Photography The enduring image In the commercial battle between digital and analogue photography, physics eventually prevailed. Here, Mike Ware reveals how chemistry shaped the history of photographic images and argues that it still has a part to play

For 150 years, the science called the effect ‘really magical’, Seeing the light of chemistry and the art of In short and asked, ‘surely you deal with Talbot made his first camera photography have been entangled in  Photography, the naughty one?’ Devilry aside, negatives in 1835 on his ‘photogenic a close relationship. A partnership once seen as a the true explanation of the latent drawing paper’ using silver chloride. that, according to a 1858 edition of black art, developed image in silver halides would have Rather than ‘develop’ these, he the literary magazine Athenaeum, through chemical to wait a century, until the quantum ‘printed them out’ entirely by the was not entirely equal: ‘The artists experimentation theory of solid state chemistry was action of light alone. This was not go on boldly and are not afraid to be  Historically, all established. an efficient process – it took him chemists; the chemists gain courage cameras used the But the successful partnership about an hour’s exposure with and long to be artists.’ photochemical of chemistry and photography the lens open (at a wide aperture However bold the artists, properties of silver halide was finally disrupted late in the of f/4) to record those first weak without a theoretical guide it was crystals but other metal 20th century by the rise of digital images with his tiny cameras that experimental chemistry – trial salts can give different technology. The convenience of his wife Constance referred to as and error – that led the way. In tones and colours when electronic image manipulation ‘mousetraps’. 1840, the polymath Henry Talbot printing the negative using personal computers moved But the chemistry improved happened upon the discovery that  Although digital the entire scientific basis for the and the cameras grew larger. gallic acid (3,4,5-trihydroxybenzoic technology has all practice of photography from Talbot’s subsequent discovery acid) could ‘bring out an invisible but won the battle for chemistry to physics. Today, of the latent image improved the dormant picture’ which had been commercial photography, images are recorded on charge- sensitivity [to light] by a factor of briefly projected upon a layer of chemical printing still coupled devices and printed about a hundred, so his ‘calotype’ silver iodide in a camera obscura. offers beautiful effects piezoelectrically by ink-jet printers. process of 1840 required camera This phenomenon – now called and permanent images Multitudes of photographers exposures of a minute or less. This the development of a latent silver have now deserted their wet (and made photographic portraiture image – was, at the time, utterly sometimes smelly) darkrooms, in possible for the first time. His inexplicable. Photography was favour of the dry comforts of their negative–positive process on paper a ‘black art’. Talbot’s friend and desktops. Moreover, many who set photographic science on the eminent colleague in the Royal were unable to print their own road to success. By the middle of the Society, Sir John Herschel, jestingly photographs in the traditional 20th century, the effective quantum way can now do so with ease. yield (the reaction produced by For most purposes, the analogue one photon of light) from gelatine- silver image on cellulose paper or silver halide camera emulsions had polymer film has been replaced by improved dramatically. A typical a computational abstraction – the camera exposure to an average string of binary digital code. Pixels subject was just 1/100 of a second at a substitute for silver grains, and the much smaller aperture (f/8). photo CD serves instead of the roll This marriage of science and of negative film. Photography has art was founded on the unique also fallen in with a new partner photochemical properties of – the internet. The processing and silver halide crystals suspended electronic transmission of pictorial in a gelatin film, the misnamed information within our culture has ‘emulsion’ of photography. Taken now reached unparalleled speed up by George Eastman’s Kodak ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY PHOTO SCIENCE / SOCIETY ASTRONOMICAL ROYAL and ease. Which raises the question: Company in the US, the technology Sir John Herschel: is there any place left in today’s became embodied in a large and inventor of siderotype photographic technology for new profitable industry, serving a photographic printing chemical science? huge constituency of users, both 62 | Chemistry World | August 2007 www.chemistryworld.org

