2018 Bead Symposium Lecture

Aventurine

Small amphora in aventurine glass ”,

Murano, Salviati.

With all its glitz and sparkle, aventurine (avventurina) stands out as a flamboyant extrovert among the varieties of glass that were developed and perfected on the Venetian island of Murano. Also known as 'goldstone', it consists of a transparent base glass with myriad reflective crystalline "spangles" running throughout. The classical version is a deep golden brown with crystallites composed mainly of metallic copper, with a few related compounds as supporting cast. However, numerous colors have been developed, including red, orange, yellow, green, blue, violet, black and white. 

Folklore holds that aventurine was discovered by accident ("a venturi") when unknown monks inadvertently dropped copper or brass shavings into a glass melt as early as the thirteenth century. [1] However, more thorough investigations have recently identified 1620 as a likely date for the first appearance of aventurine glass. [2] No example or written account has been found that dates prior to the seventeenth century. An alternate story accounts for the name aventurine being derived from 'adventure'; referring to the difficulty and uncertainty involved in its production.[3] At first, the formula was a closely held secret among a few glassmakers and subsequently it was lost and then rediscovered not once but  twice.

To complicate matters, natural minerals with a similar appearance were named after the glass, leading to the misconception that they were also discovered after the glass was invented. This is clearly not the case. Early examples of mineral aventurine artifacts date to the Neolithic era [4] and can be found throughout history. First century Roman writer Pliny mentions a type of stone with silvery flecks, a passage that was well known when the glass was developed.  The compositions of these minerals were also identified early; either species of quartz that contain flecks of mica, or a type of feldspar (sunstone).[5] Mineral aventurine turns up as the eyes of Greek statues, in stonework mosaics and later in the 'pietre dure' art perfected by the Medici artisans in Florence around the time Antonio Neri started making glass there. The chances are good that examples of the mineral were known to Neri as well as to the glassmakers on Murano, but a recipe for the glass version does not turn up in Neri’s 1612 book; he was apparently too early by a decade.  

The story of aventurine's accidental discovery by monks may well be apocryphal; nevertheless, it is a great entrée to understanding how the formulation works. First, contrary to what the story implies, aventurine is not the result of dumping metallic confetti into glass. The reflective "spangles" (as early researchers were fond of calling them) are actually uniformly sized, mirror-like crystals that are grown in the glass. In truth, the formula is quite similar to recipes already in use by Neri and others; the difference was in proportions and in how the glass was treated after it was in the furnace. The formula for aventurine calls for the addition of copper, iron and tin oxides, to a base that was a hybrid of soda, potash and lead glass. Neri’s recipe #128 is titled "A Proven Way to Make Rosichiero" [6] and provides for all of these ingredients, albeit in lower concentrations. Rosichiero was a transparent tawny red colored glass that was a staple of furnaces throughout Italy. 

The secret to producing the reflective "spangles" was to mix the glass and heat it in the furnace in a normal way, but then to slowly reduce the heat while creating a low oxygen “reducing” atmosphere. The furnace draught was shut; the glass pot was fitted with a tight lid and then covered with ashes and allowed to cool very slowly.  

Initially, the batch is saturated with copper oxide. This means the glass has dissolved as much copper, iron and tin as it can and any further addition of these powders will simply float to the bottom of the pot.  The exact amount of powdered metals able to dissolve is a function of temperature; the hotter the glass the more that will dissolve and the cooler the glass the less that will dissolve. The key concept here is that as the glass slowly cools, the metals start to come out of solution and crystals start to form. There is some complex chemistry happening at the same time; the reducing atmosphere encourage the metals to stay in a pure un-oxidized form,  Furthermore any oxygen or sulfur  that happens to be present will preferentially combine with the iron, leaving the copper crystals pristine. Once cooled to room temperature, a successful batch would be broken away from the glass pot by workers and divided into smaller pieces. Glass artisans wanting to incorporate the aventurine into their work needed to work quickly. They carefully reheated an appropriate nugget and coated (encase) it in a layer of clear glass; once molten, direct exposure to the air would destroy the glittery effect. 

Over time, it was discovered that various colors could be produced with the addition of different chemicals, but the central principal of growing tiny metallic crystals is the same.


[1]  The earliest instance of this story in print that I can find is fairly late;  Faustino Corsi, Delle pietre antiche: libri quattro (Rome: Salviuccio e figlio, 1828)  pp. 166-167.  
[2] Cesare Moretti (†), Bernard Gratuze and Sandro Hreglich,  “Le verre aventurine (‘ avventurina ‘) : son histoire, les recettes, les analyses, sa fabrication”, ArcheoSciences, 37 | 2013, 135-154.
[3] For instance see  Giulio Salviati, “Venetian Glass” Journal of the Society of Arts (Proceedings), Volume 37 (7 June,1889), p. 630
[4] Neolithic Quartz Aventurine Pendant - 7 Cm/ 2. 76 ", green - 6500 To 2000 Bp – Sahara. Item Id: 106549,  Weight: 83 gm. Sahara - Mauritania - Tagant country.
http://ancientpoint.com/inf/106549-neolithic_quartz_aventurine_pendant___7_cm_2___76____6500_to_2000_bp___sahara.html
[5] Dizionario del cittadino, o sia Ristretto storico, teorico e ..., Volume 1 pp. 38-39.
[6] Antonio Neri, L'Arte Vetraria (Firenze: Giunti, 1612).
[7] Sauzay, A. (1870) Marvels of Glassmaking in All Ages. London, 1870 pp. 173 - 175.

