Talbot's "photogenic drawing" process, which he had worked on since his return from Italy in 1834, experimented with once more in January 1839, and improved in 1839-1840, formed the basis for his future work, as, little by little, details of the daguerreotype reached him. Photogenic drawings were images formed by the action of light on sensitized paper - in other words, silhouettes on a dark background which appeared negative.
William Henry Fox Talbot | Flowers, Leaves, and Stem c. 1838
In theory, this image was reversible by exposing a second sheet of photosensitive paper placed beneath it. However, Talbot's negatives were insufficiently dense in the shadows to allow good positive prints to be produced (one negative survives, from August 1835, which shows a lattice window, in which Talbot was delighted to be able to distinguish the individual panes of glass. Apparently, however, this was taken no further.) It was at this stage, in 1835, that, discouraged perhaps, Talbot abandoned his photographic work in order to devote himself to mathematics and classical languages. Then, at the end of 1838, rumors of a French invention reached him.
At the beginning of 1839 the question of priority over Daguerre provided the impetus for research, intended to demonstrate the originality of the principles established by Talbot. In March he investigated the possibility of adding gallic acid (which John Herschel had suggested) to silver nitrate. He also began studying how to transfer from the negative to the positive in order to return to "the original disposition" of the image (in August 1839, before a meeting of the British Association in Birmingham, he referred to "a transfer or reversed image"). The method was sufficiently efficacious for Talbot's process to be considered superior to the French one, notably in its "capability of multiplication of copies, and therefore of publishing a work with photographic plates". It was practicable enough, too, for Biot to encourage Talbot to provide a more sensitive paper process intended for travelers and artists, "the rapidity of the operation being a condition of success".
In September 1840 Talbot discovered, by chance, a process "to which [he] gave the name of calotype" (from the Greek kalos: beautiful, good, useful). His laboratory notebook records the progress of the discovery, from September 20 to October 11, 1840. Exposures of one hour were suddenly reduced to a few minutes or seconds; the image support was paper prepared with silver nitrate, acetic acid, and gallic acid. An image did not appear during the exposure in the camera obscura, but the silver salts, on which the light had worked for a very short time, were darkened at a later stage during "development" in gallic acid, which speeded up the reaction.
Apart from the ease of obtaining a negative which could subsequently be used to produce as many positive images as one liked, the value of the process lay above all in the other characteristic linked to the use of an accelerating agent (gallic acid) - the concept of the latent image. This separated the two operations of taking a photograph and making the negative visible. Today this remains fundamental to the photographic process. Through Biot, the Academy of Sciences in Paris was informed of this discovery, news of which was beginning to spread: "No impression may be seen, not even the slightest beginning of the picture. And yet the picture already exists there in all its perfection, but in a perfectly invisible state. By simple procedures, which I shall later disclose, the picture is made to appear as if by magic. It really is the most marvelous thing you could ever see." Talbot immediately took a few portraits (his wife Constance on October 6, 1840; Amelina Petit, on October 13, 1840; a servant standing near a coach, October 14, in 3 minutes), and he photographed an elm tree using an exposure time of one minute.
An English patent was taken out for the calotype (Calotype Photographic Process) on 8 February 1841 (the very same day when, in the feverish quest to prove priority, Bayard made a communication to the French Academy of Sciences on a similar process), and it was patented in France on August 20, 1841. The text specifically mentions gallic acid, development, fixing, and application to portraiture, but, curiously, does not protect the one feature which, in our view, makes the calotype so original: the intermediary function of the negative. A second patent, taken out on June 1, 1843, gave a more detailed account of the procedure and introduced methods to make the negative more transparent for printing and to produce "photographs" which could be included in publications, something which Talbot would shortly achieve with his Pencil of Nature begun in 1843 (a patent was taken out in the United States in 1847 and subsequently taken up by the Langenheim brothers). The successive operations required for the production of a calotype were as follows (patent no. 8842):
1. Brush a solution of silver nitrate on to a sheet of paper. Leave it to dry, then apply a solution of potassium iodide. This paper can be kept for a long time and is not particularly light sensitive.
2. To use, coat the iodized paper, in darkness, with a mixture of silver nitrate, gallic acid, and acetic acid. This makes the paper very light-sensitive.
3. This "calotype paper" is placed in the camera obscura, in the focusing plane of the lens which has been properly adjusted, and the exposure made.
4. The exposed paper, on which no image is visible, is treated in the laboratory with a further coating of gallo-nitrate of silver, which makes the image appear in a few minutes.
5. Finally, the image is fixed in a solution of hypo, then washed in water and dried.
The procedure for printing a positive from the negative is worth describing in detail, since it would in future form the basis of most photography. Prints were made on salted paper - paper soaked in a solution of sodium chloride (common salt) then coated (using a brush) with silver nitrate solution and dried. The negative is placed in a printing frame, and exposed to the sun for about half an hour or longer, until a satisfactory image appears. The exposure time is judged by inspecting the color of the positive image. The print is then fixed with hypo, thoroughly washed, and dried.
William Henry Fox Talbot | Ships at Low Tide c. 1844