Which chemicals do I need to process X-ray film?
Making-up processing solutions
Nowadays, chemicals are supplied as a liquid concentrate, suitable for the particular type of film used. The processing solutions can be prepared either directly in the tanks or in plastic buc- kets. In the latter case each type of solution must be prepared in a separate bucket, which is never used for other chemicals.
Developer
Development fog, graininess and contrast are dependent on the type of developer, which is preferably made up to suit the film used. If a concentrated manual developer is used, for example G128 made by Agfa, and the developer tank has a capacity of, say, 25 litres, then all to do is pour 5 litres of the con- centrated developer into the tank and add 20 litres of water (ratio 1 part of concentra- te to 4 parts of water). G128 developer is also used as a replenisher, in which case 3 parts of water are added to 1 part of concentrate.
Fixer
Fixer too is supplied as a concentrated liquid (G328). The same instructions as for pre- paring developer apply here.
Developing times and bath temperatures
The film is clipped on or slipped into a frame, depending on the type of frame, and hung in the developer tank. As soon as the film is submerged in the developer, the darkroom timer is set for the required number of minutes. The optimal developing time is the time at which the most favourable “contrast to fog ratio” is achieved. Minor deviations from the correct exposure time may be compensated by adjusting the developing time. The recommended developing time for Agfa films in G128- manual developer is 5 minutes at 20°C. In the automatic process using G135 developer, the developing time is 100 seconds at 28°C. Deviating from the recommended developing times and temperatures will almost always lead to reduced image quality (e.g. increased coarse- graininess). Raising the tank temperature will speed up the development process as table 1-10 shows, but the developer will oxidise more rapidly. Should it not be possible to achieve a bath temperature of 20°C, the following developing times can be used at the temperatures as indicated in table 1-10. This applies to all D-type films.
The temperature of the developer shall never be less than 10°C, but is preferably higher than 18°C to obtain optimal image contrast. It is best to always maintain the same developing conditions, so that the exposure technique can be matched to these and uniform results obtained.
Film agitation
To prevent air bubbles from forming on the surface of the emulsion (which will cause spots on the finished radiographs), and to make sure that the developer penetrates all areas of the emulsion evenly, the films should be kept moving during their first 30 seconds in the developer. After that, it is recommended to move the film from time to time to prevent film faults such as lines or streaks.
Replenishing
Up to 400 ml of liquid per square meter of film processed may be carried over to the next tank. When developing frames used it is, therefore, preferable to hold the film 2-3 seconds over the developer tank to drip. After each square meter of film developed, 600 ml of replenisher must be added to the bath regardless of the quantity of developer lost from the tank. Up to about 4 litres of replenisher can be added in this way, for every litre of the original developer in the tank. The solution must be discarded and replaced with fresh developer when the total quantity of replenisher added is three times the original total contents, but in any case after eight weeks, irrespective of the number of X-ray films processed.
Stopbath
Before transferring the developed film to the fixer tank, it is placed in a stopbath (consisting of 30 ml glacial acetic acid to 1 litre of water) for 30 seconds to prevent the fixer solution from being neutralized too rapidly by the developer, and stripes or dichroitic fog from forming on the film. If a film is not passed through a stopbath, it must be rinsed in running water for a few minutes immediately after leaving the developer.
Fixing
Fixing renders the image formed during development permanent, by removing unde- veloped silver halide salts from the emulsion. When the film is taken from the stopbath it still has a milky appearance; this changes in the fixer and the light areas of the film become transparent. As a rule the film is left in the fixer twice as long as it takes to “clear” or become trans- parent. Fixing time (in a fresh solution approx. 3 minutes) is twice the clearing time (1.5 min.). As soon as it takes double that time to “clear” a film in G328 fixer solution at 20°C, it must be replaced. For every liter of solution in the fixing tank, a square meter of film can be treated. Films have to be kept moving during the first 30 seconds in the fixing bath.
Final wash
The final wash is intended to remove to remove the residual fixer and the soluble silver compounds left behind in the emulsion, which if not flushed out, would reduce film shelf life. Washing should preferably be done with running water, ensuring that all parts of the film are reached by fresh water. The duration of the final wash depends on the temperature of the water. See table 2-10. Temperatures over 25°C must be avoided.
Drying in the drying cabinet
When the film is taken out of the water, the water on the film, as a result of its surface tension, runs together to form droplets of varying size. The film will, therefore, dry une- venly, causing “drying marks”. For this reason it is advisable to immerse the films in a solution of 5-10 ml wetting agent to each litre of water. Wetting agent reduces the sur- face tension of the water so that, after the film has drained, the surface will be evenly wetted and will dry evenly with no risk of marks. Films should be hung to drain for about 2 minutes before they are placed in the drying cabinet. Drying should preferably be done in a drying cabinet, or alternatively in a dry and dust free room. No drops of water must be allowed to fall on films that are already drying, as this will cause marks. Wet films should, therefore, always be hung below already drying films. Drying time will depend on temperature, air circulation and relative humidity of the air in the cabinet. Films will dry more quickly when they have first been put into a wetting agent. Before a film is taken out of the drying cabinet, it must be checked that the corners and edges of the film are thoroughly dry. Air temperatures above 40°C should be avoided as this may cause ugly drying marks. There must be free circulation of air between the films in a cabinet; if they cannot dry evenly on both sides, they may curl or distort.
Roller dryers
Industrial dryers can be used to dry films quickly and uniformly after washing. This mechanised drying process only takes minutes. Dryers and chemicals should preferably be matched. There are compact roller dryers on the market which are capable of developing approx. 15 cm of film per minute and take up far less space than a drying cabinet.