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amateur and professional. This which renders it entirely reducible Chemical silver technology, most of which commercialisation gave analogue to silver by ‘developing’ agents such experimentation lies were discovered in the dawn of photography the benefit of rapid as aromatic amines and phenols, behind the magic of the photography. In 1839, the Scot technical development towards the consists of a ‘sensitivity speck’: a tiny darkroom Mungo Ponton noted a light-induced pinnacle of optical recording. cluster of about four silver atoms. colour change in dichromates coated The history of photographic on paper, corresponding to the science is studded with chemical Printing in metal photoreduction of chromium(vi). triumphs. An early example is Most photographic practice entails Ultimately, chromium(iii) results, Herschel’s use in 1839 of sodium two separable stages. First, acquire which can insolubilise a colloidal thiosulphate to ‘fix’ the silver image the image information, and then – at layer binding a stable pigment. by complexing and dissolving out some later stage – print it in a visually This technique was used to bind the excess insoluble halide. In the acceptable form. For the near- pigments in the ‘carbon process’, 1920s, Samuel Sheppard at Kodak instantaneous capture of analogue commercialised by the London- discovered that traces of thio- negative images in the camera, there based Autotype Company in organics, such as allyl isothiocyanate, has never been any viable chemical 1866, and in the painterly ‘gum- were responsible for the enormous alternative to the uniquely fast bichromate’ printing process. increase in the photosensitivity of ‘emulsion’ of gelatin-silver halide. Both processes provide images as the emulsions made with bovine But making a positive print from permanent as the artists’ pigments gelatine. As he put it, ‘if cows didn’t a camera negative is much less they employ. like mustard [plants], there wouldn’t exacting: it is no great disadvantage The seminal discovery of be any movies at all.’ to use lengthy exposures and intense ‘Because cows ‘siderotype’, or iron-based In the 1980s, the technology ultraviolet printing lights, which photographic printing, was made by of ‘tabular grain’ crystal growth open the door to a whole range like mustard Sir John Herschel in 1842: iron(iii) brought the quantum efficiency of of photochemical systems, less plants, we in certain carboxylate salts can be developed silver halides even closer sensitive than silver halides. photoreduced to iron(ii), which is to the theoretical limit. The latent For print-making there are many can go to the then reacted to make permanent image in a single silver halide crystal, alternatives to the ubiquitous cinema’ images (see box, p64): either www.chemistryworld.org Chemistry World | August 2007 | 63

CW.08.07.Photos.indd 63 24/07/2007 10:18:11 Photography Siderotype chemistry ‘By 1900 the of miniature cameras that recorded negatives onto rollfilm meant that Siderotype printing relies on the photochemistry of iron (iii) carboxylate, and its salons of enlargement by projection became subsequent reactions with gold. First, trisoxalatoferrate(iii) anion undergoes photography the norm, requiring a sensitivity an internal redox reaction induced by light with a wavelength of about 365 nm: which only silver photography can III 3– II 2– 2– hν + 2[Fe (C2O4)3] → 2[Fe (C2O4)2] + 2CO2 + C2O4 were exhibiting offer. more But during the late 1970s, The resulting iron(ii) oxalato complex can reduce noble metals from their salts: a rebellion began against the – II 2– III – – [AuCl4] + 3[Fe (C2O4)2] → Au + 3[Fe (C2O4)2] + 4Cl platinotypes uniformity of the commercial silver- on their walls gelatin enlarging papers provided But gold(iii) is sufficiently oxidising to react with oxalate anion: by the market-driven industry. A – 2– – 2[AuCl4] + 3C2O4 → 2Au + 6CO2 + 8Cl than any other few photographic artists started so the constituents of the sensitiser cannot be pre-mixed. process’ hand-sensitising their own papers, leading a contemporary renaissance in alternative photographic printing, by the reduction of noble metal similar to those highly-esteemed using long-forgotten 19th century salts such as silver (argentotype), media for other works of art on paper processes. So platinum-palladium gold (chrysotype), or mercury – etching, engraving, and mezzotint printing has regained its place as (celaenotype). – the first printing method to give a minority practice for an élite The iron(ii) can also be coupled tonal qualities. Art connoisseurs band of photographers. In this era with hexacyanoferrate(iii) to make appreciated the ‘platinum look’ and of mass production, handmade the pigment Prussian blue (iron(iii) its permanence, contrasting it with photos in platinum metals carry a hexacyanoferrate(ii)). This was the glossy, brown silver-albumen certain cachet, and can claim greater Herschel’s cyanotype process, which prints of the day, which also suffered permanence, than the silver or ink- became the first commercially from the vulnerability of silver to jet prints on commercial materials. important method of reprography sulfiding, and consequent fading – the blueprint. Entirely organic – a common fate of photographs Forgotten colour photosensitive substances were also exposed to the polluted atmospheres Throughout this history, platinum’s pressed into service, some literally: of Victorian England. neighbour gold was ignored as a Herschel extracted coloured potential image substance in its own anthocyanins from the flowers of A night at the palladium right, although the metal had been his Kentish garden to serve in his But the triumph of platinotype used extensively from the earliest ‘anthotype’ dye-bleaching process. was short-lived, thanks to the days of photography to tone and He also attempted to make prints discovery by Friedrich Wilhelm stabilise silver images by partial in platinum by his iron-based route, Ostwald in 1902 of the catalytic electrochemical displacement. but was unsuccessful. A working power of platinum in the oxidation Herschel’s pure gold printing platinotype process had to await the of ammonia to nitric acid. In 1916, process – his chrysotype of 1842 research and enterprise of William at the height of the Great War, the – never gained admission to the Willis, who made the key discovery combatant governments banned photographic repertoire, and by in 1873 that it was necessary to use the use of platinum for frivolous 1900 it had been entirely discounted the (then uncommon) platinum(ii) purposes such as photography as ‘dead and buried’. The problem salts, rather than platinum(iv) salts, and jewellery, and elevated the lay with the chemistry: the for a successful reduction. Willis metal to the status of a strategic simplistic 19th century method marketed his platinotype paper in material, reserved for making nitrate used the easily-available salts of 1879, although it took him twenty explosives. Willis immediately tetrachloroaurate(iii). This could years to perfect the process. The devised a palladium printing paper not be mixed with the iron(iii) platinum print became widely as a substitute. carboxylate sensitiser, owing to acclaimed for its permanence and His Platinotype Company redox instability which causes the artistic effect. By 1900 the salons of survived until 1937. Its ultimate premature precipitation of gold photography were exhibiting more demise was due to the limitation metal. Instead, the gold salt had to platinotypes on their walls than any suffered by all the iron-based ‘Gull, South Ronaldsay’ be employed in a developer bath, other process. The medium displays processes: the necessity for printing - printed as a cyanotype which was uneconomic. The answer, pleasing characteristics: a totally by contact, using a same-sized (left), palladotype which I discovered in 1986, was matte paper surface and subtle grey negative, to allow a sufficient (centre) and chrysotype complexation and reduction to tonal scale, with an aesthetic appeal throughput of light. The growing use (right) gold(i), to provide an efficient and MIKE WARE MIKE

