Re: Back-exposing on plastic (was: Re: Gum transfer

Mark,

thanks you very much, you have summurised what I have been saying all along from my first message here on this list about gum. Gum is just another photographic process, nothing esoteric about it and just like another process, gum as its features and true these makes gum a distinctive process but still a photographic process.

Regards
Yves

  ----- Original Message -----
  From: Ender100@aol.com
  To: alt-photo-process-l@usask.ca
  Sent: Friday, April 28, 2006 1:48 PM
  Subject: Re: Back-exposing on plastic (was: Re: Gum transfer

  Yves,

  The gum variables can be modified to change the Exposure Scale of the printing. If you first adjust the density range of the negative prior to applying a curve, then you will get the maximum from the gum print—then the curve is only used to adjust the relative tonalities between DMax and Paper White. The adjustment of the density range of the negative is pretty important for gum, since it has a shorter exposure scale than many other processes and requires a lower density range negative. Doing this is one (just one) reason why Chris Anderson is having so much success with her tri color gum thingies.

  An example might be making a negative with an Epson 2200 where the UV transmission density of a negative using all inks can be over log 4.0—so if you need a negative to match an exposure scale of gum at let's say log 1.2, then you have a mismatch of 4.0 - 1.2 or log 2.8 TOO much density in the negative that the curve has to adjust for—that is over 7 stops! This is why you often see people using curves where the endpoint has been moved to reduce the density so the highlights won't blow out... unfotrunately for every point that you move that endpoint, you lose that many tones in the negative. You can actually use this to calculate exactly how many tones will be lost.

  Best Wishes,
  Mark Nelson
  Precision Digital Negatives--The Book
  PDNPrint Forum at Yahoo Groups
  www.MarkINelsonPhoto.com

  In a message dated 4/28/06 10:43:12 AM, gauvreau-yves@sympatico.ca writes:

    Katharine,

    my first reply on this topic was probably the cause of the misunderstanding,
    when I read it back now I see what you mean. With the last one I thought I
    made all this as clear as I can but I'll try again. If whatever you do back
    exposing your print fails to give you a satisfying tonal "delicacy" as you
    put it, may be applying a different curve would help.

    If I understand normaly exposed gum printing (front exposed) you can control
    the distribution of pigment (tonal "delicacy") by the various usual means
    including % gum, % pigment, % dichromate, thickness of emultion,
    exposure(s), development and physical manipulations, etc. With back
    exposure, it seems only one exposure can be done and all I'm saying is that
    beside all the usual controls you have the possibility to change the
    negative density (distribution) by applying some curve. Can you control
    every thing with some curve, the answer is simple no. The reason for this is
    that a couple variables of the gum process are totally independent of
    exposure (negative densities), the pigment load, as you call it, is one of
    these, development and physical manipulations are other mean by which you
    can alter the tonal distribution, in the limit you can scrape it all off
    (the emultion).

    I would certainly claim that if you maintain every variables fix ie. you
    don't change anything from print to print except the curve applied to the
    negative, you can basically obtain any tone you want between the Dmax and
    the Dmin of the print. Obviously, this fix variable gum print must show
    something usable to begin with.

    Regards
    Yves
Received on Sat Apr 29 06:31:00 2006