The Classical Photographic Craft - The History of the Discovery of Cyanotypes

The Classical Photographic Craft - The History of the Discovery of Cyanotypes

A Brief History of Prussian Blue

Prussian blue has never been found in nature.

Nothing was known about Prussian blue until the 18th century, but the substance still served as one of the first synthetic pigments. During a preparation in 1706, a Swiss immigrant named Johann Jacob Diesbach stumbled upon this surprising color. Historical accounts of the original discovery, often speculative, were first written by GE Stahl and did not appear until 1731. Originally, Diesbach wanted to make a dark red liquid by adding potassium salts to a solution of potassium, alum and iron(II) sulfate. Due to a lack of potassium salts, he bought something from a chemist named John Conrad Dipper, who set up a laboratory in the same building. Dippel provided Dippel with samples of impure potassium salts, which happened to be contaminated with an animal residue known as "Dippel's oil," which provided Diesbach's chance discovery of the element - nitrogen. So, like several other important chemical discoveries, the first preparation of Prussian blue can ultimately be attributed to the dual use of an unethical chemical experiment. Diesbach identified the chemical origin of this accidental blue, and commercial production of Prussian blue began in 1708 by Berlin chemist Johann Frisch. Diesbach announced its various advantages as a pigment in 1710. In pursuit of more profits, he hid the preparation of Prussian blue. It was not revealed until 1724 in the publication of John Woodward.

Johann Jacob Diesbach

Nothing was known about this chemical reaction at the time, as nitrogen was not yet identified as an element. It turns out that any animal that contains nitrogen can be used as a source of Prussian blue: animal leather, hair, feathers, horns, hooves or meat, etc. Bovine blood in particular has proven to be a popular starting material. The methods of making Prussian blue are varied and complex, often requiring numerous procedures and complex raw materials, all of which are tiresome to operate: "Six pounds of leather shavings, six pounds of hooves and horns, and ten pounds of ordinary potash Boil together in a wok until dry. Remove the residue and mix with six pounds of leather shavings, six pounds of hoofs and horns, and ten pounds of regular potash, and boil in a wok until dry. Then add The residue is mixed with two pounds of coarse tartar and fused by a strong fire. The infusion is carried out in the usual way, adding five pounds of ferric sulfate solution, followed by fifteen grains of alum to form a precipitate, which becomes Prussian blue. Another way: Take 86 lbs of coarse tartar, fuse it with a strong fire, the infusion is carried out in the usual way, then five lbs of iron sulfate solution is added, and fifteen grains of alum are added to form a precipitate, which can also become Prussian blue. Prussian blue color quickly causes Chemist's Note.

Pierre Joseph McGuire, the author of the first chemical dictionary, took a keen interest as the government director of the French dyeing industry, and he proved that Prussian blue was a kind of fast coloring for silk and wool. of dyes. In his 1752 investigation, it was stated that the main problem with pigments is that the color is destroyed by alkaline substances. McGuire and other chemists of the time also recognized that a colorless, flammable, volatile substance (what they called the "dyeing principle") could be removed from Prussian blue by heating, leaving behind iron-containing the remains. Later, in 1782, Scherer presented a result of great significance for chemical theory, the solution of the "principle of dyeing" prepared due to the action of sulfuric acid on Prussian blue and renamed it "Prussian acid", he completed This preparation was made, but Prussian acid was not found to be a fast and deadly poison.

In 1811, Guy Lussack finally isolated pure Prussian acid and decided to make it into a simple compound of hydrogen, carbon and nitrogen with the formula HCN, later known as hydrocyanic acid. Prussian blue, which is actually the most complex of these cyanides (ferric ferrocyanide), offers a new way to isolate members of this new substance, whose name comes from its color itself. Considering that the basic unit in all chemical processes is just a simple free element consisting of carbon atoms bonded to nitrogen atoms, it was recognized that cyanide can be synthesized from pure inorganic materials by combining charcoal with potassium salts and iron filings. Roasting under red heat, followed by exposure to a nitrogen-providing atmosphere, roasting to produce potassium ferrocyanide, and Prussian blue by reacting this product with an inorganic salt of iron. This eliminates the need to use organic animal residues and endure the "weird vapors" they produce. In recognition of this achievement, the American Art Association awarded Lewis Thompson a medal in 1837 for his discovery of a new method for preparing Prussian blue.

