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, British doctor l? Leach maddox (1816 ~1902) introduced in the British photographic magazine that when gelatin emulsion containing silver bromide is prepared and coated on a glass plate while it is hot, the chemical substance will not crystallize like collodion when it is 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, it has three important advantages in use: 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? Injury (f? Hurter) and C. Derifield (C? Driffield), developed the world's first emulsion speed system with photosensitive characteristic curve, so that all kinds of photosensitive materials produced thereafter can be marked with reliable "H and D emulsion 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.