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FIXING 375
(4) A restrainer — potassium bromide. This is used to restrain very active solutions from reducing unexposed as well as exposed emulsion.
(5) A solvent — water.
The developer bath is rapidly circulated through the machine by pumps ; temperatures of developer and fixing baths are usually controlled within zb 0.25° F. Ordinarily, a fairly large storage tank is used as a reservoir ; the bath is circulated from the tank through the machine and back to the tank. The flow of fluid in the machine is opposite to the direction of travel of the film where practicable. Fluid usually enters the developer tank at the bottom and overflows at the top, returning to the storage tank.
Fixing
A fixing bath is an acid hardening bath that " clears" the film of unexposed emulsion. The principal constituent is hypo (sodium thiosulfate) — the clearing agent. For proper action in fixing and hardening, the time allowed for the fixing bath should not be less than twice the time just necessary to clear the film.
Hardeners are usually chrome alum or potassium alum. The hardening agent chosen depends upon the degree of hardening desired. To prevent precipitation of free sulfur which occurs in acid solution, sodium sulfite is added. The acids in the bath are usually acetic acid and boric acid.
Published Formulas. Photographic chemicals cannot tolerate certain impurities common in u.s.p. chemicals. It is necessary to make certain that the chemicals purchased are of photographic grade* to avoid difficulties that arise from such impurities. Table XX lists common solution formulas.
For release prints, little variation in the Eastman D-16 formula (except possibly for concentration) is ordinarily justified. There may be a change in the balance between the Elon and the Hydroquinone, but in relatively few cases are the changes significant.
For the development of negative, however, changes in the bath from the Eastman D-76 reference bath listed are very common. Many such changes show significant improvement in picture quality or in "adjusting the bath to the machine" with regard to middle-range machine de
* Specifications for chemicals of photographic grade may be obtained from the American Standards Association. 70 E. 45 St., New York. X. Y. ; there is a specification for each of the many common photographic chemicals, including all those listed in the above formulas.