In the 1780s, Prussian blue was widely produced in several locations in Europe, including the Turnbull & Ramsay Chemical Company in Glasgow, so the pigment was also known as Turnbull's blue. Another Scottish user was the inventor of the raincoat, Charles McGint, who in 1808 set up a manufacturing plant in an alum factory in Lennoxton to dye wool, silk and cotton with Prussian indigo. By the early 19th century, Prussian blue had become a popular dye. Prussian blue is also widely used in the European paper industry as an indigo substitute for the colorant in blue indigo paper, and is a component of the everyday blue commodity, Stephen's ink. In the US it is used as a laundry brightener - "Mrs. Stewart's Blue". Other chemicals such as Prussian blue produced by T. Bramwell & Co.'s Newcastle company were exported to China for colouring green tea, and for this, received the Council's Medal (the highest prize) at the Great Exhibition of 1851. It is worth mentioning that the relationship between the two was later reversed, and green tea became one of the ways to adjust the color of cyanobacteria.

The photosensitivity of ferrous salts

The cyanotype process is based on the chemical reaction of light acting on certain iron salts.

The notion that light can facilitate chemical transformations was central to the invention of photography, and the history of light is well described by Joseph Maria Ed et al. Here, instead of considering the mainstream question of the history of photographic technology of silver halide sensitivity to light, we focus our attention on iron compounds, a less widely known photosensitive phenomenon.

Another very indirect discovery, the Russian minister, Count Alexei Bestshev (1693-1768), invented a secret formula called Ding Tonic in 1725. To justify its high price, it is claimed that the tonic contains gold and that sunlight is involved in its production. When the secret of the reagent's composition was finally revealed, it was found to be no more precious than an ethanolic solution containing ferric chloride, which had discolored due to exposure to light.

Count Alexei Bestshev

Although the supplement has not been approved, it has enjoyed long-term commercial success as an addition to the "Bestshef Psychiatric Drug." Although we don't know whether it was iron or alcohol that provided the patients with greater benefit.

Until 1853, the potion could still be found in the pharmacopoeia as Spirus Matthiatus' iron-containing sulphur ethers, and Martin Kraprot replaced ethanol with diethyl ether, thus making wine a overall. In 1813, Henri August Vogel conducted a scientific study of the photobleaching of ferric chloride in ether solution and found it to be very sensitive.

While these early observations are not related to the actual invention of cyanobacteria, they may be one of the reasons for this. Some of the more misleading and non-historical statements about the process are also: "The principle of sensitizing cyanophore was discovered in France around 1725, and was discovered in England by Sir John Herschel in 1842. improved."

Ed's research on the history of photography shows that the formation of Prussian blue was due to light in 1783, when Italian chemistry professor Giovanni Antonio Scopoli exposed a solution of potassium ferricyanide and acetic acid to sunlight caused by. Prussian blue precipitated in about fifteen minutes. There is another little-known pre-photographic observation of Prussian blue formation, made by John Mercer, a Mancuna-based print dye printer, self-taught color chemist, and Get the job done as a Royal Member. In 1828, he noticed that the light reaction formed Prussian blue on cotton. The biography of Mercer's nephew, Edward Parnell, cites Mercer's description in his lab notebook: "I had dipped a white cloth with a solution of ferric nitrate and then exposed it to sunlight. The red Prussian salt solution produced blue when tested, but there was no blue before exposure. This is remarkable, and I haven't seen any chemists mention it." The pressure to maintain the business deterred Mercer from further pursuit of this An opportunity for pioneering observation, cherished by the "pioneers" of photography, was missed. This is a dilemma that financially independent Victorian gentleman scientists such as Talbot and Herschel would never have faced. Although Mercer later discovered important cyanotypes, these inventions were only disclosed in substantial form and subsequently reinvented and published by others. If we're looking for the unsung heroes of early photography, a candidate worth mentioning might be John Mercer.

