II. URANIUM DAYS: Notes On Uranium Photography
(continued from I.)
The Faustian secrets unlocked by the Manhattan Project scientists and their awesome consequences have eclipsed the earlier, more innocuous history of uranium, of which uranotype, a photographic process based on uranium chemistry, is only one episode. From the beginning, uranium was recognized as an element with a special relationship with light and color. Fifteen years after the discovery of the element by Klaproth in 1789, Adolph Gehlen had observed that uranyl chloride was sensitive to light, and the photosensitivity, fluorescence or phosphorescence of other uranyl(VI) compounds, then known as “uranic” salts periodically attracted scientific curiosity. The French scientist Henri Becquerel, looking for a link between x-rays and phosphorescence, was working with uranyl potassium sulphate when he discovered radioactivity through a sort of accidental photograph (actually an autoradiograph) which would turn out to be a signpost pointing to the interior of the atom.
Up until this moment, however, and for some time afterwards, uranium’s vividly-colored salts and oxides were much more familiarly known as workaday commercial chemicals. For a century and a half after its discovery, uranium’s main application was as a colorant for glass and glazes. Pottery and tile glazes made with uranium oxides produced vibrant yellow colors which changed, at different firing temperatures, to oranges, reds, browns, greens or black. Uranium-glazed ceramics were manufactured on a large scale in the U.S. right up through the 1960s, and many examples of such tableware, notably the popular Fiestaware shade collectors have nicknamed “radioactive red” are significantly, and even dangerously radioactive. In glassmaking, uranium was behind a greenish spectrum of varieties with names like Canary, Anna Yellow, Green Clambroth, Skokie Green, Topaz Opalescent, Chameleon, Chrysoprase, Lime Sherbet, Taffeta and Burmese. The glasses could be vermilion, turquoise or amber in color, but usually were close to the yellow-green of so-called Vaseline glass, another uranium glass.
Secrecy and uranium production did not begin with the Manhattan Project. Other metal oxides could produce yellow and green tints in glass, but uranium glass was particularly prized for its subtle green fluorescence in sunlight, candlelight or gaslight, all of which have an ultraviolet component. When this was first discovered, in the earlier part of the 19th century, the world’s only known source of uranium were the old Hapsburg silver mines in Joachimsthal, Bohemia, and the local glassmaking industry kept a tight lid on the secret ingredient and its supply as long as it could.***
It wasn’t until mid-century that uranium was tapped for photography. At this time, and in spite of the runaway success of the Daguerreotype, no definitive photographic process had yet emerged to stifle research into other techniques. During the first half of the century, many photosensitive metal salts had been identified as candidates for photographic processes, and by 1850, pioneer photographers were experimenting with most of them. In 1840, working with silver salts, Fox Talbot stumbled on the phenomenon of the latent image, the discovery that would elevate the negative-positive silver process, with its near-instantaneous exposure times, to a position of predominance among photographic processes that only digital photography has been able to challenge. For some years afterwards, however, it remained unclear whether another metal—platinum, gold, iron or mercury, for example—might not actually turn out to be the best basis for commercial photography. Even after the unique qualifications of silver halides were admitted, processes using other metals, particularly platinotype, continued to be pursued for what they could offer for fine photography or specific applications, or else were blended in to silver processes as toning, intensifying or bleaching agents.
Uranium was one of these metals. The first uranium printing processes were invented by a Scotsman, J. Charles Burnett, between 1855 and 1857, and used uranyl(VI) nitrate UO2(NO3)2, as the sensitive salt. Burnett, author of an 1858 article comparing “Printing by the Salts of the Uranic and Ferric Oxides” may have been inspired by Sir John Herschel’s work with iron-based processes, which have similar oxidation/reduction chemistries. (Incidentally, uranium’s namesake, the planet Uranus, was discovered by Herschel’s father, the famous astronomer and musician William Herschel.). The basis for uranium processes lies in the ability of the uranyl ion in the salt to pick up two electrons and reduce to the lower oxidation state of uranium(IV) under ultraviolet light:
UV light + UO22+ + 2e- + 4H+ --> U4+ + 2H2O uranyl + electrons + hydrogen --> uranium(+4) + water cation cation
The uranium(IV) cation will then in turn reduce a noble metal salt to form a metal image. In this way, by using paper prepared with uranyl nitrate, Burnett made the first palladium prints, in 1856:
U4+ + 2Pd+ + 2H2O --> UO22+ + Pd + 4HCl Silver and gold prints can be made by the same mechanism, the latter being in my experience sharply tilted towards the deeper magenta and purple end of the range of colors possible with gold prints. Both variants work better when the uranyl sensitizer is “doped” with a small amount of the developer so that a faint brown or gray image prints out, before final development.
