A Venetian Chalcedony and Aventurine Glass Bowl

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A Venetian Chalcedony and Adventurine Glass Bowl

Part of the Illuminating Objects series

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Chalcedony and Aventurine Glass Bowl Murano (Venice) 18th century

Chalcedony and Aventurine Glass Bowl
Murano (Venice)
18th century

This Illuminating Objects display investigates the cultural and scientific history of this 18th century Venetian glass bowl. Chalcedony and aventurine glass are two techniques beautifully demonstrated in this bowl. The display looks at the role of chance in scientific discovery and art, the relationship between the natural world and man-made products, and the complex techniques that went into the creation of this object.

The display was researched and prepared by Eleanor Magson, a Masters student in Science Communication at Imperial College London.

The adventures of making aventurine
A small cup cast entirely from aventurine glass (The Courtauld Gallery, London)

A small cup cast entirely from aventurine glass (The Courtauld Gallery, London)

Aventurine is the name given to a Venetian glassmaking technique developed in 16th century Murano, at the heart of the Italian glassmaking tradition. Aventurine from this period is seen as ‘inclusions’ of golden sparkles – a vitreous mass characterised by small shining copper crystals in the glass. The reflective copper crystals twinkle and are called ‘stelle’ or ‘stars’, which gives aventurine its sometimes-used second name, stellaria.

Murano’s Edge

A close-up image of aventurine inclusions within chalcedony glass

A close-up image of aventurine inclusions within chalcedony glass

According to Rosa Barovier Mentasti, a glass historian and descendent of the Barovier family of glassmakers, aventurine was one of the most ambitious of the Murano glassmakers’ secret techniques. Even during the crisis that hit the glassmaking industry in the 18th century (due to the growth of foreign competition), aventurine and other techniques including calcedonio, did not feel the consequences of foreign trade, as they were seen as prized Venetian specialities.

The preparation is long and delicate, and its success so difficult to achieve that it was said to be an adventure – avventura in Italian – a possible source for the name aventurine.

A transformative experience: copper oxide to metallic copper

A microscopic image of the Courtauld’s glass bowl: triangular and hexagonal crystals of aventurine are suspended in a clear matrix of glass (x120)

A microscopic image of the Courtauld’s glass bowl: triangular and hexagonal crystals of aventurine are suspended in a clear matrix of glass (x120)

Aventurine has a special ingredient that causes its characteristic crystal ‘stars’ to develop: copper oxide. This unassuming black powder is added to hot, malleable glass, and is reduced to metallic copper in the crucible. Reduction is the chemical process by which oxygen is removed, therefore copper oxide (CuO) becomes elemental copper (Cu). A reducing atmosphere can be created by burning organic materials, such as wood, in the furnace to produce smoke, or by adding compounds including oxides of iron, tin and antimony and even durum wheat. These reducing agents were added to the molten glass in small sachets called scartocci. Once reduced, the metallic copper particles precipitate throughout the glass in crystalline structures, which, when viewed under a microscope reveal themselves to be triangular and hexagonal in shape. Under a microscope, the crystals appear to be suspended in a colourless vitreous matrix.

Producing the copper crystals

Recent research shows that the chief difficulty in producing aventurine is not in the reduction or the nucleation (the moment at which particles begin to appear within the vitreous mass), but is in the enlargement and dispersion of the copper crystals. This process occurs during the very slow cooling period. The point at which the cooling process is begun is of high importance in the success of making aventurine. The glass must be cooled slowly until it reaches ambient temperature, and it is during this time that the crystals grow, becoming almost visible to the naked eye.

Bringing it all together: Adventurine and Chalcedony

The aventurine inclusions within the chalcedony glass are seen in this microscopic image (x120 magnification)

The aventurine inclusions within the chalcedony glass are seen in this microscopic image (x120 magnification)

In order to incorporate aventurine into chalcedony glass, another process must take place. Once a block of aventurine is produced it is crushed coarsely into small fragments. These are placed near the mouth of the furnace so that they nearly soften. The chalcedony glass is prepared onto a blowpipe, and rolled over the pre-heated aventurine, which sticks to the chalcedony glass. A technique called marvering is used – the molten glass is rolled against a hard, flat surface to cause the aventurine to become flush with the surface of the glass. The aventurine inclusions become ovals through the subsequent glassblowing process: inflating, elongating and torqueing pull the aventurine into its final shape.

Ancient recipes

A peculiar feature of chalcedony is its characteristic red glow when light is shone at it. This is likely due to the high copper content in the glass.

A peculiar feature of chalcedony is its characteristic red glow when light is shone at it. This is likely due to the high copper content in the glass.

