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Fixer

An indispensable chemical for processing B&W films


There is no one brand of fixer. A number of manufacturers make fixer, and all the well-known brand names are reliable and effective at fixing film.
There is no one brand of fixer. A number of manufacturers make fixer, and all the well-known brand names are reliable and effective at fixing film.

Fixer prevents fogging by dissolving undeveloped silver halide crystals and makes the developed image permanent. The film (or print) is said to have been "fixed" after treatment in a fixing bath.

The fixing agent in fixer is a silver halide solvent, commonly called "hypo." Hypo chemically changes silver halide to water-soluble silver sodium thiosulfate. Because it is water-soluble, it is removable by water.

But fixer does more than just remove the undeveloped crystals. It contains a neutralizer (usually acetic acid) that stops further activity of any developer remaining on the film, and also contains a hardening agent that hardens the films emulsion. (During development, the films gelatin becomes softened and swollen, and must be hardened to prevent scratching or damage from handling or during the washing process.)


FIXING TIME

The time needed for film to fix depends on a number of factors:

  • The amount of exposure of the film (The more exposure the film had to light, the less unexposed silver halide crystals there are for the fixer to remove);
  • Type and thickness of the emulsion (Thin emulsions need less fixing time, and fine-grain emulsions usually fix faster);
  • The concentration of hypo (fixing agent) in the fixer;
  • The degree of exhaustion of the fixing bath after more than one use;
  • Temperature of the fixer; and
  • Amount of agitation

Generally speaking, film is completely fixed after twice the amount of time required to clear it. "Clearing" happens when all traces of the silver halides have disappeared from the film, and it no longer has a milky appearance. You can determine clearing time in normal room lighting by agitating an undeveloped piece of the same film in the fixer until it clears. (Where do you get an undeveloped piece of the same film? You can use the film leader, snipped off before you begin film development.)


A thermometer that is designed for darkroom use is an essential film processing and printing tool. Temperature variation must be carefully controlled when processing photosensitive materials.
A thermometer that is designed for darkroom use is an essential film processing and printing tool. Temperature variation must be carefully controlled when processing photosensitive materials.

Practically, however, the label on the packaging of most fixers or the data sheet provided with them will provide recommended fixing times for film and for paper at a given temperature, generally 68 degrees F (20 degrees C), and at a given concentration. These guidelines almost always have satisfactory results. For example, Ilford Universal Rapid Fixer, when used at a 1 + 3 dilution (one part fixer concentrate + three parts fresh water) at 60 degrees F., will fix film after two to five minutes in the fixing bath.

USEFUL LIFE

An exhausted or almost-exhausted fixer can stain film. Its useful life depends on a number of factors, but the main consideration is the amount of film (or paper in making prints)that was treated by the fixer. When the clearing time for film is twice what it was when the fixer was completely fresh, it is commonly considered to be exhausted. The label on the fixer or its data sheet usually give a recommendation for how much material (film or paper) should be fixed before the fixer should be discarded.