Judy Seigel (jseigel@panix.com)
Mon, 04 Jan 1999 03:07:53 -0500 (EST)
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From: Judy Seigel <jseigel@panix.com>
To: Sandy King <sanking@hubcap.clemson.edu>
Subject: Re: POP and Super Actinics
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On Sun, 3 Jan 1999, Sandy King wrote: Sandy, I'm going to confess, entre
nous, that I didn't understand your point (a) below, but don't tell
anyone, OK? You mean the colloid in general rather than a particular
colloid? I daresay, maybe, but.....
>
> a) Research indicates that the sensitivity of dichromated colloids
> (gelatin, gum, fish glue, etc.) is influenced more by the spectral
> absorption of colloids than by the nature of the colloid. That is, the
> radiation absorbed is actually what determines the eventual hardening
> (sensitivity), of the colloid. In turn, absorption is largely determined
> more by the dichromate, and its concentration (which absorb ultraviolet,
> violet and blue radiation), than by the colloid.
==========cut======================
> >From the above I conclude that the difference in sensitivity previously
> observed between different dichromated colloid systems such as carbon and
> gum, using similar light sources, can be explained by other variables
> (concentration of sensitizer (*and accuracy of measurment of same*, age of
> materials, etc.) other than the system itself. Of these, the most
> significant would seem to be the actual strength of the sensitizer.
......... there are just so many tests a person can think of, let alone
actually perform, but I tested a particular combination of ammonium
dichromate, gum arabic and pigment on a particular paper under 3 different
fluorescent lights.... BL, daylight and cool white. The results were
distinctly different. This from memory -- the cool white were virtually
useless (I think it was the cool white, I can look it up), the daylight
did print, took 3 times as long, but the CHARACTER of the print was quite
different. Color was more intense and curve shorter (fewer steps) than
with the BL. I always meant to try again alternating the daylight & the
blacklight bulbs... maybe some day I will.
Bill asks where I got the figures on the spectral sensitivity... what a
question !! Best of my recollection is: the 360 nm figure is in Keepers of
Light. But fella from Voltarc (makes bulbs, a firm in Connecticut) said
the same thing. Or maybe he said 365. I don't remember whether he or
someone at ICP stated platinum at 412. But my experience is there are so
many variables & those figures are anyway subject to question - so I
wouldn't count on any of the numbers as definitive. To "chart" them could
be as treacherous as charting the emulsions without taking paper into
account.
Plus I'm always discovering more variables -- doesn't everybody? Since
it's got to be done empirically anyway, and I find the paper as big a
variable as the light source (not to mention the mix, & so forth), I doubt
it is perfectly significant. What I suspect is that people get their
lightsource and then tailor their procedure (negatives, paper, etc.) to
fit.
Meanwhile, the different bulbs have spikes at different wavelengths -- to
diffuse matters still more. Of course someone like Bill who uses one
emulsion might want to get the optimum bulb for that emulsion -- but it
sounds like he has already. As noted in this space a while back, I've
gotten in the habit of using only the NuArc -- for the vacuum frame.
cheers,
Judy
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