The light-induced decomposition of iron organic salts was first documented in 1831 by Johann Wolfgang de Berena (1780-1849), who observed that sunlight exposure to an aqueous solution of iron oxalate formed a precipitate of ferrous oxalate, which was associated with the mineral flood. Castle rocks do the same, releasing a gas that turns out to be carbon dioxide. The public reports of Deberena's discovery stimulated Herschel to accelerate his own current photochemical studies of platinum salts, at a time when no link between the photosensitivity of ferric oxalate had been drawn. This connection may suggest the formation of pigments as an alternative to photographic imagery.

first photo on three sheets of paper

In 1834, William Henry Fox Talbot began experimenting with silver salts as photosensitive imaging materials. He had no particular interest in Prussian blue, although he did record in his notebooks the chemical process of its formation, not the photochemical process, due to the ferric impurities in the acid, and the effect of treatment with hydrochloric acid and potassium ferrocyanide processed paper. In January 1839, stimulated by Talbert's invention of photography based on silver chloride, Sir John Frederick William Herschel (1792-1871) conducted a study of photographic phenomena. Within a week, he solved the fixing problem with sodium thiosulfate. But, contrary to Talbert's single-minded pursuit of silver chloride images, Herschel soon began to expand his research to find other viable photography methods.

Herschel's research records, the primary sources for this research are Herschel's handwritten memos, lab notes, and test prints, as well as the resulting related scientific papers. Over the years, Herschel recorded his chemical experiments in a regularized linear chronology using stationary notebooks now housed in the London Science Museum library, with four volumes chronicling Herschel from 1815 to 1870 of various experiments and tests, but there is a gap in the third volume. Between August 1841 and January 1845, two consecutive experimental numbers were recorded and dated on the same page, separated by four years. During this period, his working life changed dramatically, and Herschel began to adopt a different way of keeping records. After August 1841, they became multi-strand, a new unbound loose-leaf format, with the name of the study noted, in separate envelopes. This is a convenient way to cope with the multiplicity of parallel lines of photochemical research that he is simultaneously concerned with.

Herschel's loose-leaf notes were sold at Sotheby's on March 4, 1958, and were purchased by collector and photographic historian Helmut Gernsheim. The materials were then donated to the Harry Ransom Center for Humanities at the University of Texas at Austin, which also owns a copy of Gainsheim's original Sotheby's auction catalog, complete with his remarks to Herschel Autograph notes on the lot: "This will include many other waste products that others would use in wastebaskets, there are no interesting data or descriptions in the lab notebooks, just a few statements of fact to illustrate the obscure research that has been attempted, only with potassium Prints made with elements still show their images, while those made with others are blank."

These original loose-leaf documents cover key periods beginning in 1843 and are now kept in the Herschel Collection of the HRC Manuscript Archives. These documents will henceforth be referred to as "Memos". The HRM also has 43 sample prints of Herschel.

Herschel's 1842 sample

The rest of Herschel's experimental photographs and tests described in the memo are in Britain, divided between the Oxford Museum of Science and History, the National Museum of Photography, Film and Television (now the National Media Museum), and the Royal Society Library in London. By February 1840, Herschel's Memoirs revealed that he had performed nearly 700 photochemical experiments, almost all using silver salts. There is a chronological list of "ready papers" in the memo, with a serial number assigned to the photosensitivity properties of each chemical coated on paper that Herschel tested. Most of these coated sheets were divided into smaller pieces sufficient for several different tests.

Four Flower Juice Craft and Plant Craft

Herschel's floral print

Seeking more colourful images, Herschel soon dropped the black-brown photographs provided by silver halide, and in 1839 began experimenting with sap experiments with flower juice from his own garden. He crushed the petals with alcohol, extracted the dye (anthocyanins) into a solution, and spread it on paper. These dyed paints, which fade slowly in sunlight, come in contact with the print, and upon exposure, create a positive image directly in the vegetable pigments, which Herschel hopes will provide color for a full-color printing system. The process, which he calls flower juice or sometimes a plant-based process, requires prolonged exposure to sunlight for days or even weeks, and is made very slowly. To make matters worse, the images proved ephemeral. There is still no effective means to prevent them from fading under subsequent strong light exposure. Notably, some of Herschel's specimens still exist today. Despite its peculiarities, the flower juice process was further advanced by the talented Scottish woman Mary Somerville (1780-1872), a crafting medium in which practitioners are still active.

The splendid spring weather of 1840 brought long-lasting benefits to the embryonic science of photography. Herschel had launched a new series of "vegetable colour" exposure tests, but the work was interrupted in March as it relocated the family's home from Slough, Buckinghamshire, to Hawkhurst, Kent. Herschel resumed his experiments in August after resettling in Collingwood. In the ensuing 1841, judging by Herschel's studies of the bleaching of these coloured plant sap, it appears that he was pursuing a direct process of positive colour photography. Talbert reported these experiments in a letter to the 1841 meeting of the British Association for the Advancement of Science: Herschel's most important conclusion from his observations was that dyes tend to pass through their complementary colors The light is selectively bleached, which further proves Grotes' law of photochemical absorption.

