There are numerous substances that are altered by the action of light. We see this when leaving certain objects in the sunlight for extended periods of time, resulting in color fading or alteration. However, photographers need to employ a substance that changes more quickly to light. The salts of silver have been the mainstay of this process for over one hundred years.
In 1727 a German chemist, Johann Schulze, dissolved silver in nitric acid and mixed this silver nitrate with chalk. He placed stencils around a bottle of this solution and left it in the sun, resulting in the recording of the stenciled patterns. In this instance, the sunlit silver nitrate decomposed and was able to show up against the white chalk. The inability to fix the image made this a mere curiosity.
In the early part of the 1800’s it was found that silver nitrate’s sensitivity could be increased by combining it with a halogen element (chlorine, bromine, iodine, etc.). This combination is known as a silver halide (silver chloride, silver bromide, silver iodine, etc.).
The process was also broken down into several components. The silver halide could be exposed to light, the decomposition could be amplified by an appropriate chemical, and the unexposed halide could be removed. This is the basis of the development of photographic films and paper today.
The most important element in film and paper is, of course, the emulsion. The three major events in the creation of this material is the preparation of the emulsion, ripening it, and adhering it to the support.
Bar silver dissolves easily in dilute nitric acid. This solution is then placed in distilled water, where crystals are formed. Gelatin is then dissolved along with a halogen material (usually potassium chloride, potassium bromide or potassium iodide). When the silver nitrate solution is added, a precipitate of silver halide crystals form. There are three common silver halides. Each is used, sometimes in combination with another, for a particular purpose.
Films contain silver bromide and silver iodide
Enlarging papers contain silver bromide
Warm Toned papers contain sliver bromide and silver chloride
The result of the above process is an emulsion that is very slow, has a great deal of contrast, and is sensitive only to blue light.
As all the crystals are of equal size, at a certain point exposure to light all would decompose them equally, producing a product without any tonal range. The ripening process heats the emulsion, which dissolves some of the silver halides. These dissolved particles re-form with other grains, resulting in a larger grain size. The larger grains are more “light sensitive” because of their increased surface area. A variety of grain sizes yields differing speeds amongst grains. This not only gives the emulsion speed increase, but also a tonal range, as the grains decompose at differing light intensities. This is why “fast” films are grainy and slow films are considered “fine grain,” and explains why slow films by definition offer high contrast.
After ripening, the emulsion is chilled, shredded, and washed to remove impurities. It is again melted, which continues the ripening process, to a certain point that will determine its final speed. Finally, special dyes are added which extend its sensitivity into the green and red wavelengths. Other “minor” improvements are done at this time.
Binder And Support
Placing the silver halide on a support, such as film or paper, is not a straightforward process. In a solution of water, these crystals form a creamy curd that is impossible to be spread evenly. When dry, it crystallizes and is washed away with the developing solution.
To solve these problems, it is necessary to place the solution within a “binder” that holds these crystals (sometimes called “grains”) in an even suspension and can be attached to the support. Of course, the binder must allow liquids to enter and leave without moving the grains. The substance that combines all these factors is gelatin. It prevents coagulation, is transparent, and permeable. This combination is known as a “silver halide emulsion.”
The composition of the emulsion, a highly guarded secret by each film manufacturer, is a mixture of gelatin and silver halide. The emulsion is applied to a paper or film base, normally 120 centimeters wide and 600 meters long. Affixing the emulsion must be done in a room of pharmaceutical cleanliness.
Originally, film base was made of cellulose nitrate, a highly flammable substance. Early in this century Kodak replaced this with cellulose acetate, dubbed “Safety Film." Today, the most common film base is a non-flammable polyester. Sheet film commonly employs a polyester base for greater dimensional stability. The film is given a coat of gelatin and cellulose estar, then the emulsion is coated over this “subbing” layer. This has, in the past, been a mixture of emulsion and base material in alcohol, though now the formula is proprietary. Finally, a thin “supercoat” of gelatin is added to protect the film during handling. An anticurl layer made of gelatin is placed on the non-emulsion side of the film to prevent it from curling when dry.
The layer also includes a dye, normally dyed dark green or gray. This dye absorbs light, preventing it from reflecting off the back of the film and passing again through the emulsion. In 35mm and 120 film sizes, this dye may be situated in any of the areas of the film. As this reflection would cause “halos” to be formed around the image’s highlights, this is normally referred to as an anti-halation backing.
Cross Section of Film
_________________________________ |_________________________________| Gelatin Supercoat | Emulsion | |_________________________________| |_________________________________| --------- | | \ | | \ | | \ Foundation Layers | Tri-Acetate Film Base | --- of Gelatin and | | / Cellulose | | / |_________________________________| / |_________________________________| --------- | | | Gelatin Anti-Curl Backing | |_________________________________|
Coating paper is different in several respects. As the paper is milled specifically for photographic processes, it has no need for the anti-curl backing. Also, it includes a layer of barium sulphate, which increases the whiteness of the base and acts as a buffer between the emulsion and the paper.
