FIXING The term " fixing " is a misnomer, as, instead of making permanent something that is desired, it removes something that is not desired, and which, if left in the negative or print, would seriously impair it. Fixing, in photography, means the removal of any sensitive salt unacted upon by light, or by the developer, thus render ing the negative or print unalterable by the further action of light. The quantity of silver reduced (developed) in a negative bears a very small proportion to that originally in the plate —very small both in film depth and surface measurement, only the highest lights going all the way through the film, while in the shadows and darker parts little more than the surface is touched. Thus in the average developed film there remains a large proportion—estimated at 75 per cent.—of unaffected silver bromide which must be removed before the negative can be examined with safety in daylight, or be used for printing.
Several substances possess the property of dissolving this unreduced silver. Sodium hypo sulphite has the greatest solvent action except ammonia, which for many reasons it is not advisable to use. Although many solvents of silver are known, ammonia, potassium cyanide, and sodium hyposulphite (better known as " hypo ") are the most notable. Ammonia, however, is not practicable, because to remove the superfluous silver quickly a very strong solution must be used, and such a solution seriously damages a gelatine film. Potassium cyanide, though used as a fixer for wet plate and ferrotype work, is too expensive for modern dry plates and papers, and in addition is undesirable because of its exceedingly poisonous nature and its liability to eat into the half-tones. The colour of the negative after fixing with cyanide is by reflected light whiter than when "hypo " is used, but by transmitted light, as when printing, it is browner and consequently more actinic. All things considered, however, it is not so good as " hypo." It was in the year 1819 that Sir John Herschel discovered in sodium hyposulphite a solvent for unreduced silver, but the first use of it is credited to J. B. Reade, who in 1837 made the first fixed silver prints from paper negatives. " Hypo " was at that time a very rare salt, and expensive, costing about half a crown an ounce ; in 1845 it dropped to 6d. per ounce, in 1857 to 6d. per pound, since when it has become very cheap indeed.
The usual procedure in fixing negatives is to place them film upwards in a flat dish filled with the " hypo " solution, but while such a method serves very well, particularly when the solution is frequently agitated, the work is performed and preventing the action of the " hypo." In a vertical tank a dozen plates may be fixing at one time, and take up but little room. When a
flat dish is used the plate should be film side down, but not with the film in contact with the dish, and then impurities cannot easily attach themselves to the film ; even this is not so good as vertical fixing. A shows a form of dish for fixing plates upside down ; the long V-shaped wooden trough, covered with pitch or made waterproof and watertight in some other way, is filled with the fixing solution, and the plates rested on the sloping sides film side downwards. An advantage of such a dish is that it may be used for any and all sizes. Such a dish is more cumbersome, and takes up more room, than a vertical trough.
It is not necessary to deal here with the com position of " hypo " fixing baths, as formulae have been given under the headings " Acid Fixing Bath " and " Alkaline Fixing Bath." While fixing is apparently the simplest of all photographic operations, it is frequently done in an inefficient manner, thus leading to many failures. A common mistake is to remove the negative or print from the " hypo " solution too soon. A negative is not fixed the moment the silver appears to be dissolved away and the plate dear. The process of fixing with "hypo" includes two distinct and important functions. The first is the formation of a double salt of sodium hyposulphite and silver by the reaction of the creamy white silver bromide with the " hypo," at which stage the negative is clear and apparently fixed. The double salt so formed is insoluble in water, and therefore cannot be removed by any amount of washing ; it cannot be seen, and the negative appears perfect and ready for washing and drying, but if washed and dried at this stage the double salt will on exposure to light appear in due course as a yellowish stain, and in time the image will fade more or less to an extent corresponding to the amount of the double salt in the film. The second function is the dissolving away of the detrimental salt first formed. Although insoluble in water, a longer soaking in the " hypo " solution converts it into another double salt that water will easily remove ; hence the absolute necessity of leaving the developed plate in the fixing bath for an additional time after the plate appears to the eye to be fixed and clear. Ordinarily, the proper length of time to secure perfect fixation is double that taken by the white substance to dissolve. Thus, for example, if it takes ten minutes' immersion in the " hypo " bath to clear away the last traces of the silver bromide, the plate must be allowed to remain for another ten minutes in order that fixation may be complete.