This complex technology of creating aventurine glass was perfected in Venice in the 17th century. A glassmakers’ book of recipes, known as the Darduin manuscript, dated 1644, has been one of the chief sources in uncovering the methods of Muranese glassmakers. Interestingly, Giovanni Darduin says in the 149th recipe that the successful formation of aventurine depends more on luck than on science: La si dimanda venturina, et con ragione, perché sortisse più per ventura che per scientia’ (‘It is for this reason that we call it aventurine, because its attainment depends more on luck than science’). This suggests another possible source for the name aventurine: ventura, the Italian word for luck. Whether through adventure or chance, the development of aventurine glass was a masterwork of Venetian glassmaking.


Acknowledgments
I am grateful to the following scholars for their knowledgeable insight and help:
William Gudenrath, Bernard Gratuze, Suzanne Higgott, and Marco Verità.


Bibliography

Barovier Mentasti, R. (1983) Glass in Murano. Venice: Veneto Region and the Venice Chamber of Commerce.
Moretti, C. et al (2013) Le verre aventurine (“avventurina”): son histoire, les recettes, les analyses, sa fabrication. ArchéoSciences, 37: 2013, pp. 135-154
Washington, H. (1894) On Copper Crystals in Aventurine Glass. American Journal of Science. 3rd series, 48: 287, pp. 411-418

A sweet contest of nature and man

In Renaissance Italy, naturally-occurring stones and minerals, such as chalcedony and agate quartz, were marvelled at for their swirling, variegated patterns of colour. This was part of a wider fascination with natural phenomena in the 16th century. Renaissance curiosity cabinets or Wunderkammer contained examples of natural stones – cut, polished, set with golden mounts, and presented alongside other items from the natural world; stuffed animals, seashells, skeletons and corals, for instance.

The wonder of stone

An example of naturally occurring agate

An example of naturally occurring agate

The glass industry was keen to devise a convincing substitute for highly-prized natural stones, as they could be made to meet the demand for such natural wonders at a lower cost, and the famous glassmakers of Murano, Venice, were successful in this quest. The Venetian scholar and historian, Marcantonio Coccio Sabellico wrote in his De situ Venetae urbis(published around 1495) that ‘there is no kind of precious stone that cannot be imitated by the industry of glassworkers, a sweet contest of nature and man.’

Frontispiece of Antonio Neri’s L’Arte Vetraria, 1612

Frontispiece of Antonio Neri’s L’Arte Vetraria, 1612

The hard work that went into creating imitations of nature demonstrated to Renaissance buyers the skill, intelligence and artistry that humankind could achieve. It would have surely seemed like magic to see glass resembling stone to come out of workshops at the hands of men. Antonio Neri’s L’Arte Vetraria (‘The Art of Glass’, a collection of Venetian glassmaking recipes) of 1612 describes the beauty of a piece of glasswork in the following terms: ‘it will seem as if nature herself could not arrive to the like perfection, nor art imitate it’.

The added value of the human touch

The Madonna and Child Enthroned with Saints (c.1483) Domenico Ghirlandaio The flower vase at the centre of this painting shows the value and status of chalcedony glass.

The Madonna and Child Enthroned with Saints (c.1483) Domenico Ghirlandaio
The flower vase at the centre of this painting shows the value and status of chalcedony glass.

Calcedonio glass, named after chalcedony quartz, is an example of glass made to imitate minerals. Its swirling patterns and brown, murky colours are reminiscent of hard, polished stone. To achieve such an aesthetically pleasing final product required a labour-intensive process. Metal oxides, such as silver, tin, iron, copper and mercury, were added to the moltentransparent glass base, and would be deliberately roughly mixed to create a marbled effect. It may seem ironic that such a difficult and labour-intensive process was required to imitate stones which existed in the natural world, but glassware was continually available, unlike stone which had to be mined, and could therefore meet demand much more quickly. In addition, the skilled craftsmanship itself added to the value and made the work all the more prized, ‘this chalcedony was made so beautifully and finely that it resembled real Oriental Agate, and in the beauty of its colours, far surpassed it… [the process] is laborious and takes time, but results in something magnificent.’ The high price demanded by chalcedony glass is seen in a commission of eleven chalcedony vases in 1475 by the great Florentine banker and statesman Filippo Strozzi, who paid the handsome figure of fifty-five ducats.

Long before major industrial or technological revolutions, nature was the producer of all that was beautiful and perfect in the world. It was a marvel to own something man-made that rivalled the previously incomparable beauty of the natural world, as it demonstrated the supremacy and skill of human ability.


