Could you coat by viscocity/surface tension alone?

[grainyvision]

So I did a gelatin coating (basically more like DOP salt prints/very thick unsalted sizing) method that I thought was pretty interesting for how well it worked. Specifically, I put hot (~120F) gelatin in a tray, then touched the dry paper to the tray and simply dragged the surface of the paper along the gelatin so that none (or rather minimal due to mistakes) got on the back side. I was very impressed with just how even the coating was, even though it would be far too thick for a real emulsion. The paper curled up like I've seen when coating dry paper with emulsions, and dried into a fairly stable semi-gloss surface like homemade emulsion coatings do.

My exact recipe: bloom 40g gelatin with 400ml cold water for 1 hour. Melt gelatin (~140F), add 0.5g citric acid (for my salt print-like process), top to 600ml with water. Heat to 125F. Add 20ml of everclear. Temp reading: 120F. Pour into a dish tray. Hold paper by one edge, (width wise, curl happens width wise), drag paper carefully over emulsion without submerging.

My DOP salt print process didn't really work out, but the coating on the papers I did this with was amazingly quite even. Only real unevenness to this coating was on the edges as expected, and some I tried to place in a drying rack contacted the rack and got uneven lines.

So, my question is could this be a viable method of coating somehow? My idea for making the coating thinner on the paper would be to basically decrease the surface tension. So higher temps, lower gelatin amount (would likely interfere with silver halide process), more everclear, or addition of photo-flo or PEG or other surfacant. Notable defects was just that when the edges dripped a bit of gelatin it caused some bubbles in the emulsion. Given how commercial coating processes work, the idea of a coating thickness determined by viscosity seems reasonable, and the process is surprisingly easy, however, without making a basic silver halide emulsion, it seems impossible to know if it will actually work and provide suitable evenness in this process. I'm only going by eye and feel. The emulsion comes off quite easily when wet indicating it is definitely too thick, but could this be controlled well enough to lay down a micrometer thickness emulsion?

 

[grainyvision]

I tried it last night with a very simple and undigested silver chloride emulsion. Exact gelatin ratio was 9%, finals was 40ml additional water (to bring it to 9%), 15ml of everclear, 3 drops photo-flo. Total volume ~220ml. Coating began at 160F, but I kept coating all the way down to 90F, spraying alcohol to eliminate any bubbles. I used a small aluminum foil "trough" in a baking sheet to form a fairly narrow and shallow well.

The results are unusable, but the method has merit with some modifications. Specific qualities:

* Test strips fixed perfectly in 1m of TF-5.
* Exposing paper to sun revealed an uneven sensitivity. The paper darkened in a wavy pattern, indicating uneven emulsion thickness. This seemed to be from variations in how fast I pulled the paper over the top of the emulsion, likely due to the absorbency of the paper, which likely wouldn't happen with acetate or a non-porous base
* Other than the typical thick emulsion around the edges, the paper had minimal obvious coating defects other than the unevenness. Few/no repellency spots, few/no bubbles, few/no spots where the emulsion didn't contact the paper, etc.
* The process was messy, emulsion often got onto the back side

I'm not sure what the name is of the arcane coating method that resembled this. I believe PE talked about it once being used for production runs of film. I read it sometime back but can't find it now. It used a similar narrow trough, but had a wheel on top with the base attached which basically just barely contacted the trough. By controlling viscosity exactly, it was capable of leaving a consistent coating of emulsion on the base. If anyone has the name, patent, or any other info, I'd love to hear it. I believe that it could be possible to build a small run version of such a machine that uses this property. The key variables are speed of the base being run through, and keeping temperature under control. If the temperature problem can be solved somehow, I'd see no big barrier to 3D printing such a machine that can do something like 6" wide coatings (1/2" waste on both sides, 5" final product). The cool thing is that it could actually work with very small amounts of emulsion, as the home user would be making (whereas cascade coating etc typically operates on liters of emulsion). It would also only be limited by how much drying space you have as to how long a strip could be. The biggest barrier for paper is that grain direction would play a part. I doubt you could find a 30" roll where the grain is along the 30" side. To eliminate that problem, maybe wetted paper could be used, though in my experience that seems to cause more difficulty and problems. The other issue is that it likely requires specialized and tweaked emulsion recipes, as it really seems to need a low viscosity emulsion, though maybe such problems can always be fixed by simply adding alcohol. Either way, an interesting idea to kick around for sure. I was pretty impressed by how few flaws this experimental emulsion coating procedure produced.

 

[Lachlan Young]

I'm not sure what the name is of the arcane coating method that resembled this. I believe PE talked about it once being used for production runs of film.

Dip/ trough/ pan coating. Usually used a doctor blade (sometimes an air knife) to control thickness of layer - Efke/ Fotokemika were (I think) the last major users of this method for production emulsion coating - though it's still used for other procedures (anti-hal application I think for example) elsewhere. Main problem is that you can only apply one layer at a time. I think they're the least fussy machines about emulsion viscosity compared to multilayer etc. As for drying, grain direction is less of an issue than you might think - the paper was apparently clipped to festoons as it traveled down a drying hall. Bob Shanebrook's book has quite a bit of detail on the use of dip coating and festoon drying.

 

[grainyvision]

Dip/ trough/ pan coating. Usually used a doctor blade (sometimes an air knife) to control thickness of layer - Efke/ Fotokemika were (I think) the last major users of this method for production emulsion coating - though it's still used for other procedures (anti-hal application I think for example) elsewhere. Main problem is that you can only apply one layer at a time. I think they're the least fussy machines about emulsion viscosity compared to multilayer etc. As for drying, grain direction is less of an issue than you might think - the paper was apparently clipped to festoons as it traveled down a drying hall. Bob Shanebrook's book has quite a bit of detail on the use of dip coating and festoon drying.

Very interesting. In theory, you could likely give the emulsion a brief gel period and then run it directly into another dipping machine, but maybe that requires a full dry. Either way can see why it's not used for modern emulsions, but definitely seems to be the easiest way to approach a consistent piece of machinery that can be used at home (ie, without a warehouse or making liters of emulsion)

 

[Lachlan Young]

Very interesting. In theory, you could likely give the emulsion a brief gel period and then run it directly into another dipping machine, but maybe that requires a full dry. Either way can see why it's not used for modern emulsions, but definitely seems to be the easiest way to approach a consistent piece of machinery that can be used at home (ie, without a warehouse or making liters of emulsion)

What you describe is essentially how the machine worked - through the coater, then a chill/ set section, then another dip for a supercoat, then that set/ chilled, then dried. Most early colour materials required a separate coat/dry trip for each layer as I understand it - and the Fischer coupler materials weren't able to make the transition to slide/ waterfall coating.