MIRROR-MAKERS OF OLD DIDN’T SEE MERCURY THREAT
the treacherous mirror-makers!” That, at least according to some accounts, was the edict delivered in 1665 by the infamous Venetian Council of Ten. The victims were to be the skilled glass-blowers who had been enticed to France by Jean-Baptiste Colbert, Louis XIV’s finance minister to head up the newly established Manufacture Royale de Glaces de Miroirs.
At the time Venice was the centre of the glass and mirror trade, with artisans jealously guarding their secrets. Colbert, however, was determined to achieve French self-sufficiency in manufacturing, especially when it came to the furnishings of the Hall of Mirrors, destined to become the crowning glory of the Palace of Versailles.
Income from the export of glass and mirrors was huge for the Venetian economy. Understandably, The Council of Ten, a tribunal charged with the political, moral and financial welfare of the state, was disturbed by the prospect of the French acquiring the technology. Poisoning, mostly with arsenic, belladonna, aconite, hellebore or strychnine had evolved into an effective way of dispatching undesirables in Renaissance Italy, and it became the Council of Ten’s preferred method of dealing with enemies.
Great secrecy was maintained, but the Council did keep private records in the Secreto Secretissima, a book now on display in a Venetian museum. One entry describes how John of Ragusa, a Franciscan brother, offered his poisoning talents to the Council, with a curious price list.
The Great Sultan would be disposed of for 500 ducats and the King of Spain for 150. The pope was a bargain at 100 ducats.
It isn’t clear what the fee for treacherous mirror craftsmen would have been, but perhaps it was too high because history does not record any unusual deaths among the imported Venetians at the Manufacture Royale de Glaces de Miroirs. And it seems the French did manage to learn the secrets of mirror production from the Venetian expatriates because by 1672 the importing of glass or mirrors into France was forbidden. To this day, the French are proud of the fact that Louis and all his mistresses were able to admire themselves in Versailles’s fabulous mirrors, all of which were made in France.
What then was the secret of Venetian mirror-making? To be sure, the Venetians did not invent mirrors. Polished reflective stones dating back to 6000 BC have been found in Turkey, and by 3500 BC the Sumerians in Mesopotamia had developed methods of polishing brass with sand until it developed a reflective surface. The ancient Egyptians, Israelites, Greeks and Romans all gazed at their images in mirrors made from polished copper, bronze, tin, silver or gold.
Fragments of glass coated with tin, found by archeologists in Roman ruins, may represent the first glass mirrors, but the mirrored glass era did not start in earnest until the 14th century in Germany. Glass-blowers discovered that introducing molten antimony, tin or lead into a fresh- ly blown globe, and swirling the metal as it cooled, led to a thin deposit of the metal on the glass. Cutting the globe into pieces then produced mirrors of good quality, but the image of course was somewhat distorted due to the convex shape.
This is where the Venetian glass-blowers come into the picture. They revolutionized mirrormaking by first finding a way to cut open a globe of blown glass and flattening it and, more importantly, by managing to coat it with an alloy of tin and mercury. The process was called “silvering” because the coated glass had a silver colour, but of course this was a misnomer because no silver was involved. A piece of tin was pounded until it was paper thin and covered with mercury. The mercury seeps into the tin to form a homogeneous mixture called an amalgam. At this point a sheet of glass is placed on top and weighted down to squeeze out excess mercury. Heat from a fire or the sun then bakes the metallic coating into the glass and, presto, a mirror is born!
This is the technology that was passed on to the French, the results of which we see today in the grand Hall of Mirrors. While there’s no record of the Council of Ten making good on its poisoning threat, there is a good chance that many mirror-makers were indeed poisoned. Not by arsenic or belladonna, but by the ever-present mercury fumes!
By the mid-19th century, thanks to famed German chemist Justus von Liebig, mirror-makers were no longer at the mercy of mercury.
In 1835 while investigating the chemistry of a family of compounds called aldehydes, Liebig discovered to his amazement that treating these with a solution of silver nitrate and ammonia deposited a thin layer of silver on the inside of his glass flask. The modern mirror was born. And this time it really was “silvered.” Within five years the mercury-tin process was abandoned, saving countless workers from the ravages of mercury toxicity. Now they just had to deal with the cosmetic problem of the silver backing tarnishing if the glass chipped and air got in-between the glass and the silver backing. Trace amounts of hydrogen sulphide in air can lead to a dark deposit of silver sulphide.
Liebig’s silver nitrate reaction had an impact in the organic chemistry lab, as well. It was adapted in the form of the “Tollens’ test” for identifying an unknown substance as an aldehyde. Table sugar, for example, is a member of this family, and its presence in solution can be revealed by applying this reaction.
Many a student of organic chemistry, myself included, was enthralled by their first view of the sudden formation of a silver mirror on the walls of a test tube. I think I would have been even more enthralled if the connection between that laboratory experiment and real-life mirrors had been pointed out.
Today, the Liebig reaction is still used to produce mirrors, but it has been joined by other processes. Vapourized metals, such as silver or aluminum, will deposit on glass when introduced into a vacuum chamber.
Nice technology, but not as interesting to reflect upon as Venetian poisoners.