Bibliography
Barovier Mentasti, R. (1983) Glass in Murano. Venice: Veneto Region and the Venice Chamber of Commerce.
Edwards, G.R. and Sommerfeld, G. (1998) Art of Glass: Glass in the Collection of the National Gallery of Victoria. Melbourne: National Gallery of Victoria.
Higgott, S. (2011) Wallace Collection Catalogues: Glass and Limoges Painted Enamels. London: The Wallace Collection
Neri, A. (1612) L’Arte Vetraria. Nella Stamperia de’Giunti, Florence.
Sabellicus, M.A. (c.1495) De Situ Urbis Venetae

Aventurra: Chance and serendipity in scientific discovery and glass-making

In 17th century Italy, in the canalled streets of Murano, Venice, a glassmaker stumbled upon a new technique that seemed to create sparkling puddles of gold underneath the surface of pure, clear glass. Some legends tell how the inventor of this technique accidentally spilled copper salts into a batch of molten glass. In the low-oxygen environment of the furnace, the copper transformed from a sooty black powder into shimmering golden pools. The glass produced from this technique was named aventurine, from the Italian word aventurra, meaning ‘by chance’.  Whether or not the original creator of aventurine glass genuinely made his discovery by accident, the name stuck.

Happy accidents

Science, perhaps unlike creativity in the arts, is popularly viewed as being composed of exact, precise and thoughtfully planned out acts. Hypotheses are made, before experiments are planned in minute detail. The steps are then followed exactly as the methodology was specified, until a result is achieved. Each step is therefore of critical importance. In the case of aventurine glass, this difference between scientific precision and artistic freedom seems perfectly supported; the artistry blossomed out of a fortunate mistake. A scientific attitude to precision might not have resulted in the creation of such beautiful and prized glass.

However, science is not always as accurately predicted as some might think. Fortunate mistakes, happy accidents, or serendipity have played a large part in some of the most important scientific discoveries ever made. From Archimedes’s bath to Newton’s apple tree to Pfizer’s surprise discovery of Viagra, there are countless examples of serendipity in science throughout the ages.

The Penicillin precedent

A photograph of Fleming’s petri dish. The large white cluster on the right side is the growth of penicillin. Other white dots are staphylococci bacteria. Note the area of exclusion around the penicillin, where staphylococcus has been killed.

A photograph of Fleming’s petri dish. The large white cluster on the right side is the growth of penicillin. Other white dots are staphylococci bacteria. Note the area of exclusion around the penicillin, where staphylococcus has been killed.

In 1928, Alexander Fleming’s serendipitous discovery changed modern medicine changed forever. Although a brilliant and respected researcher, his laboratory was often untidy. When he looked at the petri dishes he had stored away over summer, perhaps much like the first person to discover golden pools of aventurine in their glass, he was confounded. ‘That’s funny’, he famously remarked. The dishes had become contaminated with a fungus, and where this fungus grew, staphylococci had been killed. Fleming isolated the fungus and grew it, noting that it exuded a substance which had the ability to kill bacteria. Fleming named his discovery penicillin.

“When I woke up just after dawn on September 28, 1928, I certainly didn’t plan to revolutionise all medicine by discovering the world’s first antibiotic, or bacteria killer, but I suppose that was exactly what I did.”

Selective serendipity

To suggest that such a monumental scientific discovery as antibiotics happened only by chance is perhaps insulting to the great minds that went into their discovery. However, Horace Walpole, the celebrated 18th century man of letters, who coined the term serendipity , described how the word referred to a specific kind of happy accident, one which only a sagacious, or clever, person can exploit. Louis Pasteur, whose discovery of the chicken cholera vaccine is sometimes cited as an example of serendipity, similarly stated that ‘chance favours only the prepared mind’.

Even if the first glassmaker to discover aventurine did so by knocking over a plate of copper oxide into the molten glass, it is only through generations of Muranese expertise (Walpole’s ‘sagacity’) that this accident was harnessed and refined to create the beautiful wares known as aventurine.


Bibliography

Colman, D. (2006) The three princes of Serendip: Notes on a mysterious phenomenon,McGill Journal of Medicine, 9(2): 161-163

Dunbar, K., & Fugelsang, J. (2005). Causal thinking in science: How scientists and students interpret the unexpected. In M. E. Gorman, R. D. Tweney, D. Gooding & A. Kincannon (Eds.),Scientific and Technical Thinking (pp. 57–79). Mahwah, NJ: Lawrence Erlbaum Associates.
Moretti, C. et al (2013) Le verre aventurine (“avventurina”): son histoire, les recettes, les analyses, sa fabrication. ArchéoSciences, 37: 2013, pp. 135-154

Walpole coined the term after a Persian fairytale he had heard, The Three Princes of Serendip. The princes, by “accidents and sagacity” discern the nature of a lost camel. ‘Serendip’ here referred to Sri Lanka.

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