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  1. #1
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    Washing and concentrating emulsions

    Well, before I start I should refer you to USP-5,248,418. This patent by Bill Munch sets the stage for all that follows here.

    Due to solubility of starting materials, an emulsion is usually restricted to about 1/2 mole (about 50 grams) of silver per liter of emulsion and between 2% and 10% gelatin. This is further diluted when the emulsion is washed free of salts. An early solution was to use Phthalated Gelatin, now sold by the Photographers Formulary and fully described in my DVD. (Yes, it is done and I've posted a picture here from the footage that shows one scene of this method.) I have also described this method here in this forum.

    The preferred method at Kodak which is now in use and described by Bill in his patent involves Reverse Osmosis (RO) and/or Ultrafiltration (UF).

    In these methods, salts and water may be removed by passing the emulsion over a semi-permeable membrane just like a kidney dialysis machine. So, you see that the emulsion can be washed and concentrated both.

    I now have access to the equipment needed for both methods and will begin adapting my emulsions to use any of the 3 methods in the future (PA, RO or UF). As I go along, I hope to document the work with suitable commentary and pictures. I'm putting this up just to let you know what I've been up to lately.

    I am also about to embark on making a new camera speed emulsion after a long series of experiments based on advice from friends. I'll keep you posted on this as well as it is integral to the new wash methods.

    PE

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    That sounds cool - and fun. Any suggestions on equipment to get to do this? And how much to does it cost?

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    Nothing to suggest yet Kirk. Give me a while to test some equipment out and compare it.

    PE

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    More information

    For RO units the small size we need for emulsions, I wanted you to know that I will be trying out a Kent unit. For UF, I will be trying a Millipore unit, but I have not determined the exact membrane size, nor have I found the exact pore size.

    For pump, I am using a Masterflex peristaltic pump. With this unit, you cannot use anything but the soft Tygon tubing with 3mm ID and 6mm OD. Other types of tubing will not allow flow due to stiffness of the tube wall.

    The tube type is selected to give proper flow rates across the pump rotation rate for small batches.

    Different types of tubing can be coupled with barb fittings or compression fittings. A pressure guage is needed to adjust pressure to prevent rupture of the membranes in the UF unit, or to cause leakage in the RO unit.

    All of the equipment must be calibrated before each use, as due to changes in the wall flex of the tubing, the flow rates and pressures can vary, and when Millipore or Kent say 40 PSI max, they mean it!

    Hope you find this additional information useful.

    PE

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    Photo of UF wash equipment in operation

    In this photo, I show a pump feeding a UF (Ultrafiltration) unit which is both concentrating and washing an emulsion. As the salts are removed, the water goes with it and the emulsion becomes more concentrated.

    You see a line going to the pump, a line going to the uf, and two lines from the UF unit. One is going to the permeat (discard) beaker. It is clear. The other line is the retentate (good stuff) returning to the source container.

    As the wash proceeds, water and salt are thrown away, and the emulsion and pure water are conserved as the concentration of silver per ml of water goes up.

    I sacrifced an emulsion make to take this picture so at the price of silver, experimentation is not cheap. The yellow opaque lines are silver halide.

    The unit is still under development.

    Oh, BTW, the aluminum foil around the edges is the catch trough for any accidents.

    PE
    Attached Thumbnails Attached Thumbnails UF1.jpg  

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    I wish to add that this is a very high end setup. My intent is to supply formulas and techniques for both low end (low cost) and high end (expensive) emulsion making methodology.

    PE

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    Could you show another photo that better shows the ultrafilter? I see the Masterflex pump (biege unit) and a Jobo behind that, and beakers in front of it, but I'm not seeing the UF unit.

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    The UF unit is in the sink submerged in a tray to maintain constant temperature. It is basically a 1x2x6 inch cartridge that is being kept at a constant 100F. There is one input (feed) and two outputs, what you want (retentate) and what you don't want (permeate). This run did not work out as I have not perfected the workflow.

    You can see the filter behind the large beaker in the foreground. It is the slim white bar in the tray. On the far upper left, you can see the colored tubing and the bottom of the reverse osmosis unit I have been using. That cannot use the Masterflex pump due to lack of pressure. To feed the RO unit you need a high power, high pressure pump.

    The clear tube running from the tray (filter) to the small beaker in the middle bottom of the photo near the big beaker is the permeate (salt) being removed along with water from the emulsion. The retentate and feed are the yellow emulsion tubes you see in the photo. They both go to a common beaker in the tray.

    PE

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    OK - thank for clarifying that.

    Quote Originally Posted by Photo Engineer View Post
    That cannot use the Masterflex pump due to lack of pressure. To feed the RO unit you need a high power, high pressure pump.
    I wonder if a used HPLC or IC pump would work. What kind of low rates are you needing? One of those pumps should be good up to about 10 ml/min at many hundreds or a couple thousand PSI, at least on less viscous fluids they are...

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    Quote Originally Posted by Kirk Keyes View Post
    OK - thank for clarifying that.



    I wonder if a used HPLC or IC pump would work. What kind of low rates are you needing? One of those pumps should be good up to about 10 ml/min at many hundreds or a couple thousand PSI, at least on less viscous fluids they are...
    Kirk;

    For the RO unit, I deliver about 100 ml/10 seconds at up to 100 PSI. That is typical. However, I cannot recommend RO until I find a membrane that stands up to the melted gelatin. Current units I've found, fail at about 90 - 100 F. See the specs at the web site I referenced earlier here for the details.

    Remember, all of my students will get complete details including sources, part numbers and work flow before I post here. I will also try to include price ranges and alternatives.

    PE

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