The discovery of five blue suns

In the early spring of 1842, Herschel stopped testing vegetable dyes, which proved unsatisfactory, and he hoped to further expand the search for new light-sensitive substances in "non-silver photography". His attention shifted from these rather perishable organic dyes (now properly called anthocyanins) to darker inorganic compounds. He later explained this shift of interest in a paper to the Royal Society in chemically picturesque terms: "The general instability of the assemblage of organic matter may lead us to discover that a large number of and striking situations occur between objects of this class. , but in inorganic arrangements of metals and other elements, many examples have emerged, and more are appearing every day, proving that it can be extended to all situations that might be thought of as chemical elements combined with some degree of relaxation, It can be said to be in a state of equilibrium.”

Alfred Smith

An earlier consultation with Dr. Alfred Smith (1818-1877) led to a heightened awareness of these "loose" inorganic compounds. Smith had a solid foundation in chemistry, when his interest lay in the exciting new science of electrochemistry, with the invention of the battery in 1800 by Alessandro Volta (1745-1827), and in 1807 by Humphrey Sir Lay Davy (1778-1829) used his electricity to "separate their elements" and isolate the reactive alkali metals, and the new science of electrochemistry has been in its golden age.

Smith is also using his batteries to create new methods for chemicals that were previously difficult to obtain. The same substance was then called differently, but we can now identify it by its modern name for potassium ferricyanide. Because the latter read his paper at the Royal Society on June 18, 1840, and subsequently published it in the journal Philosophy, Herschel was aware of Smith's achievement. The main result of this research is to make this bright red, soluble, crystalline chemical more readily available in pure form, since previous methods of preparation (dating back to Leopold Gmelin's discovery of the substance in 1822) more Easily contaminated by impurities.

An earlier consultation with Dr. Alfred Smith (1818-1877) led to a heightened awareness of these "loose" inorganic compounds. Smith had a solid foundation in chemistry, when his interest lay in the exciting new science of electrochemistry, with the invention of the battery in 1800 by Alessandro Volta (1745-1827), and in 1807 by Humphrey Sir Lay Davy (1778-1829) used his electricity to "separate their elements" and isolate the reactive alkali metals, and the new science of electrochemistry has been in its golden age.

Smith is also using his batteries to create new methods for chemicals that were previously difficult to obtain. The same substance was then called differently, but we can now identify it by its modern name for potassium ferricyanide. Because the latter read his paper at the Royal Society on June 18, 1840, and subsequently published it in the journal Philosophy, Herschel was aware of Smith's achievement. The main result of this research is to make this bright red, soluble, crystalline chemical more readily available in pure form, since previous methods of preparation (dating back to Leopold Gmelin's discovery of the substance in 1822) more Easily contaminated by impurities.

Smith's 1840 paper described how potassium ferricyanide was prepared from the common, widely used potassium ferrocyanide. He points out that this new technique of electrolytic oxidation could in principle also be applied to other similar inorganic substances, a possibility that must have inspired Herschel's initial doubts about "dark salts" for him.

Potassium ferrocyanide is oxidized to potassium ferricyanide

In early 1842, shortly after his appointment as surgeon to the Bank of England, Smith sent Herschel a sample of potassium ferricyanide. On April 23, 1842, Herschel entered the following in his memo: "Photography, light changing color, non-silver salts, red ferrocyanate washed on paper by Smith to make it pale green (April 23, 1842) Light thrown on paper works slowly, but as fast as Guyana, and when paper is thrown into water, the mould becomes firmer, loses its violet tint, and becomes Prussian blue, washed with a very dilute acidic liquid, immediately formed a strong blue image with the above characteristics."

This is the first recorded observation of Prussian blue, which is formed by the action of light on potassium ferricyanide, which he intended for photography. As such, it represents the moment when the first cyanogenesis process was discovered, although the name was not coined by Herschel until four months later. The significance of this discovery impressed Herschel enough to also include it in his diary of April 23, 1842: "Discovery of the photographic properties of reddish potassium ferricyanide."

