Photographic techniques

DAGUERREOTYPE [c.1839 and c.1860]

Invented by Jacques Louis Mandé Daguerre and publicly announced in 1839. The image is simultaneously positive and negative. Daguerreotype was the first stable photographic technique, in which a copper plate is covered by a fine layer of polished silver, whose surface has been sensitised by exposing it to iodine vapours. The exposure is made 15 minutes after this treatment. The plate is then developed through exposure to mercury fumes heated to a temperature of 90ºC, for a period of roughly three minutes. The image is then fixed in a bath of sodium thiosulphate. After this, the plate is covered in a solution of gold chloride and heated over a flame, on the reverse of the image, where a fine gold layer is deposited on the surface. Finally, the daguerreotype is placed in a decorative casing with a protective glass cover. The standard formats of Daguerre’s plates were 16.2 x 21.5 cm for the larger plates, or 7 x 8 cm for the more frequent smaller plates. [P.N.]

CALOTYPE [c. 1840 – 1860]

The first photographic process that allowed for the reproduction of multiple positive images (known as salted papers) from a negative copy on a paper support (calotype). This invention of William Henry Fox Talbot (1800 – 1877), which is often referred to as talbotype or photogenic drawing, was developed in the same period as the daguerreotype technique. The process made it possible to work with shorter exposure times, given the greater sensitivity of the paper negative; however, it had the disadvantage of resulting in an image that was less sharp than the daguerreotype. In the first phase, the paper is sensitised in a solution of silver nitrate, then dipped in a solution of potassium iodide, dried and kept far removed from the light. The reaction of the two solutions produces photosensitive silver iodide. Hours before exposure, the paper is once again sensitised in a dark room, this time in a solution of gallo-silver nitrate and the image is revealed in another bath of this solution and fixed with heated sodium thiosulphate. The translucent appearance of the paper is due to the application of a protective layer of beeswax or oil that made this support ideal for the printing of positives.  [P.N.]

WET COLLODION [1850-1895]

In 1851, Frederick Scott Archer (1813-1857) invented a method for obtaining collodion negatives on glass plates that rapidly spread throughout the photographic industry. This is a fairly complex process, as the plates have to be exposed and developed while still wet. However, the optimum image resolution obtained in a short exposure time overcame the fragilities of earlier techniques, revolutionising photography and drawing it closer to the capture of an instant image. The surface of a thick glass plate is impregnated by hand with a solution of collodion (pyroxylin or gun cotton, alcohol and ether). After drying, the plate is sensitised in a bath of silver and immediately exposed to light, while still wet. The image is then developed in a solution of iron protosulphate and washed in abundant water, after which it is fixed in an abundant bath of sodium thiosulphate and then varnished. The processes most commonly used for obtaining wet collodion positives were the making of contact prints on albumen paper, which became widely popular in the famous carte-de-visite format patented by Disdéri (1819-1889), the ambrotype and the ferrotype, or the photomechanical printing technique of the phototype. Between 1850 and 1860, various experiments were made to develop the dry collodion process, which made it much easier to sensitise the plates. [P.N.]

SILVER-SALT GELATIN [c. 1875 until the present day]

Although contributions also came from various other sources, Peter Mawdsley (1824-1881) was largely responsible for the development of the first process for obtaining negatives in gelatine silver-salt, in 1873. This process was the foundation for all the modern processes of analogue photography developed until the present day, and has had various positive applications based on the same chemical principle. Initially, it was used on glass plates and then on films made of nitrate, cellulose acetate and polyester. The photosensitive substance used for obtaining negatives was initially a warm gelatin solution to which an alkaline bromide and silver nitrate were added, giving rise to a silver bromide precipitate, which was highly sensitive to light. The first glass plates could remain sensitised for several months and could be developed some days after exposure. In the late 19th century, a new flexible photographic support was developed: the cellulose nitrate film, a light material that was available in a small roll and easy to transport, which allowed for the taking of various negatives. Other cellulose derivatives, acetates, appeared in the 1920s, replacing the highly inflammable and perishable nitrate film. Later, the development of the plastics industry brought resistant polymer supports in the 1960s. [P.N.]