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Thread: Coating Test

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    kb3lms's Avatar
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    Coating Test

    Last night I sketched out an idea for a coating block for coating emulsion on film base. I'd like to test out the idea and would like to know if there is a (hopefully common) material that I could add to the gelatin (KNOX at this point just to try) to stand in for the AgX? Or, does the addition of the AgX not substantially change the behavior of the gelatin? I don't care to waste expensive silver nitrate until I have some idea if this idea will even work!

    Also, is a chrome alum hardener necessary to add to the emulsion just prior to coating? Or, are there other hardeners to use? I tried a really rough version of my idea to coat some gelatin on some acetate base on a piece of glass last evening and it was still quite sticky (although it seemed hard) this morning after about 9 hours. Would a hardener help this or is sticky emulsion an indicator of a different problem or it is just not dry yet? The coating thickness should have been in the range of 8 - 10 mils. But again, this was a very, very rough test and I am looking for some ideas.

    Thanks!

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    Jerevan's Avatar
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    I think someone (Denise?) has mentioned gesso as something akin to emulsion, for coating practice. No point in wasting silver nitrate - if you only are going to run tests for coating.
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

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    Plain 5 - 10% gelatin is just fine, but I suggest adding some liquid (not gel) food dyes to it so that you can see the quality of the coating clearly. Clear gelatin is very hard to see when dry. A hardener is not needed, but a surfactant might be useful. I would suggest about 1/2 ml of Photo Flo 200 to every 100 - 200 ml of 8% gelatin as a test. Do it without and with and then compare the two. If the coating is worse with the PF200, then there is probably too much or too little as the response to surfactant is a curve with the optimum in the middle and worse positions to either side of it. Kind of a bell shaped curve.

    I coat at 5 - 10 mils and use about 12 ml per square foot at 5 mils gap.

    You can coat film at 100F with cold or hot film, but paper should be coated cold with hot gelatin. Heat changes viscosity and uptake into the support (if any).

    This is a good method to get started.

    PE

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    kb3lms's Avatar
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    Thanks, PE. That brings about my other question: when formulas give a percentage gelatin, for example 10%, is that by weight or volume? I'm not exactly sure how I prepare 10% gelatin?

    Thank you for all your help.

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    DAP
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    Quote Originally Posted by kb3lms View Post
    Thanks, PE. That brings about my other question: when formulas give a percentage gelatin, for example 10%, is that by weight or volume? I'm not exactly sure how I prepare 10% gelatin?

    Thank you for all your help.
    A 10% solution should translate to a content of 10g of gelatin to every 100ml of final solution. That is unless gelatin solutions are measured in a different fashion that standard % solutions (I am just starting to dip my toe into the world of emulsion experimentation so this may be the case)

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    kb3lms's Avatar
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    Hi there, fellow toe dipper! Makes sense. But is that swelled gelatin or dry? I am guessing swelled since each formula tells you to swell the gelatin first.

    Well, last night's experiment has now produced a piece of film base with a fairly uniform green tint to it since I did put green food coloring in the gelatin. It feels like a piece of film albeit a little on the thin side. The gelatin coating is hard, clear and shiny and does not seem to want to come off the base. Therefore, I will consider this a success.

    Also, remember to take the film OFF of the glass after the gelatin has set up. Otherwise it glues your base to the glass!

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    Hexavalent's Avatar
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    Quote Originally Posted by kb3lms View Post
    But is that swelled gelatin or dry? I am guessing swelled since each formula tells you to swell the gelatin first.
    Usually gelatin is weighed in the dry state. 10g of dry gelatin + 90 g of water = 10% w/w solution.

    "Swelling" gelatin in cold water allows for a better melt - dry gelatin added to a hot solution can form very stubborn globules.
    - Ian

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    Andrew O'Neill's Avatar
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    kb3lms,

    What did you sub the acetate with?

    andrew

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    What did you sub the acetate with?
    Well, I went by some information on "The Light Farm" and based the subbing on the recipe on p 187 of Wall's 1929 book "Photographic Emulsions" (with acetone variation) but took some liberties to make a very small batch. Wall's recipe makes about a liter. It was roughly as follows:

    2g Gelatine
    20ml Actetic Acid (Vinegar 5% Acetic Acid + Water)
    50ml Methyl Alcohol
    50ml Equate REGULAR Nail Polish Remover (70% Acetone + additional Gelatin (and other additives))

    Soak the gelatin in the vinegar and melt to 55 deg C. Add the alcohol warmed to same temperature while stirring. Add the Nail Polish Remover (aka Acetone) warmed to the same temperature while stirring. You need to use REGULAR style polish remover. The friendlier stuff does not contain acetone. I used equate (Walmart) but the MSDS for any brand I recognized was basically the same; about 70% acetone.

    Tape your film to a piece of glass. Use a cotton ball to wipe down the film with the subbing mixture. Do not rub, just wipe over the film and then set it aside. When dry in a few minutes it will look a little streaky but the streakyness will disappear when gelatin/emulsion is applied - just as stated in Wall's book. If you gently touch the film with your finger you will notice that the non-subbed side will still feel like smooth acetate, but the subbed side will have a little tooth/roughness to it. The "emulsion" stuck right to it when applied and did not stick well at all to a piece of the base where there was no subbing.

    Treat this as a starting point only! This was literally a garage project since the methyl alcohol is gas line antifreeze - which is just methyl alcohol anyway. It hasn't been tested at all other than I can tell you that I do not seem to be able to rub the "emulsion" off with my finger and it bends with the film base without flaking off or anything. I did not yet subject it to any liquid tests.

    If anything, I would try to get some more acid into it. I did not have glacial acetic acid which is what the recipe calls out.

    However, I think I will take this sample off of the glass and put it in some developer temperature water for 10 minutes to see what happens.

    -- Jason

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    kb3lms's Avatar
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    Quote Originally Posted by Hexavalent View Post
    Usually gelatin is weighed in the dry state. 10g of dry gelatin + 90 g of water = 10% w/w solution.

    "Swelling" gelatin in cold water allows for a better melt - dry gelatin added to a hot solution can form very stubborn globules.
    Thanks! I guess that makes sense based on your explanation.

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