How Film Works
When you look at a black and white negative or print, all you are seeing is millions of tiny particles of metallic silver embedded in a thin layer of tough jelly. This emulsion is coated onto a thin sheet of plastic material or paper.
Although this may not sound very exciting, it is the basis of all photographs, even colour films and prints. From such mundane beginnings, superb photographs can be produced, and though the process of image formation is often taken for granted, it makes a fascinating story in itself.
What is an emulsion?
The simplest type of emulsion is that used for black and white photography. Emulsion is really the wrong word, because the light sensitive layer of the film or paper consists of minute crystals of silver compounds suspended in gelatin. For historical reason it is called an emulsion, not a suspension.
It is possible to make a simple emulsion in the kitchen sink. All that is needed is some common salt dissolved in a weak glue of warm cooking jelly.
If a solution of silver nitrate is stirred into this mixture, myriad tiny crystals of silver chloride are formed, turning the gelatin white like milk. The crystals are insoluble in water, so they separate or precipitate out. if the emulsion was made in a darkroom under a safelight, instead of in daylight, it could be coated on to paper, dried and use for printing.
The light sensitive compounds used in photographic materials are made by combining silver with chlorine, bromine, and iodine. These three chemicals are called halogens, and the compounds they form when combined with silver halide, silver fluoride, but it is of no interest to photographers because it is soluble in water and would wash out the gelatin layer. Of the three useful halides, silver bromide is the most sensitive to light and the most important constituent of any emulsion.
Silver grains The top picture shows the different sized crystals of silver halide contained in an emulsion. The lower picture shows silver around a part-developed grain
If a piece of undeveloped film is left in a sunny window for a few minutes it will turn slate grey. Light converts the silver bromide to very finely divided metallic silver. Bromine gas is liberated and this can be detected by its acrid smell.
In a camera, the exposures given to a film are not enough to produce instant visible silver. Instead, on each crystal that receives more than a few atoms of silver are freed. These collect at points on the crystal surface where there are impurities—such as silver sulphide—which have been formed during manufacture.
The images formed in a camera are invisible or latent images, and can be revealed only by treating the exposed film in a developer which continues the action of the light. Every crystal with one or more groups of silver atoms on its surface is quickly changed entirely to silver by reducing agent in the developer.
If a developer is allowed to act for long enough it will reduce all the halide crystals to silver whether they have been exposed or not, but its action on exposed crystals is from 10 to 100 times as fast. In the normally brief development times of a few minutes the exposed areas of a film are blackened and most of the halide crystals are changed completely to silver. In the areas received little exposure only a few particles of silver are formed. There is thus a ‘negative’ image in which the lightest parts of the film and the dark parts appear almost transparent or clear.
The gelatin, in which the light sensitive crystals are embedded, has several things to do. It holds the silver halide crystals in place and keeps them evenly distributed on the paper or film backing. It swells in the developer by absorbing water and allows the chemicals to have free access to the halide grains. It also helps development by increasing the rate at which exposed grains develop, while slowing down the unexposed ones.
Development must be stopped before the unexposed grains start to develop by rinsing the film or paper vigorously in water. It can be stopped even more quickly by using a very dilute solution of acetic acid—spirit vinegar—as a stop bath instead of water. This has the effect of neutralising the alkali contained in every developer and the action stops almost at once. Developing agents, almost without exception, have to be in alkaline solution to work properly.
Stop bath Development must be stopped before the unexposed grains start to develop by rinsing the film or paper vigorously in water.
After development there is a considerable quantity of silver halide left mixed with the silver image. If this was left on the film or print, it would slowly darken and eventually almost obliterate the image. The silver halides are not soluble in water so a fixing bath is used to convert them to freely soluble compounds. Some of these compounds diffuse out into the fixing bath, and it is the accumulation of silver salts in a fixer that limits its useful life, rather than exhaustion of the solution.
A final wash is running water must be given to all films and papers to remove soluble silver salts and also fixing chemicals. Any silver salts left in an emulsion slowly darken in light if not removed and fixer eventually decomposes and attacks the silver image, turning it brown in patches.