How do photographic photosensitive materials go from wet plate to dry plate?

The wet printing process of collodion came into being at the beginning of 19. Its appearance makes more people enter the field of photography. In recent 20 years, collodion photography has been the only practical photography method.

Gelatin was used to replace collodion in the 1970s. It makes it unnecessary for photographers to prepare their own photosensitive materials before each shooting. At the same time, the exposure time of photography is greatly shortened, which makes it possible to take pictures with a hand-held camera.

The appearance of gelatin dry printing technology marks the beginning of modern photography.

(A) collodion wet printing process

185 1 year is an epoch-making year in the history of photography. In March this year, the collodion photography by Scott Archer (1813 ~1857) was published. As a result, in a short time, this kind of photography replaced all the previous photography methods.

Collodion is a viscous liquid, which is made by dissolving nitrocellulose in ether and alcohol. Archer invented the collodion photography method. First, collodion containing silver iodide was coated on the glass plate to spread it evenly on the whole glass surface. Immediately after that, it was immersed in silver nitrate solution to enhance sensitivity. The drier the collodion, the lower the sensitivity. Therefore, before the collodion is dry, put the wet glass negative into the camera for exposure, and then develop and fix it immediately. This method is also called "wet printing".

Collodion photography is more complicated and difficult to handle than Daguerre photography or Carlo photography, but it has four main advantages:

1. The negative image is extremely clear, and it can be printed permanently and clearly indefinitely.

2. The price is low.

Archer has no patent for collodion photography, so anyone can use it for free.

4. Its sensitivity is higher than that of silver plate photography and Carlo photography. When shooting scenery or portraits outdoors, it only takes a few seconds or dozens of seconds of exposure time. At that time, it was the shortest exposure photography.

Collodion photography not only has a clear image of Daguerre photography, but also can print cheap photos on photographic paper indefinitely like Carlo photography. Its appearance eclipsed both photography methods. A few years later, no one took pictures of Daguerre or Carlo.

When using collodion for photography, the master must be made at the shooting site. Therefore, when a landscape photographer goes out, he should take a camera, tripod and many necessary lenses, as well as many instruments: large and small bottles, scales, measuring cups, rulers, washbasins, darkroom tents and so on filled with emulsion, developer and fixative. At that time, a photographer needed photographic materials a day, weighing between 100-20 kg. Many of them trudged on the shooting scene with heavy boxes on their backs. Others rent trucks to load equipment. For example, during the Crimean War, Fenton divided the carriage into darkroom and bedroom, and rode around the battlefield in the carriage.

The collodion photography invented by Archer has made great contributions to the progress of photography technology. However, he did not apply for a patent for this invention. 1857, Archer died in poverty at the age of 44.

(2) Gelatin dry printing process

Since the invention of collodion photography, people have been exploring a better coating material, which can be used "dry" after coating to get rid of all kinds of inconvenience when using collodion. 19 in the 1970s, this wish finally came true, and the jujube gum dry printing process came out.

Gelatin is a transparent colloid extracted from animal bones and skin. In normal temperature water, gelatin is easy to absorb water and swell, and then it returns to its original state after drying. This characteristic of gelatin makes it a new sensitizer carrier to replace collodion, thus solving the problem of wet exposure, development and fixing when collodion was used before, and making the "wet version" become a "dry version".

187 1 year, a British doctor, leach maddox (1816 ~1902), introduced in the British photography magazine, prepared gelatin emulsion containing silver bromide, coated it on a glass plate while it was hot, and then used chemicals after it was dried. When washing, the emulsion will expand properly to facilitate the reaction between developer and fixer. At the same time, maddox also completely released his gelatin emulsion formula.

Maddox's invention aroused great interest. According to maddox's method, it is found that in the process of preparing emulsion, prolonging the heating time can greatly improve the sensitivity of emulsion, and the silver bromide photographic film made by this method can be stored for a long time after drying. So photographers don't need to prepare and apply emulsion before each shooting, they can buy a box of ready-made dry plates in the store.

The quality of dry printed images is as good as that of collodion. At the same time, in use, there are three important advantages:

1. You don't have to bring darkroom tents and chemicals when you go out to shoot. You can develop your own films at home or ask others to develop them for you.

2. Gelatin dry plate has a fast photosensitive speed, and the outdoor exposure time can be shortened to several tenths of a second. Photographers can shoot by hand, and tripods are no longer necessary.

3. Gelatin dry plates can be produced on a large scale, and the dry plates manufactured by the factory are more stable and have better quality than those prepared by themselves.

The appearance of gelatin dry plate also brings a problem, that is, it requires a unified standard of film sensitivity. At that time, various film manufacturers advertised their products as "express delivery" when they sold them, but it was hard to say how fast they were, which made it difficult to send them to other places for development after filming. At the same time, its photosensitive speed is fast, which requires the camera to accurately control the "fast" exposure time, and to control the exposure, the sensitivity of the film must be determined first.

1890, two British scientists, F. Fult and C. Riffield, developed the world's first photosensitive speed system by using the photosensitive characteristic curve, so that all kinds of photosensitive materials produced thereafter can be marked with reliable "H and D photosensitive speed figures". Subsequently, the exposure meter and exposure meter were also made.

In the 20th century, two kinds of photosensitive speed systems, DIN and ASA, appeared (DIN was formulated by German industrial standard 193 1 and ASA by American Standards Institute 1942). 1980, the International Organization for Standardization merged DIN and ASA into an international standard photosensitive speed-"ISO".

Gelatin emulsion can also be coated on photographic paper. /kloc-In the middle of the 9th century, according to this feature, the British "Liverpool Dry Plate and Picture Company" developed silver bromide photographic paper which can be quickly exposed. This kind of photographic paper can be enlarged on a gas lamp amplifier.

The advantages of gelatin dry printing make it popular all over the world. With the emergence of gelatin dry plate production enterprises, wet plate technology was quickly left out of the cold and quietly withdrew from the historical stage.