The idea that photography was made up of two elements, the negative and the positive, was neither obvious nor instantly grasped at the onset. Already in 1833 the inventor and amateur painter William Fox Talbot carried out experiments using sheets of paper treated with kitchen salt and silver nitrate, and exposed to light for hours, covered with small objects, such as leaves or lace, until their outline appeared on the paper. However, the negative images thus obtained were not stable, and the action of light shortly blackened them completely. Moreover, exposure times were extremely long.
The revolution came about when Talbot and the scientist John Herschel met. The latter discovered that sodium hyposulphite fixed the image obtained from light on the sheet sensitised with silver. Talbot also understood that gallic acid could chemically develop the latent image that formed on the negative after just a few seconds of exposure to light, thereby significantly shortening resting time. Thus, in 1841, he finally presented the first negative in history: the calotype (from the Greek kalos: beautiful), a sheet of refined paper treated with silver. It was placed in the camera, exposed to light, developed and fixed. Then for printing, it was placed in a press, in contact with another sensitised sheet, and was exposed again to light until it obtained the positive image onsalted paper.
Herschel also coined the terms "photography", instead of "photogenic drawing"; "positive" and "negative", instead of "reverse copy" and "re-inverted copy".
The binomial negative-positive turned out to be a winning choice because it offered the possibility of obtainingan unlimited number of printsfrom only one negative. It persisted throughout the history of analogue photography; what changed over time was the medium. In fact, in the 1850s glass was used to compensate for the lack of sharpness of the paper, and from the mid-twentieth century the use of the more practical photographic film became widespread.
The Archive hasmore than 40,000 negatives dating from the mid-nineteenth century to present day.
In the picture: negative on silver nitrate gelatine glass, with cardboard mask, albumen positive, mounted on cardboard with the name of the photographer Antonio Strazza. The picture shows the plaster scale models of parts of the façade of the Duomo designed by Giuseppe Brentano, located in the courtyard of the Fabbrica’sPalazzo dell'Amministrazione (Payment order no. 187 of 1903).