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The photographic process is fundamentally a redox process. While the redox chemistries of iron and chromium have been explored from time to time, most photography, for most of its history, has been based on silver. It is most likely that every photograph you have ever seen has been a silver photograph. We saw in Chapter 9 that silver is primarily a by-product of lead smelting, in whose ore, galena, it is a common contaminant. The production of photo-chemicals begins with the oxidation of metallic silver with nitric acid:
Silver nitrate is, of course, soluble in water. To make a photographic emulsion we need an insoluble salt, one which will not leech out of the emulsion into the paper. You will recall from Chapter 7 that silver chloride is insoluble in water:
Silver bromide and silver iodide are also insoluble in water and these salts are more light-sensitive than the chloride. Consequently they are the silver salts of choice for modern photographic emulsions. But because sodium chloride, ordinary table salt, is easier for the amateur to obtain, we shall use it in the production of our emulsion.
If our paper were simply saturated with silver chloride, the resulting image would penetrate the paper resulting in a fuzzy image. To demonstrate this, write on a paper towel with a pencil; notice that the pencil line is sharp and that it does not penetrate or bleed into the paper. By contrast, write on a paper towel with an ink pen; notice that the ink soaks into the paper resulting in a fuzzy line. Writing paper is sized, that is, it is treated with substances to prevent ink from penetrating the paper. We shall use a similar trick to ensure that our silver chloride resides only at the surface of the paper.
To complete the photographic emulsion we need something which we can apply to the paper but which will be impervious to water once it is dry. This substance should bind well to the paper and remain flexible after processing. Collodion and gelatin have been used for this purpose, but in the spirit of using as many household substances as possible, let us choose egg whites. Egg whites contain the protein albumin, which is fairly impervious to water once it is dry, an observation which will be confirmed by anyone who has tried to wash dried eggs from the side of a house. Briefly, a mixture of sodium chloride in egg whites will be applied to paper and allowed to dry. Once dry, the coated paper will be sensitized with silver nitrate solution. Silver nitrate will react with sodium chloride to produce insoluble silver chloride which will reside only in the emulsion because the dried albumin will not allow it to soak into the paper. The sensitized paper must be stored in the dark until it is used.
Now, silver ion is an oxidizing agent, that is, it is easily reduced to metallic, elemental silver. Albumin is a mild reducing agent, that is, it is easily oxidized. Place an oxidizing agent with a reducing agent and you are just asking for a redox reaction to happen. Given a day or two, the silver ion will be reduced to metallic silver:
As the silver is reduced the albumin will be oxidized to, well, to whatever it is oxidized to. Remember, proteins are polymers of amino acids; they form large, complex molecules and because the oxidation of the albumin has little to do with the resulting image, let us leave it at that and concentrate on the silver.
In the sensitized paper silver ion will be slowly reduced to metallic silver. You might expect, then, that after a few days the sensitized paper would look like a paper mirror. But you probably know from experience that powdered metals generally look black; take a file to any piece of metal and the filings will be dark gray or black. Silver is no exception. The reduced silver in the paper is not one big piece of silver to be polished to a mirror finish; it is in the form of tiny grains of silver. As a consequence, a sensitized paper left to its own devices for a few days will turn dark brown, gray, or even black.
Now for the magic! When light shines on the silver chloride/albumin emulsion the redox reaction happens more quickly than it does in the dark. Imagine now a sensitized sheet of paper, half of which is exposed to bright sunlight and the other half of which is covered up with an opaque card. The silver ions in the exposed area will be reduced to black metallic silver in a matter of minutes; the silver ion under the card will remain colorless or white. Imagine now that you remove the opaque card; what will you see? The half of the paper that was in the light will be black and the half that was in the dark will be white. This reversal of light and dark is referred to as a negative image.
But as soon as you remove the opaque card from the sensitized paper, the formerly unexposed white half will begin to turn black. In order to fix the image, we need to remove the light-sensitive silver chloride. Just washing it in water won't do the trick because silver chloride is insoluble in water. The earliest photographic fixer consisted simply of a concentrated solution of sodium chloride, ordinary table salt:
If you are looking for a household chemical to use as a photographic fixer, clear household ammonia will work better than salt. But far more effective is a specialty chemical which has been synonymous with photographic fixer since 1839: sodium hyposulfite, known to photographers as "hypo," or, by its modern chemical name as sodium thiosulfate:
By rinsing the sensitized paper with sodium thiosulfate, light-sensitive silver chloride goes into solution leaving the unexposed areas of the print white and the exposed areas black.
The image so far is a negative image. To produce a positive print, the process is repeated with a second piece of sensitized paper. The negative is placed on top of this second piece of paper and exposed to sunlight. The black areas of the negative mask the sunlight and the underlying areas of the print remain white. The white areas of the negative allow sunlight to penetrate and the underlying areas of the print turn black. Once fixed, the print bears a positive image, one in which white and black are as they should be.
The silver chloride/albumin combination produces what is known as a "Printing Out Paper," one in which the image becomes visible as the exposure proceeds. Such papers are slow, requiring minutes of exposure to bright light for the image to form. In a "Developing Out Paper," the binder used to form the emulsion is not a reducing agent, giving the unexposed paper a longer shelf life. When such a paper is exposed to light, no image is immediately apparent. The image forms when the exposed paper is placed in a developing bath, a solution of a relatively strong reducing agent. Exposed areas of the print are quickly reduced by the developer, turning black. Before the unexposed areas are reduced, the print is placed in a "stop bath," which destroys any developer remaining on the print. The print is then fixed in the usual fashion.
Photography continued to develop in the twentieth century. Color photography came into its own with the introduction of Kodachrome in 1935. In 1948 Edwin Land introduced a film which incorporated pouches of developer and fixer. When pulled from his "Polaroid" camera, the packets were squeezed onto the film resulting in a self-developing print; color Polaroid film was introduced in 1962. The twenty-first century has seen the maturation of the electronic, film-less camera. In fact, all of the photographs for this book were taken electronically. But the vast majority of photographs at the time of writing continue to be based on the fundamental principles of silver reduction first explored by Herschel, Daguerre, and Talbot.
Locate MSDS's for silver nitrate (CAS 7761-88-8), sodium chloride (CAS 7647-14-5), sodium carbonate (CAS 497-19-8), acetic acid (CAS 64-19-7), sodium thiosulfate (CAS 10102-17-7), and household ammonia (CAS 1336-21-6). Summarize the hazardous properties in your notebook, including the identity of the company which produced each MSDS and the NFPA diamond for each material.
The most likely exposure is to silver nitrate solution, which will blacken the skin. Wash any affected areas immediately with plenty of running water. If eye contact occurs, flush with water and call an ambulance.
You should wear safety glasses and rubber gloves while working on this project. Leftover silver nitrate solution should be collected for recycling or disposal. Leftover egg whites may be thrown in the trash. Spent sodium thiosulfate or ammonia should be washed down the drain with plenty of water unless prohibited by law.
|Research and Development|
So there you are, studying for a test, and you wonder what will be on it.
The NFPA diamond was introduced in Section 15.2. You may substitute HMIS or Saf-T-Data ratings at your convenience.