Judging by the notice of Burnett’s processes in later literature, development in silver nitrate and gold chloride appear to be the most commonly used variants. These prints were alternately referred to as uranium prints or urbanities, but these are both something of a misnomer, since if the uranium(IV) cation is used to reduce a noble metal salt, the uranium salts then wash out during wet processing, and the finished prints contain no uranium.
Burnett’s uranium process of 1855, however, the only one that can properly be called a uranotype, does result in an image partly formed from uranium. The process resembles Herschel’s cyanotype (blue print), using only two chemicals, one of them being the cyanotype component potassium ferricyanide, K3[Fe(CN)6]. In fact, an early formula for “uranotype” uses a sensitizer made of uranyl nitrate together with ferric ammonium citrate, the other cyanotype component, developed once again in potassium ferricyanide. In the standard formula, paper is coated with a solution of uranyl nitrate, dried and exposed under a negative to a UV source for 5-10 minutes, after which a very faint yellow image will have printed out. The exposed print is then developed in the ferricyanide to form a strong image in the pigment uranyl ferrocyanide (UO2)2[Fe(CN)6]. The unfixed prints are washed in water containing a few drops of glacial acetic acid per liter for an hour, then dried.
Due to the presence of uranium in the pigment, uranotypes are indeed mildly radioactive, and depending on the image, will emit about 300-400 cpm above background at the surface of the print, mostly in the form of low energy beta particles. This is with modern uranium chemistry which is generally manufactured from depleted uranium and so is about 40% less radioactive, with a lower gamma component, than salts made from natural uranium. While the chemical toxicity of the uranyl salts would have been a more serious hazard than radiation exposure at these low levels, all commercial processes employing uranium were thus somewhat more dangerous before the era of nuclear power.
Dry, a uranotype can vary from print to print from a more neutral, brown russet to strong Bartolozzi red, with a very long tone grade. Thus a contrasted negative with a density range of around 1.8 to 2.2 (6 or 7 stops) is required, similar to that suitable for palladium printing. Uranotypes can greatly resemble the “red chalk” drawings or sanguines that the sensibility of the latter 19th century found so pleasing, and seem to be reasonably archival. Bright sunlight or ultraviolet seems to bring out more detail in finished uranotypes, though whether this is due to stronger white in the paper or fluorescence of the thinner layers of pigment is impossible for me to tell.
One variant on the uranotype process I have not seen in any literature, but which can produce striking results, involves adding a very small, precise amount of gold chloride solution to the uranyl sensitizer before coating, as if one were planning on developing for a gold image. Developed in potassium ferricyanide, a small amount of the gold salt will be reduced by the uranium(IV) cation and the water in the developer bath, which the remainder of the uranium will react to form the pigment. Under ideal circumstances, this will result in a delicate blue-brown split toning, with pale gray blue in the highlights and a neutral brown (presumably formed by an overlay of the gold and the red pigment) in the shadows and midtones. The overlay gives a sense of depth to the interplay of these two natural complements. Color control is dependent on the amount of gold added to the sensitizer and to a lesser degree, the humidity of the paper.
***
Among the factors influencing Burnett’s the choice of metal may have been the 1854 founding of the Uran Works in Joachimsthal by the Austrian chemist Adolph Patera. The factory was designed to produce large quantities of yellow and orange uranium compounds for export to the glass and porcelain trade, and uranyl nitrate, which was used in the preparation of various liquid ceramic colors (lustres), may have been one of its products. At the time, Cornwall’s mines had not yet begun producing uranium, and the opening of the Bohemian factory, whose business in uranium chemicals quickly eclipsed the revenue from Joachimsthal’s silver mining, would have ensured Burnett a steady supply of the as yet fairly exotic chemical at the center of his process, an important condition of commercializing it.