Friar Mauritio

Friar Mauritio,

Treasure of the world, f.19v (detail)

Antonio Neri, (1598-1600).

Shortly after ordination as a priest in the Catholic Church, Antonio Neri wrote a manuscript devoted to alchemy, which he called Treasure of the World (1598-1600). It was completed a couple of years before we find any reference to his glassmaking activities. Among the recipes, is one for the philosopher's stone, entitled "Fifth way to make the stone which is very secret." Neri explains that "Friar Mauritio" was a Dominican brother, held prisoner in "The Castle of Naples" by Gilbert Montpensier. There he learned the secret of transmuting mercury into gold. Ultimately, he was freed from his French captors and went on to use his secret to create great wealth. The recipe was passed down through his family, and ultimately fell into Neri's hands. He presents the full recipe, which takes many pages in the manuscript detailing complex alchemical manipulations.

Some background is in order here. In the year 1494, French King Charles VIII amassed a very large army (twenty-five thousand men), and with encouragement  from the duke of Milan, marched straight down the Italian peninsula with the intention of annexing the Spanish controlled Kingdom of Naples, which encompassed the entire lower portion of Italy. But by the time Charles marched into the capital city, his victory was already threatened; a "League" was forming to oppose him among the states of northern Italy. Charles set up an occupation government, quickly appointed Gilbert Montpensier as viceroy and judiciously headed back toward France, but not before having to fight through the League forces blocking his way. The city of Naples would be held by the French for only three months; the occupation government barely in control. Truth be told, it was not only Friar Mauritio who was confined, but the entire French contingent was more or less sequestered to several fortresses in the area, as they were greatly resented and resisted by the Neapolitan people. Soon the Spanish military was on the scene and systematically routed French forces, reclaiming all of the lost territory.

The most likely location of Mauritio's imprisonment was the foreboding Castel Nuovo, but there are other possibilities, among them Castel Sant'Elmo and Castel dell' Ovo. In the same years that Neri was writing about the Dominican Friar Mauritio's imprisonment, Dominican friar Tommaso Campanella was held and tortured in Naples, under suspicion of heresy and conspiracy against the Spanish rule of his native Calabria. His "heretical" views on astrology and departure from sanctioned Aristotelian doctrine earned him a cell at Castel Sant'Elmo. 


The ancient Castle dell' Ovo or "castle of the egg," is located along the bay of Naples. The name derives from a story about the Roman poet Virgil, who at one time was thought to be a great sorcerer. Supposedly, he had fortified the foundations of the castle with a magic "egg," which he concealed there. Legend warns that if the egg ever were to be disturbed or broken, the entire Castle would self-destruct, and a series of great calamities would befall the city.

The endeavor of history rightly demands strong attention to demonstrable facts. Unfortunately, folklore is often a casualty of that process, because, by its very nature it has little to contribute that can be vigorously verified. The identity of Neri’s Friar Mauritio and, for that matter, the existence of Virgil's egg in the foundations of a Naples castle, in all likelihood, will never be confirmed. But these stories have qualities that solid evidence often does not capture. Regardless of its factual content, folklore tells us about the hopes and fears of the people who repeat it; it is a taste of their culture. Folklore also emphasizes that the way we see history is very different from the way contemporaries saw it. We have no choice but to view events of the past through a lens of the present, just as Neri viewed it through the lens of his time. His view of the events was very much colored by legends about characters like Virgil and Friar Mauritio, but also by the promise of transmutation, and fear of the Inquisition. 

* This post first appeared here on 12 March 2014.

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.

Reticello

Reticello by Aaron Tate (detail),

Photo by David Lindes.

In fine art of the early twentieth century, ‘cubist’ painters and sculptors caused a sensation by pioneering a style that was fluid and emotional yet rigorously geometric. Earlier still, eighteenth century Baroque musicians thrilled audiences with ‘counterpoint’; music that was melodic and harmonious yet structured with an almost mathematical precision. The recurrence of similar themes in widely differing arts is perhaps not so surprising. When successful, these are themes that resonate deeply within us; they amplify what is common to our nature and remind us that we are in the world together. 

In sixteenth century Venice, a form of glasswork emerged that anticipated these juxtapositions to similar popular acclaim. In fact, the technique of ‘reticello’ went on to become an enduring trademark of Murano technical skill and artistry. To this day, well executed pieces of reticello glass are coveted by collectors and displayed with great pride. 

