Developing Film

The film is loaded into the developing tank before or after the solution is brought to operating temperature depends on circumstances and individual methods of working. In any case, a final check of solution temperature must be made before it is poured. Loading a film spiral for the first time can be a nerve-wracking business or it can go with extraordinary ease only to lead to problems on the second or third occasion. The metal type raises no problem beyond making sure that the film is squarely located under the central clip. Thereafter, light pressure on the edges with forefinger and thumb bows it sufficiently to fall easily into the spaces between the metal coils as the reel is rotated. The plastic spiral is theoretically easier to load because it generally has an auto-loading system. Loading roll film is more or less the same procedure except that the backing paper has to be removed first. It is a matter of choice whether the paper is simply folded back and left until all the film is in the spiral or whether it is torn off as a first step. With modern very thin films the latter course may be preferable because the edge from which the paper is detached is thereby stiffened and is easier to push into the spiral.

Equipment for Larger - Scale Working

The individual users of a relatively large amount of film can usually cope with processing in multitasks. Where processing is undertaken for more than one photographer or where sheet film is regularly used, a rather larger installation becomes necessary. The general practice in this case is to use large box like tanks with a capacity of about 7 or 15 liters. The tanks are available from many manufacturers in various forms and with their own individual refinements. Basically, however their construction is simple. The main requirements are that the material from which they are made should be reasonably robust, resistant to all photographic chemicals and totally opaque. A typical gas burst agitator to be fitted to IS-liter or larger tanks consists of a frame of six pipes with small perforations through which an inert gas (usually nitrogen) can be forced at intervals. The method is used for sheet film but is not considered suitable for roll and 35 mm films on spirals because the gas bubbles could be trapped between the closely-spaced film surfaces. The duration and frequency of the burst can-be critical for some processes. The duration is rarely greater than two seconds because a longer burst might create flow patterns that would lead to streaky or uneven -development. The frequency of, or interval between, the bursts depends largely on the process and the amount of agitation required.

Useful Accessories

The useful accessories are really essential for black and white film processing but several other items may be found in the well-equipped darkroom. Some workers for example, have adapted their processing methods to those used for color work. There are various ways of mechanizing the routine, some elaborate, some quite simple. With modern small tanks this is usually achieved by inverting the tank two or three times at the end of each minute or so, but there are various power-driven machines on which the tank can be placed so that it rotates and either rocks from end to end or reverses its rotation ‘periodically. Washing the film is an important part of processing and there are various accessories to aid that routine from a simple length of tubing to lead water to the bottom of the tank to various designs to increase the water force and to mix air with it. After washing, the film has to be hung up to dry. Film clips are supplied for that purpose. Drying can be hastened by directing a hair dryer or fan heater on to the film but that can be asking for trouble. Any dust, fluff or other foreign bodies in the air may be driven into the emulsion with sufficient force to make it irremovable. Various designs of film-drying cabinet are available, most based on tall cupboard like containers, with gentle heat (from lamp bulbs, perhaps) at the bottom, protected from drops and with screened air inlets at the bottom and outlets at the top.

Temperature and timing control

Processing tank is naturally the most important piece of equipment but various other items are necessary and there are a few others that can be used to make the process easier. A vital addition is a good photographic thermometer. The modern film processing is a time and temperature operation. A given film developed in a given solution has an optimum development time at a given temperature. The preferred developer temperature for black and white films is 20° C (68° F). Few photographic thermometers are in fact completely reliable, not even those certified as such. However, any error they may show is generally small and can be tolerated by black and white films if the processor takes pains, as he should do, to establish his own development times. If his thermometer has a significant error, he will find that the manufacturer’s recommended development time needs to be adjusted to his operating conditions. The spirit thermometers are perfectly suitable and in practice are generally preferable. The main function of the thermometer in black and white film processing is to check the developer temperature before it is poured into the tank. Some tanks allow thermometers to be inserted to monitor the temperature during processing. The photographic timer can provide the same facilities but the operator has to reset the time after each step in the sequence. The digital watches so popular and it is possible to use the elapsed time facility in a similar manner but on most watches that is much more troublesome to set than on a simple clockwork timer.

Film Processing

The Black and White Film Processing Equipment of Current camera films is mostly small-format (35 mm or 6 x 6 cm). They are roll films up to 165 cm long which means that they can conveniently be processed only by suspending them in deep tanks or by coiling them into a spiral. The suspension method is rarely used these days because even where large tanks are the rule, it is preferable to put the films into spirals and suspend them in baskets. The film spiral is the basis of the most commonly used processing method. They are made of plastic or stainless steel. It has a diameter of 8-9 cm and is in the form of two grooved discs separated by a central core. The grooves are simply spaces between the spiraling steel or plastic and the film is slid or sprung into them in such a manner that successive coils are about 3mm apart. That is sufficient for the processing chemicals to circulate freely when the spiral is immersed in a tank. The film must be loaded into the spiral in darkness but once the spiral is in the tank and the lid is firmly in place light can no longer reach the film until the lid is removed after completion of processing.

Constituents of Fixer Solution

The modern fixer constituents are the fixing agent, acid and a hardener. The fixing agent is generally sodium thiosulphate but ammonium thiosulphate is used in some faster-working solutions. The acid is commonly acetic acid but it cannot be used alone because the thiosulphates are rapidly decomposed by direct addition of any but the weakest of acids. The proportions of the various constituents are not critically important and innumerable formulae have been published with minor variations. Kodak’s F-5 is typical one. Acetic acid is usually supplied in the glacial form which is converted to 28 per cent by diluting three parts with eight parts of water. The boric acid is believed to increase the hardening capacity of the bath and to minimize the formation of aluminum sulphite sludge. The fixing time for this type of solution is generally quoted as 10 minutes with a clearing time of 5 minutes. The Rapid fixers based on ammonium thiosulphate are available from most manufacturers but their formulae are not generally published. There was a suspicion when they were first introduced that they did not provide stable images and some also seemed to exhaust rapidly. The suspicion has slowly evaporated and appears. In fact it is not to have been soundly based. The arguments for and against ammonium thiosulphate depend ultimately on speed of reaction. The fixing time is reduced dramatically from 10 minutes to perhaps 90 seconds or less but there has to be some doubt about the efficiency of the hardening process. If speed is paramount and hardening is not vital then the ammonium thiosulphate solutions are invaluable. For many workers however, the longer fixing time is better suited to their working methods.