Fixat

In brief, the fixing process is the removal, after exposure and development, of unused silver halides, such as silver bromide, from the paper or film. This is necessary because unused silver bromide particles will eventually ruin the image. For this reason proper fixation is as important to the print-making process as proper development. Fixing can make the difference between an image of lasting value and an image that doesn’t last. The fixing process involves a series of chemical reactions in which the silver bromide is converted into complex argentothiosulfates, which are then dissolved by contact with fresh fixer and finally washed out of the fi lm or paper. Upon immersing an emulsion (film or paper unless otherwise noted) into fixer, the first reaction is the conversion of unused silver bromide into an insoluble but not very stable compound. This compound can be seen by looking at negatives (not prints) after only a few seconds in the fix. They will appear milky in appearance. If fixation is not continued and the compound not completely dissolved, the negatives will rapidly degenerate. As fixing progresses, the complex compound reacts with fresh hypo to form a soluble compound of sodium argentothiosulfate that can be removed by washing. In other words, fixing creates by-products; more fixing eliminates them. The actual rate of fixation is relatively fast. It is the breaking down of by-products (complex argentothiosulfates to soluble sodium argentothiosulfate) that takes time. The fixing process for paper and film is similar, but film and paper have their own unique characteristics and requirements. Until the 1970s, the primary agent used for fixing was sodium thiosulfate (hypo). Other fixing agents, though not as common, include ammonium thiosulfate, alkali thiocyanate, thiosinamine, cyanide, sodium sulfite, ammonia, thiourea, and concentrated solution of potassium iodide. Except for special purposes (for example, ammonium thiocyanate can be used to achieve fi xing times of a few seconds; Formulas: Fixers: Defender 9-F Rapid Thiocyanate Fixer), only two fixing agents are of interest to the general darkroom worker today, sodium thiosulfate and ammonium thiosulfate. Those fixers using ammonium thiosulfate are commonly referred to as “rapid fixers.” For practical purposes the difference between the two thiosulfates is a matter of fixing time. According to Pierre Glafkides, in Photographic Chemistry, Vol. 1 , ammonium thiosulfate is 400% faster than sodium thiosulfate.2 At some point, ammonium thiosulfate gained a reputation for producing less-than-permanent images. The problem was that ammonium thiosulfate, even in an undiluted concentrate, was not stable. Therefore, photographers desiring a more rapid rate of fixation would add ammonium chloride to their standard hypo solution just before use. This produced ammonium thiosulfate in solution. Today, a stable form of ammonium thiosulfate is readily available in two forms. The preferred form is a 60% solution. However, when it is inconvenient to ship or store liquids, it is also available in photo-grade crystalline form. Although there are fi ve published ammonium thiosulfate formulas, the two most used in contemporary photographic practice are ATF-1 and ATF-5.