Things I have not discussed so far

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by Photo Engineer, Aug 25, 2009.

  1. Photo Engineer

    Photo Engineer Subscriber

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    Well, in making hand coatings, I usually stress how the emulsion is made and coated and don't delve too deeply into keeping. Although the emulsions keep for up to 1 year with no appreciable change and the coatings keep for up to a year with no appreciable change, there are things you should know about "real" products which will influence your work.

    All real products have an overcoat. This overcoat is needed in machine coatings due to the fact that the winding operation at the end of the machine causes cinching of the film or paper and abrasion. Both of these can cause scratch marks and fog. Also, there are some things best not placed in the emulsion layer during coating, but rather are placed in the overcoat.

    So, here goes.

    An overcoat is used to insulate the emulsion layer from cinching caused fog and scratches. It is NOT needed for hand coatings because there is no rough abrasion of front to back surfaces during the winding process, but on a machine you are required to have either 2 coating stages or a C2 multi hopper coating stage for B&W or color products.

    In addition, lubricants are added to the top layer to help eliminate any cinching that may take place.

    But, in addition, this top coat may contain hardener, antioxidants, buffers and mattting agents. The hardener was added to the overcoat because in many cases, hardeners such as formalin were reducing agents which could fog the emulsion. Adding formalin to the overcoat reduced hold induced fog.

    Antioxidants and buffers were added as preservatives and could not be added to the emulsions in some cases due to the possible effects of pH shock on a hot emulsion or the fact that antioxidants were not good during the hold.

    Matting agents were used to create just the desired surface and so had to be on top.

    Well, there you have a minor introduction to overcoats and things not really brought to the fore before in this forum.

    PE
     
  2. eli griggs

    eli griggs Member

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    Nice rundown on stuff I did not know. How thick are these layers, including a typical ground?

    Eli
     
  3. Photo Engineer

    Photo Engineer Subscriber

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    Ground? Not sure what you mean.

    Usually 10 microns to 100 microns depending on intent.

    PE
     
  4. AgX

    AgX Member

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    Of course the coating layer thickness is dependant on the crystal size in the emulsion and on the mechanical stress it has to bear.
    But 100 microns is in the range of the base thickness.
     
  5. Kirk Keyes

    Kirk Keyes Member

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    You mean you were holding out on us?!
    ;^)
     
  6. eli griggs

    eli griggs Member

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    Ground meaning the plastic base for a sheet or roll film, onto which the emulsion and other coatings are applied.

    Eli
     
  7. Photo Engineer

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    The suppor thickness is about 5 mil (0.005") for 35mm film and 7 mil (0.007") for sheet film.

    And no Kirk, I was not holding out. You must have slept through that one. Maybe Bill or his friend kept you awake all night. :D :D :D

    PE
     
  8. Kirk Keyes

    Kirk Keyes Member

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    It must have been those chirpless crickets, I swear I could hear them chriping!
     
  9. Photo Engineer

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    To be more clear, I'll repost the above.

    Ground has no meaning to me in this context. Support is the correct term for the film or paper base. Support for roll film is usually about 0.005" (5 mil) or for sheet film it is about 0.007" (7 mil). Coating layers themselves are anywhere from 10 microns to 100 microns depending on the intent of the layer.

    PE
     
  10. WolfTales

    WolfTales Member

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    Cool - would that mean a hardener added directly to the silver emulsion would fog the film? It's only added as a seperate layer?

    Thanks!
     
  11. Photo Engineer

    Photo Engineer Subscriber

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    It depends on hardener, emulsion and hold time. I have seen hardeners such as formalin cause an increase in fog if the emulsion is held for hours prior to coating. This is usually only a problem in production if something backs up the schedule or whatever... With hand coatings, over say 4 hours, I see little or no problem with severe fog but do see some changes including a slight increase in viscosity.

    So, it depends. At Kodak we often added hardener in a separate layer just as a precautionary measure.

    PE