The form is characterized by a transparent glass base which is embedded with a network of crisscrossing threads of opaque glass, forming a lattice of diamond shaped pockets. Classically, white 'latimo' glass was used in a clear 'cristallo' base.The overall effect is reminiscent of fine lace or of fishing net, both of which are strongly evocative in Venetian culture. When executed in the classical technique of the island’s glassblowers, each diamond in the pattern contains a single bubble of trapped air, perfectly centered and uniformly sized. 

In general, the latticework theme in art traces to much earlier times. It is common in Hellenistic and Islamic art. To an extent it is a natural consequence of mosaic making. However the application of the pattern in glassblowing requires a completely different approach and a complex series of steps. First a ‘filigrana’ bubble is formed. To accomplish this, a series of pencil thin glass rods called ‘canes’ are laid side by side, touching each-other, in a pan ('piera' in Venetian, 'pietra'=stone in Italian) and partially fused together in the furnace.[1] Each cane is made of transparent glass with a core of opaque glass (a thread) running its entire length. Glass artist Emilio Santini writes:

On Murano, the "piera" is coated with a thin layer of clay from the laguna marshes. This is rich in salt and does not stick too much on the glass even if overheated. Then they preheat the, piera (called a 'plate' in the US) to dry the clay. While it is still hot but not scorching they lay down the canes so they are partially warmed before they go in the furnace. Remember that they are not annealed [and could easily shatter from thermal shock]. Then two little square metal pieces are placed at the two ends to hold the cane in place. These are called fereti (V) ferretti (I) . Some of these same terms are also used in the US by skillful glass blowers.[2]

On the end of an iron blowpipe, the fused mat of glass rods is wrapped around into a hollow cylinder. Next the open end is gently worked closed. This forms a sealed bubble of glass that can be manipulated by standard glassblowing techniques. Soft from the heat of the furnace, the glass can be given a twist so the parallel threads form a loose spiral. This piece is stored in a ‘garage’ kiln while a second bubble is formed in the same way but with the spiral running in the opposite direction. The first bubble is opened wide at one end, removed from its iron rod and placed in a cradle on the floor which holds it upright. The second bubble is carefully lowered into the open end of the first bubble. The glass artist stands above, with the second bubble inside the first and blows, inflating the inner glass until it comes in contact with the outer bubble.

Because both glass bubbles are formed with canes, they have a ribbed texture both inside and outside. When the two glass bubbles contact each other, the high-points of the ribs meet first, which is where the threads of the two bubbles cross. The valleys of the ribs are where one cane is fused to the next; these areas cross between the bubbles at the center of each diamond in the pattern. The two bubbles fuse together trapping air in the valleys. As the glass is worked and heated these regions form small, perfectly round air bubbles trapped inside the glass. You can see this for yourself by taking two or three fingers of each hand and crossing them against each other. Imagine the threads of opaque glass running down the center of each finger. The air bubbles are trapped where you can see light between your fingers.

Reticello ("fillacello") style flamework pendant,
by Adam Reetz 2015.

The distinctive diamond pattern of reticello has been successfully achieved with other glassmaking techniques. In flameworking, glass is manipulated using only a torch and handtools. The torch is fixed to a bench where the artist either stands or sits. In general, this is a more accessible technique because it does not require a glass furnace. Here, the reticello pattern is accomplished by starting with glass tubing. The crisscross pattern is ‘painted’ onto the outside of the tubing, one line at a time; with very thin rods of glass known as “stringers.” In one version, evenly spaced straight parallel lines are drawn along the length of the tube. The tube is then reheated in the flame and twisted. Next, a second set of lines are drawn twisting in the other direction, forming the diamonds. The ends of the tube are drawn down and one end can be mounted to a rod or tube of glass and further manipulated in the flame. 

In a testament to the continued popularity of reticello, a new colorful variation of the flamework technique has emerged within the past decade, among American flameworkers. [3] This has been playfully dubbed “fillacello.” After painting the fishnet pattern on tubing, and further working the glass, the individual diamonds are “filled” with various colors using stringers. The resulting effect recalls mosaics and the ancient inspirations of reticello.

[1] The iron tool used to move the 'piera' of filigrana cane in and out of the furnace is called a 'pasorale' (V), 'pastorale' (I) = pastoral, named after the staff carried by the pope. It consists of a straight rod with a U-shaped fork in the end. Thanks to Emilio Santini for his kind assistance with Muranese terminology and knowledge of hotshop technique.
[2] Private correspondence, March 2015. Here is a video of the reticello technique as executed by American glass artist Dante Maroni. http://www.youtube.com/watch?v=WJ3DDon23Lc
[3] My research point to the first examples of this technique emerging on the west coat of the United States around 2005-07. (Further information on its origin would be greatly appreciated).

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

Early Modern 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 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 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