Invention in Camera: the Technical Achievements of Henry Talbot
To capture the first photographs on paper in 1835, William Henry Fox Talbot brought together two resources that had been known separately since the late Renaissance: the camera obscura fitted with a convex lens, and silver chloride, a substance that is darkened by sunlight. The early availability of both these items has raised the historical issue: why, therefore, was photography not invented much earlier? In the context of this exhibition of Talbot's photographs, it is appropriate to re-phrase this question: if, by 1835, there was little novelty in the idea of 'fixing' a silver image in a camera obscura, then what exactly was Talbot's contribution to the invention of photography? In seeking an answer, this essay will consider the two different photographic processes that he invented, and explain the highly varied appearances of Talbot's silver images, which any viewer of his work cannot fail to notice, and to wonder at.
As early as 1725, a primitive photographic effect with silver had been demonstrated by Johann Heinrich Schulze, who prepared a suspension of chalk in an aqueous solution of silver nitrate contained in a glass vessel. On exposure to sunlight, the surface of the chalk darkened where the rays fell. In subsequent experiments, Schulze proved that the phenomenon was indeed due to the light, not the heat, of the sun; and that the silver salt was essential. By masking the vessel with stencils, he created dark projected images of letters and words upon the temporary white surface. Schulze's own report of this experiment reveals, to our surprise, that he did not undertake it in the hope of inventing photography, but in order to prepare a luminous 'phosphor'. His observation of a darkening was the complete converse of what he anticipated, but nonetheless he pursued this unexpected result until he understood its cause. Then, having reached the brink of inventing photography in 1725, he gave up the entire study. Why? A motive can be inferred from the title of his publication of 1727, which translates thus:
'The discovery of the bearer of darkness instead of the bearer of light; or, a curious experiment concerning the effect of the solar rays.'
There is an hint of self-mockery in this description; Schulze had set out to investigate a light-emitting substance, but his endeavours were rewarded with nothing but darkness.
The negative view that Schulze took towards his own discovery illustrates a response that is deeply embedded in the human psyche: both literally and metaphorically, we avoid the dark, but we seek the light which is always seen to represent the spiritual, the divine, the good. In this context, therefore, it is historically significant that the best primary photographic processes are negative-working. That is, light is absorbed, and darkens a pale substance, rather than bleaches a dark one. Investigators like Schulze were reluctant to explore phenomena which they instinctively felt to be unattractive, or even repellent; it may have been this innate aversion to the dark that delayed the realization of camera photography. Several other claimants to the invention of photography, including Samuel Morse, also showed an intolerance of the negative image, which was strong enough to discourage them entirely from pursuing it further. The idea that had eluded these men did, however, occur to Henry Talbot in 1835, as evidenced by this now-famous quotation from his Notebook 'M':
'In the Photogenic or Sciagraphic process if the paper is transparent the first drawing may serve as an object to produce a second drawing, in which the lights and shadows would be reversed.'
This marks the conception of the negative-positive idea, by means of which a photographic image could be rectified and multiplied. In contrast to the antipathy of other investigators towards the tonally negative image, it seems that Talbot was entirely at ease with it, because he knew how to transform it back to normality. It is a matter for conjecture whether there was also some attribute in Talbot's personality that enabled him to contemplate the negative, as expressed for example in the words of Mike Weaver:
'...as a photographer-poet he was highly responsive to the melancholic strain in the mid-eighteenth century sensibility.'
Talbot may have experienced some echoes of the saturnine melancholia of the alchemical adepts, to whom blackening ­p; the nigredo of the magnum opus - represented death and putrefaction. Existing portraits of Talbot disclose a sad demeanour, which suggests the speculation: was Henry Talbot ultimately successful where others had failed, not only because he was a fine scientist, but also because of his preparedness to contemplate the dark side of things? While we may only guess at the psychological factors contributing to his invention, there are some physical facts of geometrical optics and photochemistry that we can take into account, for they presented a substantial impediment to the initial accomplishment of camera photography, and it was Talbot's first great achievement to overcome them all.
The photographic effect was first observed with substances in which the action of light alone suffices to produce a visible image by 'printing-out' ­p; in contrast to the chemical development of an invisible latent image, as in modern photography, which demands an altogether more subtle and complex procedure for its realization.
