If wet, it may be pressed firmly to a sheet of glass, and the potassium hydroxide solution applied, care being taken to prevent the solution from flowing onto the glass plate or in any way coming in contact with the bottom emulsion. Care must be exercised to prevent the dry chemical or the solution from coming in contact with the bare skin (use rubber gloves), clothing, or the emulsion surface that is to be preserved. After the film has been rubbed with the swab for about 1 minute, the emulsion is usually soft enough to be scraped off with a smooth, dull implement that will not scratch the film base-for example, a plastic windshield scraper. After the emulsion has been removed, the film is rinsed in running water, removed from the glass plate, and immersed in the fixing bath for a few seconds to neutralize any remaining caustic. It is then washed for about 20 minutes and dried, although a shorter washing period is acceptable if the film is not to be kept indefinitely.
Alternatively, the unwanted emulsion may be covered, prior to development, with some waterproof sheet material that will protect it from the action of the developer. The protective material is removed after development but before fixation. The time of fixation should be extended, since the undeveloped emulsion clears more slowly than does the developed one. Wash and dry according to standard procedure.
If the derived area is narrow, as in the case of a microradiograph or a powder x-ray diffraction pattern, it may be covered with a waterproof adhesive tape. It is inadvisable to overlap strips of tape to cover wider areas because the adhesive coating of the tape does not always adhere tightly enough to the back of the adjoining layer to prevent leakage of the developer. Any tape used should be tested for its impermeability to developer, and to be certain that the adhesive adheres to the tape when the latter is removed rather than to the emulsion.
For larger areas, as in conventional radiography, two films may be taped together, with the unwanted emulsions in contact, and developed together. After development the films are separated, fixed, and washed individually. Alternatively, films may be taped, unwanted side down, to a discarded radiograph or a piece of film base, and removed after development but before fixation.
Tray Processing
This method is not as efficient as the tank system for the manual processing of x-ray films. However, when tanks are not available, satisfactory results can be obtained by employing trays and exercising the necessary care. The time and temperature recommendations for tank processing apply to tray processing.
Several glass, hard rubber, plastic, or enameled trays are essential. They should be large enough to accommodate the largest film that is used. One tray is used for developer solution, a second for stop bath or rinse water, a third for fixer solution, and a fourth for wash water.
In the tray system, a quantity of solution should be mixed at regular intervals and kept in glass bottles or glazed jugs. Then enough solution to cover the films to a depth of at least 1 inch is poured into the proper trays just before processing.
When the film is removed from the cassette, film holder, etc, a stainless steel clip should be attached to one corner to facilitate handling. The film is then immersed in the developer solution using a quick, sliding motion. If the emulsion is not covered evenly, a dark line will show in the radiograph where the solution pauses. Likewise, dark spots will appear on the film at points where spattered drops of developer strike the dried emulsion. During development, the film should be moved frequently and turned over so that the under side does not adhere to the tray and thereby retard the action of the chemicals. The tray should also be rocked in an irregular manner to provide continual mixing and redistribution of the solution over both surfaces of the film. Similar agitation is necessary in the fixer. Provision should be made for a constant flow of water in the wash tray. Care should be taken to be sure that radiographs do not cling to one another or stick to the bottom of the tray during the course of the washing process.
Commercial Washing
Films intended for ordinary commercial use should show no image change for several years under normal storage conditions. Adequate washing reduces the residual fixer content of a processed film to an acceptable level. The KODAK Hypo Estimator used with the KODAK Hypo Test Solution HT2 provides a simple, convenient method for measuring washing efficiency and can be used for cursory estimates of the keeping quality of films. It is especially useful for comparing variations within a test or for comparing several films in the same process. It has the additional advantages of being fast and easy to do.
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1To make approximately 28% acetic acid from glacial acetic acid, dilute 3 parts of glacial acetic acid with 8 parts of water.
Store the solution in a screw-cap or glass-stoppered brown bottle away from strong light. Avoid contact of test solution with the hands, clothing, negatives, prints, or undeveloped photo- graphic materials; otherwise, black stains will ultimately result.
The KODAK Hypo Estimator consists of four color patches reproduced on a strip of transparent plastic. It is used in conjunction with KODAK Hypo Test Solution HT-2. For use in the test, an unexposed piece of film of the same type is processed with the radiographs whose fixer content is to be determined. After the test film is dried, one drop of the KODAK HT-2 Solution is placed on it and allowed to stand for 2 minutes. The excess test solution is then blotted off, and the stain on the film compared with the color patches of the KODAK Hypo Estimator. The comparison should be made on a conventional x-ray illuminator. Direct sunlight should be avoided since it will cause the spot to darken rapidly.
For commercial use, the test spot should be no darker than two thicknesses of Patch 4 of the Hypo Estimator. Two thicknesses can be obtained by folding the estimator along the center of the patch.
Storage Conditions
The residual fixer concentration for commercial use can generally be tolerated in areas where the average relative humidity and temperature in the storage space are not excessive. These quantities may, however, be excessive when storage conditions are worse than average for temperature and humidity. Archival processing should be the rule whenever it is known that relative humidity and temperature are likely to be constantly excessive, as is the case in tropical and subtropical areas.