Re: Gum and contrast (was: UV printing systems)

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From: Sandy King (sanking@clemson.edu)
Date: 10/17/01-08:42:45 PM Z


Judy Seigel wrote:

>
>Sandy, I've read Kosar, for gum found it perfectly useless. I daresay
>Katharine will correct me if I'm wrong, but from MEMORY, it was Kosar who
>said the dichromates have a straight line, which they do in gum on
>alternate Mondays when your mix has angel dew. I don't recall if he
>included gum specifically, but he didn't exclude it -- which is to say I
>wouldn't take whatever in Kosar on faith (yeah, I repeat myself) unless
>I've tested it myself, and maybe not even then.

It is known that many gummists, past and present, engage in such
practices as mechanical development, forced development with water
sprays, rubbing the image with sponges, brillo pads and other
abrasive tools, etc. etc. Some have even been know to illustrate the
borders of their prints with hand drawings, painting, and various
other kinds of decorative information!!! Such practice (excluding
the latter) *distort* the basic sensitometric principles of colloid
processes so it is hardly surprising that some of the photochemical
laws described by Kosar for photosensitive systems do not apply in
your case.

>
>I think, suspect, assume, the "point of greatest sensitivity" is not the
>(entire) issue, because there are in fact SPIKES at other wavelengths that
>are higher, the different media and different colors have their own
>sensitivities, etc., etc. In other words, these generalisations are not
>definitive, may send us down dead ends, or cause us to overlook something
>better.

Virtually all light systems emit spikes outside of the area of major
radiation of the light source, and virtually all photographic systems
have some sensitivity outside the area of greatest sensitivity. That
is why we can print carbons and gums with light sources which emit
little or not UV light, i.e. daylight tubes. The bottom line,
however, is that as the wavelength of the exposing light falls into
the UVB range, and lower, the contrast of the image decreases. This
is a basic photochemical law of dichromate colloid processes.

>
>> Speed of the printing source, within a certain range of wavelength
>> (say 300-450 nm) appears to be more dependent on wattage than the
>> exact wavelength of the light source, or so my tests show.
>
>Oh yum, yum... Did you put those tests on the list?
>

Not yet, but coming soon to a theatre near you!!

BTW, I must correct a previous observation. As you might recall, in
comparing the basic pros/cons of my bank of BL tubes with a new metal
halide light system recently acquired, I stated that contrast was the
same with the two systems. In fact those earlier observations were
incorrect. Further, more definitive tests have shown that the ES of
carbon prints made with the metal halide unit is about log 0.30 less
than that of prints with the BL unit. That is, the MH unit is in fact
a more contrasty light by more than the equivalent of one full
stop/grade.

Sandy King

-- 


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