Prints of Gold: the Chrysotype Process Re-invented
Throughout the traditions of alchemy, metallic gold has always enjoyed a mystical identity with the sun. Although the alchemists seem to us now irrational in their view of gold as the mineral embodiment of the sun's light, it is a view which proved prophetic, at least in a poetic sense. In 1842 the strongest source of light known to humanity was the sun, and the brilliant summer of that year provided enough sunshine for one of the finest scientists of the day -Sir John Herschel- to invent a novel method for making photographic images in gold. He named his new process the Chrysotype (from the Greek word for gold, which has also given us 'chrysalis' and 'chrysanthemum'). Herschel's discovery followed just one year after Talbot's first announcement of the invention of photography using silver. At that time it was not obvious which of the many light-sensitive substances discovered by the experimentalists of that era would provide the best foundation for a viable photographic process -whether it should be platinum, mercury, gold, silver or iron.
Talbot soon discovered the chemical fluke -the 'latent image'- that can make silver halides uniquely sensitive to light, and thus he set the negative-positive silver process on the road that reaches down directly to us today. No viable chemical alternative to the silver halide emulsion has yet been found for capturing an image 'instanteously' in the camera. But when it comes to making a photographic print from the negative, the circumstances are less demanding, and long exposure times and intense light sources are useable, so bringing into play many possible alternative substances which are much less light-sensitive than the silver halide print-making technology used exclusively by the photographic industry today. Historians of photography will know that over the last 150 years many alternative processes have been devised for making photographic prints, many bearing the picturesque names coined by their inventors. Besides Herschel's Anthotype, Phytotype, Cyanotype (forerunner of the blueprint) and Argentotype, there was Willis's Platinotype, and Nicol's Kallitype, to name only a few. Most of these alternative processes have languished in obscurity for many decades, eclipsed by the speed, convenience and quality of modern industrially produced photographic printing papers. But today we are seeing a minor renaissance in alternative photographic printing, brought about by photographers who are dissatisfied with the monotony of the commercial product, and wish to make prints which are beautiful as objects in their own right. Foremost among the re-discovered processes is the Platinotype, but Cyanotype, Salt Prints and Van Dyke prints have also attracted practitioners. Herschel's Chrysotype process is one of the least-known of these 'lost' options, totally by-passed in the mainstream of photographic development. It was never carried into successful practice owing to technical difficulties in controlling the contrast, colour and fogging of the image, despite sporadic attempts during the 19th century to resolve the difficulties. Today the only historical specimens of the Chrysotype process known to me are those made by Herschel himself, which are in the collections of the Museum of the History of Science in Oxford and the National Museum of Photography, Bradford. Few of Herschel's images are actually continuous tone photographs; most were made by using an engraving as a 'negative'.Can this use of gold for photographic image-making amount to anything more than just an historical or romantic curiosity? I believe the medium does offer some specialised advantages, and the first of these is permanence.
The besetting problem of conventional photography has always been its relative impermanence. The minute silver particles which make up monochrome images are very susceptible to attack by atmospheric pollutants and residual chemicals in the paper and mount, especially sulphur compounds. Many readers will have seen faded specimens of yellowing albumen prints from the last century, and have regretted that so much of our visual history is in jeopardy. Most present-day colour prints will also prove ephemeral unless great care is taken with their conservation, because they use organic dyes that have a limited life from the archival point of view. It was the recognition of impermanence during the earliest days of photography that induced the Photographic Society to set up the (delightfully named) Fading Committee of 1850; whose recommendations included the 'toning' of prints with gold salts, to coat the silver particles with a protective layer of gold, which is quite impervious to attack. Albumen prints that have been treated in this way usually have a rich purplish-brown colour, with little evidence of fading. Later achievements in the quest for photographic permanence include the carbon process, due to Swan in 1864, and the platinotype process invented by William Willis in 1873, which has the justifiable reputation of providing the most beautiful of all photographic images, in the most permanent of metals - platinum.
It was my enthusiasm for updating the platinotype process that eventually led me to investigate the possibility of printing in pure gold, which resembles platinum chemically. Herschel's Chrysotype recipes are quite well documented, but their use unmodified leads to a disappointing quality, compared with the platinotype. I found it necessary to apply some 20th century chemistry to the problem in order to 'tame' the vigorous and unpredictable behaviour of the gold salts. After six years and many thousands of experiments, I arrived at formulations for making gold prints of good gradation in a wide range of colours. It must be stressed, in view of common misapprehensions, that this is not gold toning -in the sense of a retrospective manipulation of an existing silver image- but a 'straight' printing medium. The colours so obtained, including pink, magenta, brown, purple, violet, blue and green, are not of course 'true to life' -they might even be described as surreal- because this is still essentially a monochrome printing process, using a conventional 'black and white' negative. One of my aims in using this medium is to challenge the current presumption that monochrome photographs should always be black and white; in the last century, brown and white was, as an inevitable consequence of the chemistry, the fashionable colour! True black and white, in the form of the platinotype, was so contrary to popular taste when first introduced that William Willis had to work hard to modify his process to yield an acceptable brown, in the form of his Sepia Japine Platinotype paper.
