William Henry Fox Talbot was an English chemist, who lived in the 1800s, who invented the process negative to positive for to make photographic copies of the same image.
Today on Wikipedia he is called a photographer. But back to those times, the word ‘photography’ was not yet invented. Fox Talbot was one of the many scholars who tried their hand at researching a possible chemical way to record an image with the light. He discovered and described a procedure that then became the most widely used for printing photographs, the negative to positive process.
The salt paper process
The process, called salt paper, was consisting on a salted paper (salted with table salt, hence the name ‘salt paper’) to be sensitized with silver nitrate. The compost so formed, silver chloride, is sensitive to the light. It has the attitude to become black when exposed to light. So he put such a sheet into a camera and obtained a negative picture. By putting that negative in contact with another salt paper and exposing such a sandwich of sheets under the light, he could achieve the positive image on the second sheet.
In those years other scholars and chemists were working on a method to record the image produced by the action of the light in a ‘camera obscura’.
A notable process was the daguerrotype. Invented by frenchman Louis Daguerre and presented to the world in 1834, it achieved immediate success. It was consisting of a silver plate inserted in the camera for to record an image using silver salts and a mercury process. The final result was an original picture on metal plate that encountered immediate success amongst the public.
It became so popular that the French Academy awarded Daguerre with an annuity for the merits that this discovery had as an impact on the whole nation.
The popularization of Daguerre’s method prompted Fox Talbot to publish his own discovery as well. In 1835 he presented it to the Royal Society and he too achieved immediate success.
But the daguerreotype was unveiled seven months earlier than the salt paper process, and had already become a commercial success. It was quick and able to deliver an on-location finished and direct positive picture on a stunning silver plate. The salted paper needed a reprint phase and was made… on paper. So, Fox Talbot’s negative did not meet the hoped-for worldwide success.
However, his salt paper marks a milestone in the evolution of photographic technique.
How did Fox Talbot arrived to the negative? A step by step short history of the invention of the calotype.
Johann Heinrich Schulze and silver nitrate
Everything originates with the studies on the properties of a compound of the silver: the silver nitrate.
The first scholar to discover the photosensitive properties of silver nitrate was the chemist Johann Heinrich Schulze, about a century before the discoveries of Niépce, Daguerre and Fox Talbot. It appears that he was making a silver nitrate mixture in a glass bottle that he left by a sunlit window. The sunny side of the bottle turned dark.
This effect is due to the reaction of silver nitrate with sunlight. Light, especially the ultraviolet component of sunlight, is capable of oxidizing silver nitrate, turning it into silver oxide, which is black.
This reaction is also called “direct blackening” in photographic technical jargon, because it does not require the use of developments or detectors.
Later on Schulze carried out some experiments by spreading a solution of silver nitrate on sheets of paper, on which he placed other sheets with drawings, in contact. On the sheet treated with silver nitrate the image of the drawing of the above sheet magically appeared. Its tones where inverted. The white areas of the upper sheet turned black, while the black marks of the drawing turned clear on the paper. The image was a negative.
However, such a craft was not lasting over time. After a few minutes the sheet was turning completely black.
William Henry Fox Talbot on the tracks of Schulze
A century later William Henry Fox Talbot resumed Schulze’s experiments by following the silver path. He tried all the silver compounds, but it was with the chloride that he got the best results.
Silver chloride is obtained by mixing sodium chloride with silver nitrate. Sodium chloride is the common table salt. So he basically spread a sodium chloride solution on a sheet of paper, then applied a silver nitrate solution to it. This led to the formation of silver chloride.
He placed a leaf on a sheet of salted paper and placed it all under the sunlight. After a few minutes the sheet turned black all around the leaf. When Fox Talbot lifted the leaf he found the outlined image of the leaf with its veins printed on the sheet of salted paper.
Nothing new under the sun, so far. The image was decaying very quickly. As like as his predecessor Schulze, Fox Talbot had the same problem. The image was not stable. After a few minutes the whole sheet was turning black.
John Herschel and the discovery of fixer
Much of the work of the chemists of the time focused on finding a method of fixing the image so that it became permanent over time. The definitive solution was found by the chemist and astronomer John Herschel. He suggested Fox Talbot to use a sodium hyposulphite solution which was supposed to remove all the sodium chloride that hadn’t reacted to sunlight. The method worked so well that hyposulphite is still used today to fix chemical photographs.
The positive image
The image of the light, semi-transparent leaf, against a black area, was evidently the negative image of the leaf. The areas directly exposed to sun became dark, the textured subtle details of the leaf resulted in a clear image. Fox Talbot then performed the second step: he took a new sheet of sensitized salted paper, put the negative with the image of the leaf in contact above it. He put such a sandwich under the sun again. Finally on the second sheet he got the inverted image of the upper sheet.
The dark areas of the negative sheet blocked the penetration of light and the underlying sheet remained clear. The clear and semi-transparent textures of the leaf on the negative let instead a certain amount of light passing through. The dark image of the leaf was imprinted on the underlying sheet.
The positive picture made from a negative was so invented.
In the early times, the process involving salted paper was pretty slow. The exposure of the salted negative in the camera was requiring very long exposures, in the order of many minutes. Fox Talbot then decided to use a ‘developer’ after the exposure of the negative, to help the formation of the image. In this way he reduced exposure times a lot.
Fox Talbot called ‘calotype’ the procedure involving a development of the negative. The calotype term comes from the Greek “kalos”, which means “beautiful”. Later this process will also be renamed talbotype in honor of its inventor.
One thing is certain: that day a technique was born that would then lead to the development of the entire photographic industry throughout the 20th century, namely the negative to positive process.
Printing of serial copies
The calotype did not encounter great commercial success among the public. The daguerrotype was pretty useful and nice. A paper negative was instead such an advanced means that many did not have an idea what to use it for.
The calotype, on the other hand, was widely used in the nascent printing and newspaper industry. Roger Fenton used it to photograph the Crimean War. The calotype, or salt paper, was a very convenient method of making pictures. It was easy for transporting the necessary materials to war zones. That was consisting ultimately in carrying sheets of paper and bottles with salt solutions. Least but not last, for the easiness of its preparation in the field.
Furthermore, the ability to print unlimited numbers of copies from a single negative original made the path to commercial photographic printing. The calotype could be reproduced indefinitely.
In the second half of the 1800s, almost all photographers engaged in travel used the calotype technique for their shots. The photographs taken by the photographers of the Fratelli Alinari of Florence for the albums of the Grand Tour are very famous.
The next step on improving the photographic process will arrive with the wet plate collodion, in 1851. The collodion process and the glass coating technique mark the definitive affirmation of the negative to positive process in the world of photography.