"Discovery" certainly more appropriately describes this moment of innovation in photography than the word "invention". But the next day, Herschel was already looking for ways to improve this non-silver photography process. At the same time, he also began to make images in this way, which is evident in the following entry in the Memorandum: "Photography, non-silver salt type. Potassium salt of ferrocyanide, experimented with various changes in color and Ways to enhance quality

1. Pure water first destroys and washes away the yellowish red border, it also darkens the image to a good blue.

2. Washing with dilute sulfuric acid will make it darker blue.

3. The sulfate of soda water is weak, which can wash off the excess salt well without harming the cyanobacteria, which is completely insoluble in neutral salts.

4. With the very weak perchlorate of acidified iron, the deepest and most brilliant blue will be formed. But the surface is also a little blue. Then tried using it for a replica, but the negative in the replica was blue in the lights and white in the shadows. They are best fixed with very weak sulfuric acid (barely visible acidification). If ferric chloride is added, the surface will turn blue. "

Sir John Herschel, Cyanotype (1842)

So while it was well suited for making positives from camera negatives, Herschel's preferred method at the time was to make "positive" image copies through the upright process. The picture resulting from the negative-making process would require a second print, or "retransfer," to restore its correct tone and texture, which he realized would lose sharpness. Herschel will soon devote considerable energy to devising a viable Prussian blueprint to try to circumvent what he sees as this disadvantage. But before that became a possibility, there was another chemical ingredient he didn't know before that was destined to be introduced into the process, greatly increasing the sensitivity to light and the speed of imaging with Prussian blue.

The discovery of hexaferon salts

The "document list" in the memo shows that Herschel received another sample of potassium ferricyanide from Smith on April 27 and immediately used it to make more "Prussian blue" copies from the engraving , and he sent one of them to Smith. This was acknowledged by Smith on May 10, 1842, which showed that it was clear to him that Prussian blue would result from the action of light on potassium ferricyanide, especially when mixed with iron salts: "Allow me to tell you Thanks for the beautiful and interesting photo, I think the photo was made with red potassium salt and iron salt together, because the compound always deposits Prussian blue under the combined action of air and light. I believe by this operation , you'll be able to apply it to photography."

Smith was no stranger to newly invented photography. As early as May 1839, a few months after Talbot first made his achievements public, Smee published "Light Paintings" in the Literary Gazette, in which his optimal ratio of photosensitizing chemicals and the selection of papers were observed. Given previous photographic experience, and his knowledge of ferricyanide chemistry in the quote above, we can speculate why Smith did not use potassium ferricyanide to make photographic images. It appears that he has unfortunately missed his illustrious moment as the inventor of the cyanotype. Had things been put on hold, Herschel's discovery of the "cyanolysis" process could not have developed into an important invention, a commercially viable cyanolysis process, because potassium ferricyanide itself was very sensitive to light, and Inconvenient for long exposure. But Smith went on to propose key chemicals in his May 10 letter that would open up a whole photographic treasure trove for Herschel. "Two salts have recently been modified for use in medicine by chemists and pharmacologists. Ferric ammonium citrate and ammonium tartrate, which dissolve completely and produce dark solutions. When I mention their ideas, if my expectations are Right, it's almost impossible for them to get into your lab, and it will make me happy to advertise the name of each of these salts."

That dignified tone is what we expect from Smith, now 23, a newly elected Fellow of the Royal Society. He was speaking to 50-year-old Herschel, already one of the most prominent senior scientists in British science. Presumably, Smith didn't realize that he was putting the key to the best method for cyanolysis and several others in Herschel's hands, only thinking that "dark solutions" might make them susceptible to photobleaching . Although not difficult to make, "ferric ammonium citrate and tartaric acid" are fairly new substances in the field of chemistry. The well-known medicinal benefits of ferric ammonium citrate have sparked interest in it.

Different varieties of ferric ammonium citrate

It is evident from Herschel's reply to Smith that Herschel had no knowledge of these salts, which is not surprising since they did not appear in the pharmacopoeia until after 1841 and were then promoted by pharmacists as "anemia" therapy", in fact, they are still considered iron supplements today.

After discovering the chemical "ammonium ferric citrate" on a pharmacy shelf, Herschel soon followed Smee's advice, tried it photographically rather than medically, and found it immediately. It is very sensitive to light. He has successfully printed it in various ways, but his tests on substances abbreviated as "ACI" and "ATI" are yet to be dated. It was an unusual blunder for such a scientific record keeper. Might the discovery intrigue him so much that it distracts him from the systematic records he's been carrying through his life? Herschel's letter to Smith is missing, but fortunately he gave a substantive reply to Smith's letter of June 15, 1842, which clearly shows the richness and sense of diversity of his discovery process to excitement and joy.

