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  1. #1
    Jerevan's Avatar
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    Silver content in emulsion

    Okay, this may be flogging the proverbial dead animal, but...

    I have been looking at the Bruce Kahn formula. To me, it looks like a gelatin-sized anemic salt print (The BK formula has a silver content of 5.1g to make a solution of 100ml whereas my salt print solution has 12g as per Reilly). I know that in salt prints, an excess of silver is needed to get the desired results as some of it washes out.

    Is the silver content related to what other chemicals there is in the emulsion, i.e can I get away with less silver using something else that is active in the emulsion and still get the same results?

    A standard (old technology such as say, Tri-X) film has, as far as I get it, a silver content of 1,5g per square meter. The same content of silver could also coat the same amount of paper. I am not sure of the Ag content of T-grain films but I believe it is lower due to some manufacturing differences.

    What I am really trying to figure is this idea of "silver-rich" materials. Let's say one makes a film with 3g silver per square meter. Apart from the obvious rise in costs, does it make any sense altering the levels of silver - and is there any limit to how much you can use in an emulsion?
    Last edited by Jerevan; 10-02-2006 at 02:28 AM. Click to view previous post history. Reason: typo
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

  2. #2
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    Actually, I'm all in favor of flogging "dead" topics. Those of us who love silver printing are probably going to be revisiting texts from the 1880's. It will be hard to resist looking for canned answers from the current experts, but I can't help be feel that, in the end, we'll all know and understand a lot more if we dig into the questions ourselves. Thanks for throwing your question into the ring. I'm getting back to work on silver gelatin emulsions tomorrow. When I had to leave off a month ago, I had just started pushing up the silver content, along with the NaCl and gelatin. My impression (subjective, but running toward certain) was that the higher the "octane" the more the whole print just glowed. I'm looking forward (to say the least) to seeing how far it can be pushed. I'll post my results here, and I would love to see the results of others. If we're going to reinvent (and improve!) the wheel, teamwork can't help but help.

  3. #3
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    The silver content of albumin and silver halide in gelatin is judged and adjusted in a different manner in each case.

    In the Kahn formula, when coated by the dip method as in albumen, the content of silver cannot be judged or adjusted. When coated in a normal manufacturing process, it can be.

    Therefore, to coat the Kahn formula, you have basically 2 options. You use a brush or you use a coating blade. In either case, the goal is to spread about 12 ml of the emulsion over each square foot of paper. This works out to about 500 mg of silver in each sheet of 8x10 paper. With a brush this may require from 1 - 3 applications, and with a blade it requires a gap of 0.005".

    This is silver rich, but that is an artifact of early emulsions. Only a small part of the silver was used in the imaging. The rest was washed out in the fix.

    In addition, the old emulsions were rather low in gelatin wrt silver but high in overall amount per square foot.

    The overall combination gave old prints a rather velvety look, especially on the old baryta papers or untreated papers which had less surface repellancy.

    PE

  4. #4
    Jerevan's Avatar
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    PE:
    Okay, that confirms a bit of my assumption what "silver rich" really means, as regards the gelatin/silver mix of the BK formula. If one brushes it on, does it need to dry in between coatings?

    The artifact problem with (too much) silver washing out of the emulsion, is there any chemically easy way to remedy this? Is it meaningful to even pursue that? By the way, I like the velvety appearance of non-glossy/untreated papers.

    dwross:
    I think the questions sometimes needs to be asked again. The old books are full of good things but sometimes just thinking out loud gets you even further along the road.

    I suppose a higher content of NaCl and AG may give better results as more reactions between NaCl and silver can take place, but then there's a price point somewhere along that line...

    Show us something when you are ready!
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

  5. #5
    Photo Engineer's Avatar
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    By washing out of the emulsion, I mean in the fixer. The dmax of the silver can only reach about 2.2 on reflection support regardless of the amount of silver. Older emulsions had a lot of 'dead grains' that would not develop and were there as an artifact of the older methods of emulsion making. They were just fixed out in the fixer. If you did develop them, you would still see no increase in dmax, but you run the risk of blocking up high density areas.

    As for coating, I can coat enough by just multiple brushings while wet. I don't need to allow the coating to dry in between. I do this by horizontal and then vertical brush strokes reversing direction and sometimes do 2 - 4 coats just as long as total consumption is about 12 ml / 8x10 or per foot squared. The problem with brushing is that you most always see brush strokes with silver halide in gelatin due to the nature of the beast. It was not designed for this method of application.

    PE

  6. #6
    dwross's Avatar
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    Jerevan:

    Det ska jag. Tack!

  7. #7
    Jerevan's Avatar
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    Ska bli kul att se, dwross!

    ("will be fun to see" in swedish)

    Thanks for the information and input, guys!
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

  8. #8
    dwross's Avatar
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    I'm finally back at emulsion cooking. I bought a 8x10 size Mowrey blade from Photographers' Formulary. While I was doing my fall shows, all I could think about was getting back to emulsion research (and stereography, but that's a different story). Working with the 4x5 blade convinced me that this is for real and worth doing. I can enthusiastically recommend the purchase to anyone thinking of learning silver gelatin coating. If you start with a 4x5 blade, the initial expenditure to see if the process is for you shouldn't be much more than $500 (assuming a basic darkroom is in place.) I don't know what to say about learning the basic technique. A workshop from PE isn't possible for everyone, and to be honest, the process takes a bit of darkroom or chemistry lab experience. Maybe something can be worked out. I know that I'd be happy to demonstrate the blade (Ron:without giving away proprietary recipe secrets :-) to anyone who wants to come to Newport, Oregon, and I'm sure that former Mowrey workshop students in other locations feel the same. It's a cart or horse situation. How do you know you want a blade unless you know what to do with it? For now, I'm blogging my results here:
    http://dwrphotos.com/blog/EmulsionResearch.htm#Current

  9. #9
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    Congratulations Denise, it really looks good. The wider the blade, the harder it is to master the technique.

    I coat an 11x14 sheet with 12 ml of emulsion to cover an area of about 8.25" x 14" The 4" at each end are 'startup' and 'shutdown' artifacts due to the way the paper swells and the dropoff of the blade at the end of the traverse. I take the 8x10 out of the center. You saw some of those at the workshop.

    For gapping the blade, cut two strips of 35mm film and put one under each end and then regap. You will have about 0.005" at each edge and with a tolerance of about 0.001" the center should be no more than 0.006".

    If you put down too much at too low a viscosity, it does run as you state, but if everything is just right, it will not run significantly.

    I found, to my sorrow, that if you don't clean all of the adhesive tape from the coating block when you move a sheet, it can build up and cause a bump to form which increases coatings defects at the leading edge.

    Also, some papers are more repellant than others and can cause runs and do buckle as you describe. When I have that take place and cannot solve it by other means, I move the sheet right away to a temporary resting place and tape down the edges. This helps.

    Make sure that the paper you use is not less than 100# paper and it should be hard press. Cold press are good, but the texture on them often shows up in the final print.

    PE

  10. #10
    Jerevan's Avatar
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    Denise,

    it's looking really good! Worst thing about this, is that I am getting more and more tempted try myself...
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

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