newspaper in January of 1743. The writer is so taken by Mrs. Johnston’s demonstration that he or she was moved to compose a poem. In terms of documenting eighteenth century glass artists, it simply does not get any better:

“To The Publishers of the Caledonian Mercury. Reading a former letter of Leonora’s, curiosity inclined me to see Mrs Johnston the glass spinner, and was agreeably surprised to find the encomiums given her fall short of the character she justly deserves; so I hope the gentlemen, as well as the ladies, will solicit in the behalf of the celebrated artist, as is due her merit.  Therefore,

Let Britain quite enjoy its transport round,

Or Johnston’s praise to all the nation sound;

For me, to humble distance I’ll retire,

There gaze, and with secret joy admire:

My native Scotland such a one can boast,

On whom the praises of the world are lost,

For her own works do justly praise her most.

By giving this a place in your paper, you will oblige, yours, etcetera  -- Torisment. [1]

Two weeks later, appearing in the same paper is Mrs. Johnston’s reaction:

“When a person is obliged to persons unknown, the best way is to return them thanks in the most public manner: therefore Mrs. Johnston, the glass blower and spinner, returns thanks to all the gentlemen and ladies who have honored her with their presence; but more especially the gentleman and lady who did her that honour in the public paper: She cannot show her gratitude in any other way than by her best prayers for their felicity, which she shall always think herself to do both for them and all other her benefactors. Her stay being short in this kingdom, she performs now for the small price of sixpence per piece. [2]

[1] The Caledonian Mercury, Edinburgh, Scotland, 10 Jan 1743, p. 3.

[2] Op. cit., 24 Jan 1743, p. 3.

Weights and Measures

 

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny caused by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Turquoise Glass

 

Turquoise glass stamp

of calif Mustadi  c.1170.

It is estimated that turquoise is among the earliest gems ever mined. With colors that vary from pastel green to a bright sky blue, it has adorned Egyptian sarcophaguses of 5000 years ago, 3000-year-old Chinese art, Aztec death masks and the domes of Persian palaces. 

When traders brought it to Europe from the Mideast, it became known as "turks" or "turquoise" after the old French for "Turkish." While it has never been mined in Turkey, the most highly valued Persian stones were imported there and used extensively for trade. Polished pieces were famously mounted on Turkish equestrian saddles, in the belief that the material conferred sure-footedness and protection from injury during a fall.

As one of the first gems to be collected and traded, turquoise was also one of the first to be imitated. Egyptian faience blue is an early forerunner of glass. It is more porous than glass, but it contains all the same ingredients and could be cast into forms that look just like solid turquoise. In the seventeenth century, the genuine mineral and its imitation continued to hold importance. In Antonio Neri's book L'Arte Vetraria, the subject is mentioned several times; he offers one recipe to restore faded stones by soaking them in almond oil. For turquoise colored enamels he presents two different shades. On the subject of glass, he notes that "Sky Blue, or more properly turquoise, is a principal color in the art of glassmaking" and "I have made this color often, because it is very necessary in beadmaking and is the most esteemed and prized color in the art."

To make his imitation turquoise glass, Neri starts with a batch of high quality transparent aquamarine blue, to which he adds a specially prepared variety of common salt. "Add it little by little, until the aquamarine color loses its transparency and diaphany becoming opaque."

Take the sea salt known as black salt or rather coarse salt, since the ordinary white salt that they make in Volterra would not be good. Put this salt in a frit kiln or oven to calcine, in order to release all moisture and turn white. Next, grind it well into a fine white powder. This salt now calcined should be stored for the use of making sky blue or rather turquoise color as described below.

Sea salt is mostly composed of sodium chloride, which is like table salt that we use for food. However, it can include significant additional minerals, as implied by Neri’s description of it as "black salt." Additional elements can include sulfur, potassium, manganese and more. Regrettably, he leaves us with no further clues to its identity, nor does he explain why the recipe would not work as well with the salt available from Volterra. He goes on to advise that the mix should be used quickly, because if left to sit in the furnace, the glass would start to revert to an ugly transparent color. The remedy for this is to add more salt. He finishes with some practical advice for glassmakers about adding salt to molten glass:

The furnace conciatore should take careful note here, when you add this salt, if it is not well calcined it always bursts. Therefore, you should be cautious and shield your eyes and vision, because there is a danger you could be hurt. Add the doses of salt little by little putting in a bit at a time pausing from one time to the next until you see the desired color. With this, I do not rely on either dose or weight, but only on my eyes. When I see that the glass reaches the desired level of color, I stop adding salt. This all comes with experience. 

* This post first appeared her in a slightly shorter form on 9 April, 2014.

A Philosophical Explosion

 

Glass drops demonstration in slow motion
(Starts after the ad)

Previously, 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, take a look at the dramatic video (above) of an actual glass drop exploding in slow motion.

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.

Thomas Hobbes
by John Mitchel Wright,
(National Portrait Gallery)

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, as the still molten interior cooled more slowly, the drop shrunk, leaving the inside compressed. 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.

Video Credit: Glass drops demonstration in slow motion. courtesy of the American Ceramics Society http://ceramics.org/ceramic-tech-today/video-glass-science-of-prince-ruperts-drop-captured-with-high-speed-cameras

* 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