Darkroom Digest: Stop Baths - Water vs. Acid

August 08, 2017  •  1 Comment

In this article, I thought it may be helpful to compare water versus acid-based stop baths and open up the conversation with others photographers.  

I should first cover the basic function of the stop bath.  It should be obvious, but I will cover it anyway.  The function of an acid stop bath is to stop development as completely and quickly as possible.

There is a relationship between development times and the stop bath that should be briefly mentioned, whether we are discussing film or prints.  In this article, I focus on black and white film development.  The shorter your development time, the more critical the stop bath becomes.  The longer the development time, the less likely it is for the performance of the stop bath to have a visible impact on your intended results. 


There are advantages and disadvantages to acid stop baths that darkroom photographers should be aware of include:

  1. When you are using an alkaline developer and move your film in an acid stop bath, unwanted pH variations are likely to occur.  
  2. If you black and white film developer contain carbonate, an acid-based stop bath can cause unwanted pinholes in some films.  It is important to perform tests with non-cortical films before working on anything serious. 
  3. And the most important concern for me is that acid stop baths can cause swelling of the gelatin in some films which can lead to loss of image quality.  

On the other hand, when you use a water stop bath in place of an acidic stop bath, it can create a different set of circumstances that you should be aware of.  

  1. It should be obvious, but when you place your film in a water stop bath, development does not immediately cease development.  
  2. In fact, the developer becomes very dilute in the water and the first place it exhausts is in your highlights, but it will continue to develop a little longer in your shadows.  This may not be a bad thing, just something to be aware of.  
  3. The biggest benefit of a water stop bath vs. an acid-based stop bath is the sharpness-enhancing adjaceny effects that will occur.

If you are interested in the information in this article, you will likely want to read my article: Exploring D-23 Split Bath Large Format B&W Film Development.

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-Tim Layton 

The Darkroom Underground is your analog photography magazine produced on a quarterly basis serving photographers, artists, collectors, and readers around the world. Publications are released on Jan 1st, Apr. 1st, July 1st, and Oct. 1st.  The Darkroom Underground publishes a balance of technical and creative articles in every issue along with featured photographers and their portfolios. We are pleased to offer editorial from internationally recognized photographers and writers and also publish articles and portfolios from our readers. 

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B&W Fine Art Analog Photography
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1.Chris R(non-registered)
That's interesting Tim. It helps make sense of a comment by my lab, when they had to hand-develop a roll of the new Film Ferrania P30, that they would use a water stop bath "as it tends to hold back the highlights a bit".

When I dev my own fils, I use an Agfa Rondinax daylight tank. This is a continuous rotation system, and I think that might mitigate the first of your acid stop bath disadvantages.

One thing I haven't seen information on is how long to leave the acid stop bath in there. I've tended to use 30 seconds for no very good reason.
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