Cross Section of Paper
_________________________________ |_________________________________| Super Coat | Emulsion | |_________________________________| |_________________________________| Baryta Coating | | | | | Paper Base | | | |_________________________________|
When film is exposed, a latent image is formed on the silver halide crystals within the emulsion. Although refrigeration of the film will allow for the retention of this image for years, it is a good idea to convert this latent image soon after exposure through development.
Development is the process of chemically reducing the silver halides in the emulsion to black metallic silver. This is done by donating electrons to the grains of silver that have been exposed to light, which is an all or nothing process -- the grain is either reduced or it is not.
There are many chemical agents capable of reducing the silver halides to black metallic silver, though most act equally on the unexposed, as well as exposed, halides. Acceptable developing agents work more quickly on the exposed halides than the unexposed halides. The most common of these are Metol, hydroquinone, and Phenidone, which may be used alone or in combination.
Each has its own characteristic effect upon development. For instance, developers incorporating Metol alone in a low pH solution tend to raise shadow values and lower middle tones. This is a characteristic often favored by photographers. The tradeoff is that the effective speed of the film is substantially reduced. Developing agents by themselves are not active enough to yield acceptable results. They would take hours to complete their task, are quickly oxidized, and act on the unexposed halides to the extent that a fog is created. To correct these flaws, the agent is combined with an accelerator, preservative and restrainer.
The activity of the developer is increased by combining it with an alkali, which will increase the solution’s pH. This is known as an accelerator, contracting development time from an order of hours to minutes. The higher the pH, the more active the development. Common accelerators are borax (pH 9), sodium carbonate (pH 10), and sodium hydroxide (pH 12).
A developing agent will quickly lose its ability to work once dissolved in water. This is because it reacts with the oxygen in the water. Sodium sulfite (or sometimes sodium metabisulphite) is often added to prevent it from oxidizing, and is referred to as a preservative.
Although developing agents are selected because of their ability to work on exposed halides more quickly than unexposed halides, the unexposed areas can still create a fog. To combat this, a restainer such as potassium bromide may be employed. This chemical restrains development more in the low density than the high density areas, thus helping suppress fog.
When the development of the film is complete, a stop bath is used to halt the development. Some individuals use water to halt the development, but use of an acidic stop bath has two advantages: 1) exact control over ending the developing process can be employed, and 2) a stop bath keeps developing agents from carrying over into the fixer, preventing dichroic fog and preserving the fixer.
The idea of the stop bath is to induce an acidic element into the emulsion. As developers need to be accelerated by an alkali, they are not able to work in an acidic environment. Strong acids cannot be used, as they might bleach the silver or damage the emulsion. A convenience of the indicator stop bath is that it may be reused until it changes color, indicating exhaustion.
The purpose of the fixer is to give the silver stability in light. The two most commonly used fixers are plain hypo and rapid fixer forms. Plain hypo is simply a 30% solution of crystalline sodium thiosulfate, a cost efficient choice. Alternately, a 20% ammonium thiosulfate solution may be utilized. This is more expensive, but the time required for adequate fixing of the film is significantly lessened.
Fixer works by diffusing through the emulsion and reacting with the silver ions, forming insoluble compounds. The halide ion, liberated by the removal of the silver ion, diffuses into the solution. Fine grain emulsions have a larger surface area, thus are quicker to fix. Accumulation of this soluble iodine eventually reduces the silver ion concentration, slowing the fixing action.
Fixer exhaustion may be determined two ways. The first of these methods is to check for the clearing of the film. When initially placed into the fixing bath, the unexposed silver will show a milky color, which eventually clears. Adequate fixing time may be determined by doubling the amount of time it takes for this to happen. This is not always convenient when fixing roll film, as the film must be removed from the tank repeatedly for inspection (though I find it convenient when fixing sheet film with safelight illumination). When this time becomes inconveniently long, the fixer is considered exhausted and should be replaced.
The other means is to understand that exhaustion is a result of excessive silver within the fixer. If 1cc of a 5% potassium iodide solution is dropped into the fixer, excessive silver will be noted if this mixture turns cloudy.
The purpose of washing is to remove the soluble silver salts and fixer from the emulsion. If this is not done properly, the remaining fixer will decompose and react with the image. Also, the silver salts will decompose and stain the emulsion.
When the film is placed in water, the by-products will leach out of the emulsion. This is why running tap water quickly over the film is less efficient than simply allowing the film to soak, occasionally changing the water. Archival washing of prints can be achieved by allowing the paper to soak in water for sixty minutes, dumping the water and refilling every five minutes. Wash aids can reduce the washing times dramatically.
The purpose of drying is the allow the emulsion to “set” evenly and cleanly. This should be done in a dust-free environment, as the dust can cling and dry into the emulsion. It is also very important that film dries evenly, as water droplets can cause patches of uneven density. This can be avoided by soaking the film in a wetting agent before hanging up to dry.