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  1. #11
    RalphLambrecht's Avatar
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    Quote Originally Posted by Ian Leake View Post
    I suspect that you're over-engineering a solution to a problem that is purely theoretical. Ordinary glass is fine, and the simplest way to decrease your exposure time is to add more lights.
    sounds reasonable to me.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  2. #12
    Hexavalent's Avatar
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    Most alt-processes do just fine with UV-A. Save your eyes/skin and don't worry about UV-B transmission - glass that passes 360 nm and longer is totally usable.
    - Ian

  3. #13
    David A. Goldfarb's Avatar
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    Realistically, what you do is pick your light source and expose and develop your negs to give you a reasonable exposure time for your process.

    I make albumen prints and use the sun. A good neg for me prints in 10-15 minutes in direct sun, 30-60 minutes in indirect sun. Some can be as short as 5 minutes or as long as 90 minutes, but anything longer than that, and there start to be heat artifacts from the rivets that keep the springs on the back (maybe I need to design a back without rivets that go all the way through, hmmm....). I haven't started albumen printing yet since relocating to Hawai'i, and our balconies face north, so no direct sun, but it's very consistent. It would be ironic if I had to build a UV exposure unit out here, but I might have to.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  4. #14

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    DAS-based processes are sensitive at 335 nm (compared to 360-365 for dichromate). I've looked at super-white and borosilicate glasses as an alternative for my NuArc. Some examples are Schott B270, Borofloat and BK-7. They have transmittance of 70-80% at 335nm, compared to 40-50% for plain-old soda lime glass.

    Given that there are 2 sheets of glass between the light source and the print in my unit, that can make a 1-stop difference in printing time. (0.7^2 = 0.49, 0.5^2 = 0.25). I haven't actually tested this, it's just theory at this point, but I hope to once the weather cools and the nights get longer.

    --G

  5. #15

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    I went to the glass shop and asked for a glass that would pass UV to replace the regular glass I had been using in my print frame. They called it "low E" glass and charged me about $17/sq ft. It made less than 1/2 stop difference in exposure, so if it breaks, I will just replace it with plain glass.

  6. #16

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    You have to select carefully. One ultra-clear glass I looked at, PPG Starphire, actually blocks *more* UV than soda lime.

    Also for dichromate or iron-based UV processes there won't be as much of a difference. 1/2 stop sounds about right, based on the transmission curves I've looked at.

    --G

  7. #17

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    I can just confirm that normal glass works perfectly well. Iím also using DAS.
    With a face tanner as light source at a distance of ~1m I get exposure times in the range of minutes. (Iím using 200mg DAS and 500mg of lamp black for 5g of gelatin)

  8. #18
    Jim Noel's Avatar
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    I have non-reflective framing glass in all of my contact frames and it works well. The slightly rough surface is against the negative and helps prevent Newton Rings which I used to have occasionally.
    [FONT=Comic Sans MS]Films NOT Dead - Just getting fixed![/FONT]

  9. #19

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    Quote Originally Posted by chrisaisenbrey View Post
    I can just confirm that normal glass works perfectly well. Iím also using DAS.
    With a face tanner as light source at a distance of ~1m I get exposure times in the range of minutes. (Iím using 200mg DAS and 500mg of lamp black for 5g of gelatin)
    Your face tanner probably puts out more UV-B than my NuArc. The NuArc lamps just weren't designed to have much output in that region. It's been a while since I've done anything with DAS, but my exposures were more like 20 minutes, and the tissue/negative sandwich was getting *hot*. If I could cut that by even 5 minutes (10 would be better) it would be a huge improvement.

    --G

  10. #20

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    Those face tanners are quite cheap second hand. My one consists of 6 fluorescence tubes (15W each).
    I did need quite long exposure times at the beginning until I noticed that I have to use more lamp black to get into a good gamma range. This reduced then the exposure time automatically.

    Chris

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