"I can't help but thank you for mentioning ferric ammonium citrate and ammonium tartrate as light-sensitive color salts in one of your latest notes to me. I have purchased and checked the ingredients in these salts and it works with ferricyanide and other ingredients Offers me limitless photographic possibilities. I would like to enclose a sample or two for enjoyment. Wet them, place them on white paper, and examine them with a fairly large magnifying glass. If you happen to be attending next week Session of four, do this process. By using gold as an oxidizing agent to create a still image, on paper prepared with this salt, the action of light creates a still image. It's the most dramatic magic in photography One of the effects, while Carroll photography and Agello photography, daguerreotypes themselves are not excluded.”

Thanks to Smith, Herschel was able to devise a more rapid cyanotanning formula that has remained virtually unchanged to this day. In addition, his keen chemical reasoning soon came up with ways to use the same iron salts to image in precious metals (silver, gold, and mercury) among others. So, in 1842, three years after the first public announcement, Herschel proposed a new process, a photosensitive salt based on iron, see table below.

time	inventor	technical name	image
1842	Herschel	blue tan	blues blue
1842	Herschel	Agellofa	silver salt
1842	Herschel	golden seal	gold
1842	Herschel	kelainotype	Mercury
1859	Poitevin	Ferrogallate	Iron-gall ink
1873	Willis	Platinum	Platinum
1877	Pellet	Pellet print	Prussian blue
1889	Nicol	Kallitype	Silver
1889	Arndt	Van Dyke	Silver
1917	Willis	Palladium	Palladium

early photography

The gold-printing process that fascinated Herschel, while striking in color, never took its rightful place in photography. As Herschel expected, it was nowhere near comparable to Talbert's Carlo-style photography, destined to be ignored by all the major photographic authorities of the 19th century. However, Herschel's initial beliefs about the excellence of the gold seal process were eventually confirmed by the author. All crafts proved to be more photosensitivity than Talbert's first 'carlo' craft, so once Talbert discovered a faster mass-producing craft, they would not have been in 1840 Commonly used as an effective means of fixing an image in a camera. Of all sunlight scattered by a subject, only a small fraction (about 1 percent to 1 in 1,000) is captured by the camera lens and projected onto the photosensitive On a photosensitive surface. Exposing the same photosensitive surface directly to the incident light of the sun according to the law of reciprocity will only take about one percent to one thousandth of the exposure time of the camera to clearly show anything placed on it. So , it's much easier to make such "photos" with direct contact than with a camera.

Disclosure of Hepta-Iron Salt

Herschel submitted a second lengthy and important paper to the Royal Society detailing his new photographic discoveries, accompanied by 43 specimens, all made from engravings, in his several processes. His paper, entitled "On the Action of the Rays of the Solar Spectrum on the Colour of Vegetables, and in Some New Photographic Processes", was accepted on 15 June 1842 and published in Philosophical Works of the Royal Society. Herschel read part of the article at the Society on June 16, but it wasn't published until September. It would be convenient to call this seminal work "the 1842 treatise". Herschel's first public description here of the preparation of Prussian blue from potassium ferricyanide alone is as follows: "...recently, I have An interesting salt (potassium ferricyanide) was studied and this experiment was found on the deoxygenation of non-silver salt compounds. Paper simply washed with this salt solution was highly sensitive to the action of light, and Prussian blue appeared. After half an hour or an hour of sun exposure, you get a really nice negative photo." It should be noted that the relatively long exposure time required here (actually, Herschel's "half an hour or an hour" tends to be an optimistic underestimate), in contrast to very short exposures of only a few minutes. When using ferric ammonium citrate, contact printing takes only a few minutes. It can be deduced from the 1842 paper that by June, Herschel also made a cyanobacterial (but not yet named) from a mixture of ferric ammonium citrate and potassium ferricyanide, a recipe that is still used today. If instead of iron chlorate, we replace the solution of that strange salt with iron citrate, the photographic effect is the most diverse and remarkable, and in this novel field of study the image effect has been brought to our attention. The two solutions mixed without causing any precipitation and produced a brown liquid which was green after washing on the paper. If this is done under a prism, the effect of the spectrum is almost instantaneous and most intense. Richly colored Prussian blue deposits formed above blue, violet and hyperspectral rays.