Non-literal colours, which have their own emotive connotations, can reinforce an artist's expressive intent and offer another dimension for the image-maker to explore. To draw a musical analogy: the choice of colour for a monochrome print can be likened to the choice of key for a musical composition.
In ordinary experience we are accustomed to different substances, for example artists' pigments, displaying different characteristic colours. It may therefore seem incomprehensible that a single, pure substance -gold, in a finely divided state- can appear with so many different colours. The scientific answer to this puzzle lies in a rather heavy piece of mathematical physics worked out by Gustav Mie in 1908, but it is possible to gain a qualitative understanding of it without a technical background. The key to the phenomenon lies in the size of the microscopic particles of the metal that form the image, because their ability to absorb different wavelengths of light depends on their dimensions. The 'sea' of electrons in a particle of metal resembles the water slopping to and fro in a container; it is intuitively apparent that the natural wavelength of this motion will be smaller in a little basin than in a large bath. Now light is absorbed when the electric field associated with it can excite the motion of the charged electrons of the substance; thus the smallest gold particles absorb short wavelength light (blue), so take on the complementary colour - pink or red, and the larger particles absorb long wavelength light (red), so appear blue. Mixtures of particles, or a departure of their shape from spheres to ellipsoids, can produce purple, violet or even green colours. The differing shapes and sizes of the gold particles are the result of the chemistry and conditions of the process, and so a control of the resulting colour can be established.
The use of colloidal gold as an enduring and refractory red pigment has a long history, dating back to the seventeenth century, at least, when it was first employed to make a beautiful ruby red glass, which can still be seen today in many stained glass windows. This preparation of colloidal gold and tin hydroxide, called the Purple of Cassius, is also used as a colorant for decorating glazed ceramics; I am exploring the possibility of using the chrysotype process for making photoceramics in a new way; this is a beautiful process which was very popular at the turn of the century using platinum, iridium or refractory pigments.
In addition to permanence and colour, the other characteristic of Chrysotypes is their surface texture. All commercial photographic printing papers yield images consisting of small silver particles bound within a layer of hardened gelatine which coats the paper and confers on the print its uniform glossiness. In contrast, the Chrysotypes consist of minute particles of colloidal gold trapped within the uppermost fibres of a fine-art paper. The surface is therefore totally matte and non-reflective, and displays the characteristic texture or 'tooth' of the paper, which can be chosen by the print-maker. Such images may seem to have more in common with etchings or engravings than with conventional silver-gelatine photographs.
The procedure for making a Chrysotype closely resembles that for the more familiar Platinotype or Cyanotype: a sheet of artists' watercolour paper is hand-coated with a solution of light-sensitive chemicals, dried, and exposed through a negative in a contact-printing frame to a source of ultra-violet light such as a sun-lamp or even -reverting to the earliest photographic technique- the sun itself. A wet-processing procedure then follows in order to develop the image and remove all excess chemicals. The print is finally washed and dried. There is no need for a darkroom, thanks to the low sensitivity of the material.
The chief disadvantage of this, and other alternative processes is the need for a full-sized negative: direct projection enlargement of a miniature negative is not possible with accessible technology. This limitation runs contrary to the direction of popular 35mm photography and may have dissuaded many photographers from trying any sort of alternative 'non-silver' printing processes; it is also probably the main reason for their commercial unprofitability. However, enlarged internegatives can be quite easily made from 35mm or roll-film originals onto material such as Kodak's Professional Direct Duplicating Film, Type 4168. The other strategy of using a large format camera is admittedly costly, and confers its own special limitations and modus operandi, but the quality of the results can be breathtaking. If the process seems labour intensive, it should be remembered that time spent handcrafting the picture provides a great source of satisfaction in being 'true to ones' materials' when pursuing this alternative way of making photographic images.
First Published in The Scottish Photography Bulletin no 1 (1991).
A complete history of the discovery and development of the chrysotype process, together with much more information on the role of gold in the graphic arts, is contained in my book Gold in Photography obtainable from the Siderotype website.back to top