Talbot prepared his paper by first soaking a sheet of fine writing paper (J. Whatman's 'Turkey Mill') in a solution of common salt (sodium chloride), drying it, then brushing it over on one side with a solution of silver nitrate, before drying again. Thus, light-sensitive silver chloride was precipitated within the cellulose fibres of the paper. However, pure silver chloride will only yield a very feeble print-out image, no matter how heavy the light-exposure. To print-out a vigorous image, having a dense deposit of silver, requires the assistance of an appropriate 'impurity' to absorb the reactive chlorine which is also formed in the photochemical reaction. The paper itself, or its sizing agent, can fulfill this role to some extent, but Talbot knew nothing of this at the time. It was only his keen powers of observation and inference that led him to adopt the most effective formulation. He noticed that one of his images was stronger where, due to a chance uneveness in coating, there was a deficiency of salt in the paper, compared with the amount prescribed for chemical equivalence. Accordingly, he ensured that henceforth his solution of salt was dilute (ca. 1%) and his solution of silver nitrate was concentrated (ca. 20%). This counter-intuitive preparation he called his Photogenic Drawing Paper, and he used it consistently over a period of several years for making photograms, camera negatives, and positive prints from those photograms and negatives. In practical terms, to print-out a photogram of an object in contact with such a silver chloride salted paper, requires a direct exposure to bright sunlight of a few seconds for a perceptible image, and of about two minutes for full darkening of the background. This practice, to obtain photograms of botanical specimens or fabrics, or copies of engravings, was successfully employed from the earliest days of photography.
However, photography in the camera obscura must record an image projected by a lens, the aperture of which can only admit a small fraction of the total light scattered by the subject. Geometrical-optical factors will therefore attenuate the brightness of the image compared with contact-printing. Simply placing a paper coated with silver salts at the focus of a camera obscura does not inevitably guarantee a photograph. Thomas Wedgwood and Humphry Davy discovered this, to their disappointment, when they made the attempt around 1800, following their successful preparation of photograms on leather and paper:
'The images formed by means of a camera obscura have been found too faint to produce, in any moderate time, an effect upon the nitrate of silver.'
To make a camera negative, the duration of the exposure has to be about one thousand times longer than that needed to make a contact photogram of comparable density, given the same illumination and sensitive recording material. To secure even a feeble camera image with photogenic drawing paper, at an aperture stop of f/4, demands a typical exposure of about one hour in bright sunlight, but in duller light it may take a day. Photography with a camera is therefore barely feasible in practice using 'print-out' sensitizers. If the experimenter is to succeed with this marginal technology, at least seven requirements must be observed. Talbot's chosen modus operandi satisfied every one of them, as follows:
- He discovered that his 'Photogenic Drawing paper' sensitized with silver chloride and excess silver nitrate gave a much denser image than either compound alone.
- He adopted a very small camera format (Talbot's wife, Constance, referred to them as 'mousetraps') to concentrate the available light on a small area at the focal plane.
- He used lenses of relatively large aperture (ca. f/4).
- He exercised patience, aware that a long exposure might be necessary (ca. one hour to one day).
- He chose a very bright subject, rich in the ultraviolet and blue light to which silver chloride is most sensitive, namely: the sky. This served to outline, in dark contrast, either the roofline of his home, Lacock Abbey or trees on his estate, or the latticed panes of windows, viewed from indoors.
- He could see the potential of the resulting negative for re-printing, to reverse both the tonal scale and the handedness of the image.
- He discovered a method of fixation to stabilise his images sufficiently, as enduring proof of the achievement, as follows. Talbot reasoned that if a deficiency of salt made his paper more sensitive, then an excess of salt should achieve the converse. Accordingly, he treated his exposed photogenic drawings with a concentrated solution of common salt, and found that this did indeed inhibit their further darkening in the light.
But such prints do remain slightly sensitive to ultra-violet light, especially in the clear regions where there is much silver chloride. On their first exposure to daylight they acquire a slight deposit of silver, which appears as a lavender- or lilac-coloured 'veil' in the highlights, and which Talbot considered a rather attractive feature, contrasting pleasantly with the dull magenta in the shadows. Unfortunately this deposit of 'photolytic silver' tends to sensitize the print to visible light (a phenomenon known as the Becquerel effect) and so chloride-fixed photogenic drawings tend to fog on exposure to visible light, even when it is ultra-violet free. This sensitivity is sufficient to preclude them from being exhibited, even under the most strictly-controlled gallery illumination, which explains why this exhibition has made some use of facsimiles to avoid risking damage to the originals. The characteristic colours of such 'salt-fixed' photogenic drawings are due to the small sizes of the silver particles which, unlike modern silver prints, are much smaller than the wavelength of visible light.
