S. Carl King (sanking@hubcap.clemson.edu)
Tue, 05 Jan 1999 10:32:49 -0400
Judy Seigel 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.
Darn, why did you confess in public? Now I am expected to explain it!
The purpose of the original post was to make the following assertion,
supported by research, though perhaps not accepted by all.
In dichromated colloid systems it is primarily the dichromate which
determines sensitivity, not the colloid. That is to say, if used in the
same manner (same % solution, identical strength sensitizer, similar kind
of development) gum arabic, gelatin, fish glue, and other colloids would
show similar sensitivity. In practice the different way we use the colloids
creates variables which would suggest that the sensitivity is different,
when in fact it is not.
>......... 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.
There is no question but that different results will be had from light
sources operating at a particular nanometer range, all other variables
being even. What I am suggesting is that the major variable (given same
light source) is the actual strength of the dichromate sensitizer. In
carbon this is definitely the case, and the results are both easily
observable and repeatable. I observed the same results in carbon printing
with BL, cool white and daylight tubes you report above. However, by
adjusting the strength of the sensitizer it was possible to make prints of
a similar density range with the BL and daylight tubes. In gum work,
because of the way the sensitized emulsion is actually prepared, one is not
as conscious of the exact % dichromate solution that is being used as in
printing with a more mechanical medium like carbon.
>
>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.
Clearly no one person is gong to be able to do definitive tests for
different light sources with all these processes. The best that we can hope
for is that persons with expertise in specific processes will conduct
controlled experiments with different light sources and make that
information available.
>What I suspect is that people get their
>lightsource and then tailor their procedure (negatives, paper, etc.) to
>fit.
I agree that it works this way in most cases, and the bottom line is that
excellent results can obtained with a wide variety of light sources (sun,
mercury vapor bulbs, plate burners, BL tubes, super actinic, etc). So we
adapt our working procedures to the light source and accept those
conditions as our standard. However, for every process (can we presume
there is an ideal negative for this process?) there should be an optimum
light source. Unfortunately, anecdotal reports are often contradictory and
not particulary useful in making this determination.
Sandy King
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