Carl Scheele’s pigments
The intense experimental activity of Carl Scheele (1742-1786) embraced the fields of organic and inorganic chemistry, obtaining incredible results in his short life (he died at the age of forty), having the back rooms of the pharmacies where he worked. There are Scheele’s discoveries that have had a great impact, such as the discovery of oxygen on the development of chemistry, his research on phosphorus in the Swedish match industry, or his investigations on the photochemistry of silver salts on the development of photography etc. But there is an experimental result by Scheele, not among the most important in terms of scientific relevance, which however had the most impact on art, fashion and costume in general. It is the invention of the green dye which then took the name green of Scheele, the link between the pigments and Scheele was the unexpected result of research that had a different purpose. As early as 1770, in an attempt to obtain soda directly from common salt, a field that has always been tempting to a chemist, Scheele had mixed powdered litharge with an aqueous solution of sodium chloride, and the caustic soda solution formed was carbonated. with exposure to air. In the process, a modest quantity of soda was obtained, together with lead oxychloride which appeared of a beautiful yellow color. The resulting pigment was patented in 1801 in England, many years after Scheele’s death, by James Turner, a chemical manufacturer, later known as Turner’s Yellow.
However, this yellow pigment left little traces in the art of that time, its sensitivity to light and sulfur fumes limited its use and was replaced by yellow chrome pigments. Five years after the synthesis of the yellow pigment, while Scheele was intent on experimenting with arsenic compounds, he obtained a green pigment, later called Green Scheele. It is a discovery that does not seem to be the result of a preordained plan as well. But his resonance seems greater this time than the first pigment, so much so that the Swedish Academy of Sciences invites him to publish an account of his discovery in the proceedings of the same academy. Scheele will do it only after three years, overcoming her proverbial refusal to publish his research. He does it on a short note, addressing the academy with the usual courtesy and modesty.
Scheele’s short memoir:
Below is the full translation of his memoir from the Swedish language. The units of measurement for weights and volumes that are used were those in use in Sweden before the introduction of the International System. They have been converted into the S.I.
Royal Swedish Academy of Sciences 1778
Preparation of a new green color by Carl Wilhelm Scheele
That one can hardly conceive of the help of chemistry in the preparation of colors for painting, and that with its help one can discover new colors again, is a truth that no one doubts. To comply with the request of the Academy of Sciences, namely that the color green, which I discovered in my experiments with arsenic, as well as its method of preparation, should become more familiar to the public; I have the honor of being of the same opinion, even more, because I have discovered that the color is not only useful, so for an oil color similar to water, but also that now, after three years, it has not changed minimizing.
Two skålpund [1 skålpund = 0.425 kg] of blue copper vitriol [copper sulphate] are dissolved in a copper kettle over the fire, with 6 kannor of clean water [1 kannor = 2.6 liters]; when the vitriol has dissolved, the kettle is removed from the heat.
Then, in another copper kettle, 2 skålpund of dry white ash and 22 lod [1 lod = 0.013301 kg] of powdered white arsenic * are dissolved in 2 kannor of pure water over the fire; when all is dissolved, this lye is filtered through the flax into another vessel. Gradually this arsenical lye is poured into the container of the solution of the aforementioned copper vitriol, stirring constantly with a wooden spoon. ** When everything has been added, the mixture is left unchanged for a few hours, while the green color settles at the bottom; then the clear liquid [supernatant] is poured out and a few more jugs of hot water are poured, which are then well mixed; when the paint has settled again, clear water is poured off; continue two more times in the same way, pouring hot water. After the paint is well leached, everything is poured onto a stretched linen cloth and, when the water has drained, the paint is spread in small lumps on gray paper and dried on a light heat. From the specified quantity you get 1 skålpund and 13 lod [total 0.560 Kg], of a nice green color.
* It is always safer to pulverize whole arsenic yourself than to buy it previously in powder, because it happens that the support is mixed with grated plaster. One can be convinced of it, if one puts a pinch of it on a blazing fire: if then it smokes completely bare, without anything being left behind, then this arsenic is pure.