Talbot also discovered that his photographs could be fixed with potassium iodide, which confers a characteristic primrose-yellow colour on the high values due to the presence of silver iodide. Although this substance is quite impervious to further darkening by light in the presence of excess potassium iodide, the silver in the shadows of these prints tends to be susceptible to bleaching by light; so, in contrast to the chloride-fixed photogenic drawings, which fog, these iodide-fixed prints tend to fade in the light.
This was the extent of Talbot's technical achievement by 1835, but even so, he only just succeeded. Severe under-exposure is typical of his photogenic drawing negatives made in the camera at this period; all are more-or-less outlines. Talbot admits as much retrospectively in his Introduction to the Pencil of Nature:
'But when the sensitive paper was placed in the focus of a Camera Obscura and directed to any object, as a building for instance, during a moderate space of time, as an hour or two, the effect produced upon the paper was not strong enough to exhibit such a satisfactory picture of the building as had been hoped for. The outline of the roof and of the chimneys, &c. against the sky was marked enough; but the details of the architecture were feeble, and the parts in shade were left either blank or nearly so.'
An entry in Notebook 'M' also implies that he yearned for more detail. To continue the passage quoted earlier:
'If an object, as a flower, be strongly illuminated, & formed in a camera obscura, perhaps a drawing might be effected of it, in which case not its outline merely would be obtained, but other details of it.'
We can now appreciate that Talbot's initial success in 1834/5 was achieved against the odds. Camera photography on photogenic drawing paper remained only a marginally feasible procedure for a further four years. It was a curiosity, of little practical value, known only to Talbot and his immediate circle. His reluctance to publish this discovery probably stemmed from dissatisfaction with his camera negatives, which lacked sufficient optical density and stability to permit the printing of many good positives. For this purpose, his method of fixation by excess halide was inadequate, and his text in The Pencil of Nature accompanying Plate XX makes this admission too, in retrospect:
'In the commencement of the photographic art, it was a matter of great difficulty to obtain good positive images, because the original or negative pictures, when exposed to the sunshine, speedily grew opaque in their interior, and consequently would not yield any positive copies, or only a very few of them.'
To his eternal regret, Talbot delayed publishing - until obliged to by Daguerre's surprise announcement of January, 1839. In August of that year at the meeting of the British Association for the Advancement of Science, Talbot mounted an exhibition of photogenic drawings which betrayed the limitations of his early camera photography. Of 93 listed pieces, the majority were photograms: same-sized images of botanical specimens, textiles, facsimiles of printed pages, or painted glass. All these were easily created by contact-printing in direct sunlight. The only images that were relatively easy to obtain by projection were those from the solar microscope, which employed directly focussed sunlight to provide a very high light intensity. Just one camera negative was displayed, and 21 transfers (Talbot's name for positive prints) from camera negatives, recording subject matter only of the highest contrast against the luminosity of the sky.
In February1839, Sir John Herschel introduced Talbot to the more efficient fixing agent, 'hypo' (sodium thiosulphate) for 'washing out' the excess silver chloride in an exposed photogenic drawing. Because it entirely removes the light-sensitive material, this method of fixation is technically superior to Talbot's halide fixation; indeed, thiosulphate remains the only fixing method still in use today. Talbot, however, showed a marked reluctance to adopt Herschel's 'hypo' fixer. His reasons appear to be both technical and aesthetic: if carelessly performed, without adequate washing, 'hypo' fixation could lead to a serious fading of the image; moreover, it invariably yielded an image colour that was a rather dull brown ­p; the beautiful colours of his salt-fixed silver images were completely lost in the process. Accordingly, Talbot persisted in employing chloride fixation for many of his 'transfers' for several years more. A positive print made on photogenic drawing paper is now generally referred to as a 'salt paper print', especially if it has been fixed with thiosulphate.