At any rate, supply of uranium chemicals was up, and others were quick to exploit the potential of uranium for photography, most notably Niépce de Saint Victor, the cousin of Niecéphore Niépce, who succeeded in taking out an English patent on an identical process in 1858, launching a paternity feud over the uranium process that briefly threatened to go critical. Other photographic processes employing uranium, growing ever more elaborate (and one thinks, rather pointlessly), sprung up and vanished throughout the second half of the century. In 1864, the Wothlytype appeared, a printing out process employing a mixture of uranyl nitrate, silver nitrate and collodion. While more sensitive than the albumen papers of its time, it was doomed to be as short lived as the images printed on its paper proved to be. The Mercuro-Uranotype had a brief life in the later 19th century; another printing out process, it employed a sensitizer made from a 10:1 mixture of saturated uranium chloride and mercuric chloride solutions. It became obsolete by the end of the century, perhaps because its purveyors all poisoned themselves with the extremely toxic sensitizer formula. Others included the Platino-Uranotype and the Auro-Uranium process, and after the beginning of the century, much faster uranium-silver processes for gaslight prints, like Bartlett’s Process (1906). Rather anecdotally, it also popped up around this time in a few complex homebrew recipes for imitating platinum’s deep, rich blacks at lower cost.
Uranium papers were manufactured commercially at least until the end of the 19th century. Soon afterwards, however, uranium printing, along with numerous other non-silver printing processes quickly became scarce, bowing to the practical advantages and comparative safety of working with “instantaneous” cameras, silver halide films and papers and the enlarger technique than silver’s vastly superior sensitivity to light made possible. Another factor was a diminishing taste, in the new century, for the colored monochromes that the previous century had generally preferred over black and white. But much of the non-silver chemistry worked out in the previous century found a second life as adjuncts to silver printing processes, and uranium was chief among these.
Niépce de St. Victor is credited with first experimenting with uranium as a toner for silver prints, though the chemistry is essentially just a modified form of the image-making reaction from the uranotype process he wrestled over Burnett with. Whether used on a negative or a print, an acid, usually oxalic or acetic, is added to the uranyl nitrate-potassium ferricyanide combination, occasionally with some balancing agents. Used on silver prints or negatives, a small amount of silver ferrocyanide is created and uranyl ferrocyanide pigment is deposited on the silver, intensifying the image and gradually toning it through a warm black to red-brown shades, depending on the time the print spends in the bath. On platinum prints, the toning can produce red, green or blue tones. Photographs thus toned will contain up to about ten milligrams of uranium metal, and like uranotype prints, will be mildly radioactive.
Uranium toning and mordanting treatments were quite common through the first half of the 20th century, as indicated by the long tenure of a uranium toner (K-9) on Kodak’s product list. Prior to WWII, it was a process familiar enough to the general public for some studios to promote uranium toning as a special service. I have even encountered a framed and faintly-toned print at an antique store in New England that bore a label from the early 1930s advertising “uranium-toned prints” as the studio’s specialty, not a marketing strategy that is likely to meet which much success today. Happily, and somewhat surprisingly, however, uranyl nitrate, the primary metal salt for all uranium processes, is still within reach of those who wish to experiment with uranium toning or uranotype, and glass and ceramic artists, probably with more frequency than alternative process photographers, still have recourse to uranium compounds to create the glazes and fluorescing greens that once represented the primary destination of all uranium taken from the earth. While the consumer uses of uranium chemicals have waned, their supply is kept open to artists by a variety of industrial and scientific uses, notably the use of uranyl acetate and uranyl nitrate as analytical reagents or stains for electron microscopy. And the feedstocks for these products remain secure, as long as uranium is enriched for use in the reactors or weapons that the heaviest metal has come to be identified with since 1945.
(Thanks to Clay Perkins and Mike Ware for their attention, advice, and assistance relative to this project)
Uranotype
Uses uranyl compounds as the sensitive salts; the image is formed by the orange pigment uranyl ferrocyanide
Chrysotype
An iron compound is the sensitive salt, reducing gold salts to make a metal image of pure gold. The varying sizes of the colloidal gold particles produce absorb and reflect light so as to give split tones ranging from pale blue in the highlights to bright magentas and deep purples in the midtones and shadows.
Uranium Chrysotype
A uranyl compound is used to reduce the gold salts instead of an iron-compound; developed in gold chloride, it produces a pure gold image of a different character.
Uranium Gold Print
A hybrid process using a mixed uranium and gold sensitizer, which then developed as a uranotype, giving split brown-blue tones in uranyl ferrocyanide and colloidal gold.