** Given the effervescence that is produced here, the kettle in which the mixture is contained should not be too small, but it should be able to accommodate 16 kannor [about 35 L]
*** All the water with which the paint was leached contains some arsenic; therefore it should be taken to such a place, which after the cattle cannot have access
Carl Wilhelm Scheele – 1778
The weight ratios present in the memory:
The most representative formula we can assume is:
The insoluble compound acid copper arsenite II is not the only component of Green Scheele, the composition of which tends to vary according to the method of preparation.
The beautiful emerald green conquered Europe in the nineteenth century. However, two clarifications must be made before talking about the use and the consequences that resulted from the indiscriminate use of the pigment. The first is linked to some statements we find in some blogs about Scheele’s role in marketing his pigment and making a profit. Nothing is further from the truth. Scheele has led an existence as a shy honorable scientist, all devoted to research, refusing remunerative positions. He died long before his pigment was released. The second clarification concerns the toxicity of the pigment of which he is aware and warns in the note ** that the washing water must be discharged in places not accessible to animals!
The explosion of greenery
In the nineteenth century we have a revaluation of the green color. It was Goethe, who was the first, in his “Theory of colors”, to consider the green color “soothing” and to recommend its use in rooms intended for rest and conviviality. Its diffusion was favored by improving the characteristics of Scheele’s green at the beginning of the nineteenth century. Vienna green or Paris green (two of the many names that modified Scheele green will take) was produced which was acetate arsenite of copper (II), Cu (C2H3O2) 2 · 3Cu (AsO2) 2, similar in shade to the pigment of Scheele, but more lightfast. The new pigment was immediately commercialized and adopted in virtually all branches of the industry. The green pigment shone in women’s fashionable clothes, vests, shoes, gloves and trousers, in candles, like paint in children’s toys and even in the confectionery industry with the beautiful green sugar leaves placed on the frosted cakes and in the wallpaper.
Scheele’s green became so popular that it could be found throughout Britain and much of the continent throughout the nineteenth century. It was so popular that Britain was said to be immersed in it. The toxicity of arsenic was deliberately ignored, Scheele’s recommendation that accompanied the preparation of his pigment was useless. Scheele had imagined that her color would grace the painters’ palette. He certainly did not imagine the extensive use that Victorian England would make of it and to follow Europe and America.
Green in painting
In painting the use of green pigment was employed in their palettes by artists exponents of the Romantic movement at the beginning of the 19th century such as Georg Friedrich Kersting (1785 – 1847) up to exponents of impressionism such as Monet and Renoir and post-impressionism such as Gauguin , Cezanne and Van Gogh. But, as we have seen, its use went far beyond the field of painting.
The green in the wallpaper
Georg Kersting’s previous painting, Woman Embroidering, effectively photographs the fashion of the time: in the homes of the bourgeoisie the walls of the living rooms and sitting rooms were colored green Scheele. Especially green wallpaper became very popular at the time, and as the romantic movement began to take hold, it became even more fashionable to decorate the living room or sitting room with scenes of stylized strawberry vines and green fluttering-headed tulips. The burgeoning wall paper market, where green dominated, was the cause of the most widespread contact with the pigment, it is estimated that in 1858 there were one hundred million square miles of green wallpaper in Britain alone.
The paper flowers
But, as we have seen, its use went far beyond the field of painting. In the burgeoning paper faux flower market, green pigment became all the more valuable. Many manufacturers relied on child labor to curl the petals, sew the flowers, and perform other tasks that required skilled little hands. These flower workers slowly poisoned themselves by their contact with the green pigment and by inhaling the dust, dusting the artificial leaves with green to make them look more lifelike. It is one of the most severe occupational diseases recorded in the 19th century.
As early as 1839, the German chemist Leopold Gmelin had noticed that damp rooms carpeted with paint produced a toxic acid. In 1891, the Italian doctor Bartolomeo Gosio confirmed that the humidity of the walls of the houses and the molds that arose in the wallpaper paste metabolized arsenic to produce a poisonous gas, then identified in 1933 by the chemist Frederick Challenger who recognized it as trimethylarsine.