By 1840 the challenge of Daguerre's achievement appears to have spurred Talbot into attempting a wider range of photographic subjects. With the arrival of the excellent spring weather, he was able to progress his negative-making both technically and aesthetically, by studies of brightly sunlit scenes in the landscape, architecture, and still-life. The relative motion of the sun during these hour-long exposures causes both a multiplication of the highlights and a diffusion of the hard-edged shadows, which softens the contrast of these beautiful images, embuing them with an almost surreal light. Talbot's camera negatives on photogenic drawing paper record the illumination of scenes in a way that cannot be perceived by the human eye, or by a modern camera with its 'instantaneous' vision.
This widened scope of Talbot's pictorial work was enabled by some improvements in the sensitivity of his photographic papers, including the application of multiple coatings, and the addition of gallic acid to his sensitizer. But most significantly, in 1839 Talbot made the first photographic use of the more sensitive salt, silver bromide, which has since become the major component of all modern photographic emulsions.
After five years of sporadic experimentation, determinedly worrying at the problem of camera negative-making, Talbot was eventually rewarded by chancing upon that wonderful phenomenon, so rare in photochemistry, and so peculiarly well-developed in silver halides: the 'latent image'. This is now known to be a tiny, invisible cluster of a few atoms of silver produced at the surface of each crystal of silver halide that has absorbed some light. The image is only rendered visible when the affected crystals are 'developed' with a suitable chemical reducing agent, which transforms them entirely into grains of silver metal, but leaves the unaffected crystals unchanged. The latter are subsequently dissolved out by sodium thiosulphate fixer. Thus, the effect of the light is amplified enormously, enabling much shorter exposures. Chemical theory in Talbot's day lagged far behind the practice of photography, however, which is why the apparently inexplicable phenomenon of 'latency' was referred to by Talbot and his peers as 'natural magic'.
Talbot's discovery of the photographic development of a latent image in silver iodide by means of gallic acid, dates from the days following 20 September 1840, and immediately enhanced the speed of his photography by a factor of about a hundred. He could now direct his camera towards any static subject, confident that the details of the mid-tones, and even the shadows would be recorded. Most importantly, he knew that a stable and printable negative, which he called a calotype, would result from an exposure of a minute or two in bright sunlight. It will be noted that calotypes are usually neutral grey-black in tone, in contrast to the colours of photogenic drawing negatives, and the brown tones of salt paper prints. This is due to the larger size of the particles of silver metal yielded by the development process, which also confers on the image a greater resistance to deterioration.
Talbot's shortened camera exposures no longer entailed blurring of the shadows by the sun's apparent motion; accordingly, strong chiaroscuro effects can be seen in some of his early calotypes, compared to their mellow precursors in photogenic drawing. The sensitivity of the calotype also enabled negatives to be made 'without sun', and in larger formats than hitherto. Paper photography at last became a viable practice, even for portraiture and 'conversation pieces', promising economic commercial applications that would eventually displace the well-established, but expensive and unique daguerreotype. In 1844 Talbot set up the 'Reading Establishment' to produce the editions of prints needed to illustrate his publication, The Pencil of Nature, which displayed the full range of photography's capability. In the interests of best stabilising the salt paper prints to light, sodium thiosulphate fixation was used exclusively, but this was not without its problems too, because inadequate washing of the prints could leave excess thiosulphate in the paper. This impurity would ultimately react with the silver to form silver sulphide, which resulted in a yellowing and fading of the image. This problem was later overcome by others with the introduction of toning with gold.
Thus, Henry Talbot set negative-positive silver photography on the road towards technical refinement and commercial exploitation for the next 150 years. The process has arrived at a point today when our camera exposures are close to the lower limit achievable in theory, and are about ten million times faster than those that Talbot had originally found necessary in 1835. Although Henry Talbot was not the first to make camera photographs, his determination and powers of inductive scientific reasoning, coupled with a stroke of serendipity, enabled him to create a visionary photographic oeuvre, and an unsurpassed pictorial legacy of the mid-nineteenth century.
My book 'Mechanisms of image deterioration in early photographs
- the sensitivity to light of W.H.F. Talbot's halide-fixed images 1834-1844',
is published by the Science Museum and the National Museum of Photography,
Film & Television, 1994.
ISBN 0 901805 78 5, paperback.
To purchase 'Mechanisms', please visit the Siderotype.com website.back to top