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Stop bath

It stops development of the film.

Although the main function of stop bath (also known as Acid stop bath or Acid rinse) is to stop development of the film or of the print, it also removes calcium scum that may form on the film's surface during development. Stop bath stops the action of the developer because developing agents cannot work in an acid solution.

But there's more. Stop bath also prevents excessive swelling of the gelatin layer of the film. And because the developed film is in an acid condition after using stop bath,

  • it helps to preserve the acidity and hardening properties of the fixer,
  • protects and prolongs the life of the fixer by neutralizing developer carry-over,
  • and even prevents the formation of alum scum and sludge.

    Plain acetic acid stop bath does not take on color after extensive use. It remains colorless, in other words, and therefore gives no visible indication of its condition and ability to function at normal effectiveness. The only way to determine its condition is to test its acidity. Rather than having to do this, you can purchase Indicator stop bath, which contains an additive that "indicates" the effectiveness of the solution by turning a purplish-blue when it should be changed.


    You should only use a weak acid stop bath between development and fixing. A strong acid bath combined with the acid in the fixer may tend to form carbon dioxide gas bubbles in the film's emulsion. Why is this bad? When the developed film is placed into a strong acid fixer, the bubbles may burst, causing small, rounded holes in the film's emulsion. Should this occur, you may mistake the bubble holes for pinholes such as those caused by particles of dust on the emulsion before the film is exposed in the camera.

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