Unwashed emulsion

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by kevin klein, Feb 14, 2007.

  1. kevin klein

    kevin klein Member

    Mar 20, 2006
    Large Format
    This is a question for the Photo Engineer.
    In the preperation of the unwashed bromide emulsion, would it be neccisary to add potassium bromide at the end as a stabeliser,if so what is the amount based on?

    Also, will washing improve anything in its working properties?

    Thank you

  2. Photo Engineer

    Photo Engineer Subscriber

    Apr 19, 2005
    Rochester, NY
    Multi Format
    If you leave an emulsion unwashed, it will not coat well on film support or plates because crystals of bromide and nitrate will form, and the extremely ionic nature of the emulsion will lessen the adhesion to the support. Even if crystals don't form, sometimes the coating can be quite tacky due to the salts present.

    This is also the case when coating on RC support, but not on FB paper support.

    If it is unwashed, it will react more slowly and less efficiently to sulfur sensitization, but will not need (usually) the addition of more bromide, provided you have used the right amount of excess salt to start with. You must always have excess salt to some degree or other.

    Leaving some salt present improves keeping, so it is useful to wash and then adjust, the problem is that you never know how much to adjust by. This is why we used a conductivity meter at Kodak to test the actual salt level.

    If you test the wash water for salts as you wash, and wash until the wash water has no more salts (as indicated by adding a drop of silver nitrate to a beaker of wash water), then you know you have washed to the end. Then you can add salts to adjust the conductivity to a good value. Unfortunately, I cannot tell you what that might be for you without extensive calculations. There is a good example in the emulsion posted by Jim Browning on his web site that should help.

    For those interested in the average values, and those who can measure it, it translates into a vAg (voltage as conductivity measured based on silver ion) of +50 to +80 millivolts. Using vX (voltage measured based on halide ion) as suggested by others is entirely misleading here. The reasons are way to complex to get into here, just as this paragraph has gone way outside what you would normally want to know.

    Just FYI, a vAg of 50 is about 0.00194 m/l or 0.1996 g/l of NaBr at 40 deg C. That is a good starting point. Or, less. For example, a vAg of 80 is 0.00062 m/l or 0.0642 m/l at 40 deg C. That should start you off. That is the final concentration range I would aim for.

    For those interested, this latter example works out to a pAg of 8.3936.

    Now, I'm going to put my slide rule away.