Here again, the chronological order of Herschel's experimental memoranda is anomalous, as they did not record such a test until August 10, 1842. This may indicate that the work at hand has made it exciting again, and no doubt Herschel has made the first "standard cyanotype" in June. This time, the serendipitous discovery has matured into a planned invention using the ferric process. Herschel performed a simple experiment to show which of the two components was more sensitive to light: ferric ammonium citrate was isolated and exposed separately, then developed in a potassium ferricyanide bath, which Herschel demonstrated so far. So far, most of the photosensitivity can be attributed to the former iron salts, but not the latter. "At first glance, it seems natural to attribute these strange and complex changes to the instability of cyanide compounds, which is true to a certain extent and is demonstrated in the described photographic images. There is no potassium ferricyanide, and other additives are added in addition to the ferrous salt itself, but the following experiments fully prove that in the above several changes, the effect of solar rays is not on these salts, but on the added Iron contained in a ferric solution. It is deoxygenated or otherwise changed so that it is present in some form in ferrous salts, so that the acid binds to iron peroxides or propoxides. Just in the paper Ferrocyanate is omitted from the fabrication process, reducing it to a simple process of washing with an ammonium solution." Herschel's photochemical fundamental understanding of the "deoxygenation" behavior of ferric ammonium citrate allowed him to use several Different ways of using this knowledge of chemistry rationally, such as his gold stamping process. Herschel's important observation about the high sensitivity of ferric ammonium citrate to light led to the invention of these new photographic methods. Prussian Blue Image For a month in July 1842, he again used the wonderful ferric ammonium citrate to make a photo. His account in his 1842 treatise revealed the seductive pleasures of mercury development, which he called Kelainotypes. "Can deliver such true-to-life color and depth, such soft material richness, such perfect detail, and preservation of the relative intensity of light that infinitely surpasses any photography I've ever seen, which in fact seems to be possible.

Lest readers feel tempted by such eulogies, put the book aside and, instead of looking for mercury, pay careful attention to Herschel's next sentence: "The most unfortunate thing is that they cannot be preserved." , in the second week of August, Herschel returned to the making of Prussian blue and made new attempts to perfect the craft. His chemical logic suggested that a complementary, efficient working system should be achieved by using potassium ferrocyanide and ferric ammonium citrate instead of ferricyanide. On August 16, 1842, he first wrote the term "cyanotype" in a memorandum to describe the working process of this erection, but he later admitted in his 1843 treatise that the results of the process were given only to the royal family. learn. On November 17, 1842, "My paper describes the cyanotype process, and while the effect is beautiful (especially in the first few minutes of the picture), the final result is very unstable... The process, although very Easy, but not recommended for practical use." Today, in our nomenclature, all we have to do is respect Herschel's preference (contrary to what some bibliologists think, cyanotype only means positive like processes), but where necessary, proper nouns should be used to distinguish the different processes. Herschel then goes on to exaggerate: "There seems to be an infinite variety of cyanobacterial processes..."

From the observations in the 1842 paper, as well as the notes to the memorandum, we can distinguish at least 15 Prussian blue image-making methods devised by Herschel. It was found in a letter that Herschel, who seriously admitted that his companions first discovered it, had to ask Smith if he was the discoverer of ferric ammonium citrate and ammonium tartrate to determine whether he should be given a "confirmation of title" or wanted to use them for his own photography. . Smith rejects any prioritization, but one wonders if perhaps he knows more than he admits. In any case, Smith should have guided Herschel on such a fruitful path.

Herschel's original reason for looking for "dark salts" was that he hoped that some of them might be susceptible to photobleaching, providing a direct positive image. As a result, successful photochemistry turned out to be the opposite, but it was a sign of Herschel's great talent as a scientist, his ability to turn these unexpected serendipitous discoveries into excellent inventions.

8 Pursuit of Permanence in Photography

Herschel copied Schiller's blueprint

Surprisingly, in the three decades since its invention, the cyanotype method has hardly been used. Herschel's achievements were not strongly supported by his bibliography authors, most of whom only reported his unsatisfactory attempts at feature production. Given the enormous commercial importance of the later negative print media in photocopying technology, Herschel himself has demonstrated the feasibility of copying the text. In 1867, Herschel translated a poem by Schiller from German into Latin, then copied the manuscript as a blueprint.

Herschel's son Alexander also used the process to replicate his astronomical records.

Part 2 Reesearch

Confirm copying