The Right Chemistry: Reflections on the manufacture of mirrors

Among the many achievements of 19th-century German chemist Justus von Liebig was a new method that eliminated the need for mercury.

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The Hall of Mirrors in the Palace of Versailles is magnificent. Commissioned by Louis XIV, the Sun King, it was designed to pay tribute to his political, economic and artistic successes with paintings, sculptures and of course, the famous 357 mirrors. Louis had stipulated that everything in the hall must be made by French artisans, which presented a difficulty when it came to the mirrors. At the time, the Venetian island of Murano had a monopoly on mirror-making and the secrets of manufacture were so tightly guarded that any glassmaker who was suspected of disclosing them was dispensed with by hired assassins. As a result, Venetian mirrors were literally worth their weight in gold.

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The task of producing mirrors in France fell to the king’s minister of finance, Jean-Baptiste Colbert, who, in a clandestine operation worthy of a spy-thriller, managed to spirit away several Murano glassmakers to establish a factory at Saint-Gobain. It was here that the mirrors for Versailles were produced. The secret was now out! Venetian mirrors were made by coating glass with an amalgam of tin and mercury and then heating until the mercury evaporated, leaving behind a reflective coating of tin. It comes as no surprise that mirror makers suffered from mercury toxicity and a shortened life expectancy. German chemist Justus von Liebig was aware of this problem and in the 1850s sought to solve it.

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Von Liebig, already an established professor of chemistry at the University of Munich, was consulted by instrument maker Carl von Steinheill, who was attempting to improve the mirrors used in reflecting telescopes. Liebig went to work and discovered that coating glass with a mixture of ammonia, silver nitrate, sugar and a small amount of copper resulted in the deposition of a blemish-free surface of silver. Having secured financial backing from industrialist Johann Beeg, Liebig worked out the details of the process and produced several test mirrors he proudly presented to the wives of his friends. He also recognized that replacing the mercury process had the potential of being financially lucrative, and wrote to a colleague:

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“I want to drive out mercury mirror-making and its injurious influence on workers’ health and will offer replacement mirrors for them in the home which are more durable, cleaner and clearer than the amalgam ones and at the same time cheaper.”

He went on to describe that while his position at the university was “a splendid one,” he struggled financially and was destined to keep lecturing “until his teeth fell out.” Liebig took out patents, and partnered with Cramer & Vetter, a company that had been making tin foil and obviously had a vested interest in getting involved in an industry that sought to replace the one to which it had been supplying tin. Unfortunately for Liebig, the factory ran into production problems and closed after just two years. Ironically, soon after his death, safety regulations led to the banning of mercury mirrors and led to the worldwide use of Liebig’s process until it was replaced in the 1930s by the modern process of vacuum-deposition of aluminum.

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By the time that Liebig dabbled with mirror chemistry, he had already made a name for himself as a scientific pioneer. He and Friedrich Wohler were the first to argue against the theory of vitality, which stated that “living organisms are fundamentally different from non-living entities because they contain some non-physical element or are governed by different principles than are inanimate things.” Liebig asserted that “the production of all organic substances no longer belongs just to living organisms but it must be seen as not only probable, but as certain, that we shall be able to produce them in our laboratories.” He of course was correct, as numerous naturally occurring substances can now be produced synthetically. Liebig also laid the foundations for agricultural chemistry, being the first to recognize that nitrogen, phosphorus and potassium were essential for plant growth and that a plant’s development is limited by the one essential mineral that is in the shortest supply. This led to the development of nitrogen-based fertilizer and recognition that chemical fertilizers could be substituted for animal dung.

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In the early 19th century, chemists understood that all matter was composed of elements and had already developed means to identify the elements that made up a substance. But they struggled to determine the exact composition. This is where Liebig made a huge contribution. Having developed expertise in glass blowing, in 1830, he designed a small apparatus, the “Kaliapparat,” that consisted of an array of bulbs filled with an alkaline solution of potassium hydroxide, hence “kali” from “alkaline.” This could be used to determine the amount of carbon in a sample!

Combustion of a sample converted its carbon content to carbon dioxide, which was then be passed into the kaliapparat where it reacted with the potassium hydroxide to yield potassium carbonate. Weighing the kaliapparat before and after gave the weight of the carbon dioxide absorbed, and by knowing the percent of carbon in carbon dioxide (27%), the amount of carbon in the original sample could be determined. Hydrogen content was determined from the amount of water produced during combustion. The combustion gases were passed through a tube filled with calcium chloride, a substance that absorbs water. Weighing the tube before and after gave the weight of water produced from which the weight of hydrogen was calculated. Any nitrogen released by the sample passed through the apparatus and was captured by another glass bulb. In 1831, Liebig was the first to accurately determine the composition of morphine, and his analytical methods were quickly adopted by chemists around the world and were in use for close to a hundred years. The kaliapparat turned out to be so instrumental in the progress of chemistry that the American Chemical Society, the largest professional society in the world, chose to incorporate it into its logo.

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Liebig was also a profuse writer with many memorable quotes. Let me leave you with my favourite, given that I often talk about the importance of coming to the right conclusion based on an observation. “Observation is like a piece of glass, which, as a mirror, must be very smooth, and must be very carefully polished, in order that it may reflect the image pure and undistorted.” Especially appropriate coming from the man famous for producing mirrors that did not distort.

Joe Schwarcz is director of McGill University’s Office for Science & Society ( He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 pm

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