The death of Napoleon and the arsenic in his hair
In 1961 an analysis conducted on a sample of Napoleon’s hair found a very high concentration of arsenic. The result called death from natural causes into question and the hypothesis of poisoning was put forward. Thus arose a lively debate on the causes of Napoleon’s death. The results of an interdisciplinary study, published in Advances in Anatomic Pathology March 2011, entitled The Medical Mystery of Napoleon Bonaparte An Interdisciplinary Expose puts an end to the diatribe. The study makes use of the point of view of the gastroenterologist, the pathologist, and above all the determination of arsenic in the hair samples of Napoleon and his family, through Neutron Activation Analysis, NAA. The results achieved exclude the emperor’s arsenic poisoning. But then why is the concentration of arsenic in Napoleon’s hair in all cases much higher than the norm? The emperor shared the tastes and trends of the his time: he loved the color green and his clothes, upholstery and the walls of his living rooms often sported this color. But this color was Arsenic-based Scheele Green, and Napoleon had extensive exposure to it. Finally, most cosmetics and hair powders at that time contained arsenic and Napoleon made abundant use of hair powder. Therefore, the potential for direct and indirect contamination of the emperor’s hair was very high.
The lack of awareness of the danger
Throughout the nineteenth century, there have been countless reports of diseases and deaths related to the use of the pigment. People wasted away by frequenting green rooms or when green candles were lit. It is difficult to imagine today how such a widespread use of arsenic compounds has not caused an awareness of its danger. We can think that since the symptoms of arsenic green poisoning, which began with headaches, drowsiness and gastrointestinal problems, were not acute, but remained latent for a long time and manifested gradually, they lost the cause – effect link. The most visible symptoms of acute arsenic poisoning – nausea, vomiting, diarrhea, and abdominal pain – could easily be confused with other common diseases of the time (eg, cholera and pneumonia). The dust of the pigment or the gas that emanated from the wallpaper of the living rooms of the good bourgeoisie did not show itself concretely. In the young workers, who were employed in the processing of fake flowers, the symptoms were more acute, but occupational medicine had yet to be born, as well as union protection.
Last but not least, many businesses had arisen that used the green pigment and the alternatives were not ready. An emblematic case is the manufacturer of fabrics and wallpaper William Morris (1834 – 1896), a British artist and writer who, among the main founders of the Arts and Crafts movement in England, is considered a forerunner of modern designers and had a considerable influence on the architecture of his time. He continued to use both Scheele green and Paris green in his hugely popular line of wallpaper, rugs and textiles. William Morris was very skeptical of claims that arsenic could be dangerous. Around 1870, Morris bowed to public pressure and began using arsenic-free greens in his labs. By the time the European, British and American governments began regulating arsenic, the vivid green wallpaper had already gone out of style.
Arsenic to be more beautiful
But there is another reason that hindered the association of arsenic with a danger. Unusually for a poison, arsenic has had many other common uses throughout history since its discovery. Arsenic was a common treatment for the skin until the early 19th century. In the Victorian period, arsenic was taken as a supplement for internal use to improve the complexion resulting in a bluish and translucent skin. Victorian and Edwardian doctors prescribed it for asthma, typhus, malaria, menstrual pain, syphilis, neuralgia, and as a non-specific remedy. The following poster from 1896 advertises arsenic-based waffles and soaps for skin beauty!
The last act of this story
It is unlikely that many of the Scheele or Paris green products are still around. However, Victoria Finley writes in The Brilliant History of Color in Art, “As late as 1950 the US ambassador to Italy, Clare Boothe Luce, fell ill with arsenic poisoning. The CIA suspected the Soviets and sent a team to Rome to investigate. Eventually they discovered that her bedroom ceiling was decorated with arsenic-filled pigments. A new washing machine had been installed in the room above. Her gasp had released arsenic powder, which she breathed in while she slept. ”
Another article about a Prussian Blue pigment
Laboratory experience: the analysis and synthesis of Prussian Blue dye