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CW.08.07.Photos.indd 64 24/07/2007 11:49:09 economic mixed sensitiser (see box, below). The new chrysotype process is a close analogue of the celebrated platinotype, but brings one exciting extra benefit: colour. Since the Middle Ages, it has been known that gold salts can impart rich ruby reds and purples to ceramic glazes and glasses. Purple of Cassius, discovered by the proto-chemist Johann Rudolf Glauber in 1656, is now known to be nanoparticle gold in a matrix of tin(iv) hydroxide. The origin of this colour was first established by Michael Faraday in 1856 as due to the formation of highly dispersed gold, but it was not until 1904 that Gustav Mie explained its visible absorption spectrum with a classical scattering theory based on the application of Maxwell’s equations to spherical metallic particles. The strong absorption band of nanoparticle gold in the visible region around a wavelength of 520nm is due to a surface plasmon resonance – a collective oscillation of the delocalised conduction electrons – and its wavelength is dependent, among other things, on the radius of the metal nanosphere. Thus the colour of gold images may be tuned by reactions yielding nanospheres of differing sizes. If the particles are ellipsoidal, rather than spherical, two absorption bands are generated and green gold can result. These developments have provided creative photographers with a new palette of non-literal or ‘surreal’

colour to enhance the expressiveness WARE MIKE of their imagery, while also forging an invigorating new link between any prevailing computer technology Chrysotypes like to many historical photographic the graphic arts and the burgeoning ensures that, providing they are well ‘Il Salvatore, Noto archives. science of nanotechnology. prepared, they will remain humanly Antica, Sicily’, show good Although ink-jet printer pigments readable forever. It is hard to see tonal range and surface are improving in their endurance, Enduring images how machine-dependent, digitally- quality. A variety of some photographers prefer a hybrid Analogue photographs – negative or encoded images can ever lay claim colours can be produced practice. They employ digital positive – are repositories of pictorial to such robustness and accessibility. depending on the size of techniques to ink-jet print large information in a form easily handled The end of the traditional the gold particles negatives onto transparent film and stored as permanent, flat objects. photographic negative and printed (which may well prove ephemeral) Their complete independence of record would prove a great loss but then use them to print positives in gold or platinum upon a substrate of the finest cellulose paper. Such a The new chrysotype process print can endure for a millennium. This union of the noblest metals with This process relies on the ligand 3,3'-thiodipropanoic acid, S(CH2CH2COOH)2, the commonest organic substance on used in the neutralised form of its disodium or diammonium salts, which are earth also provides the artists with a highly soluble in water. The reaction of this thio-ether (written below as SR2) satisfying embodiment of being true with tetrachloroaurate(iii) takes place in three steps: to their materials.

– – i) Coordination to gold(iii): [AuCl4] + SR2 → [AuCl3SR2] + Cl Mike Ware is honorary fellow in chemistry, University of Manchester ii) Reduction to a two-coordinate gold(i) complex by a second molecule of + – ligand: [AuCl3SR2] + SR2 + H2O → [AuClSR2] + OSR2 + 2H + 2Cl Further reading http://www.mikeware.co.uk/ M Ware, Gold in photography. Brighton: Ffotof- iii) Excess ligand may displace the chloride ion from the gold(i) complex: + – film, 2006 [AuClSR2] + SR2 → [Au(SR2)2] + Cl M Ware, The chrysotype manual. Brighton: Ffotoffilm, 2006

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