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

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From: Sandy King (sanking@CLEMSON.EDU)
Date: 10/18/01-06:12:28 AM Z


Judy Seigel wrote:

>
>> 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,
>
>Which as I recall worked with carbon, not gum. I think lumping the two
>together is mistaken in many of these respects.

Actually the daylight tubes worked very well with gum for Phil Davis,
from whom I got the idea to try them for carbon. Later we set up one
of these units with daylight tubes at Clemson but Sam eventually
changed the lights because times with cyanotype were so slow. I think
they were pretty slow also with gum.

>
>> 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.
>
>Ah, law... these laws are entirely manmade (probably in this case, tho
>maybe "human made" is nicer). So did you test that with gum?

Looking at the literature in Kosar indicates that a vast amount of
testing of colloids was done in the past and there are apparently far
more similarities than differences in the way different ones react to
light and the dichromates and chromates.

And no, I have not done any testing with gum. However, the
literature suggests that all of the colloids have similar spectral
sensitivity requirements (and thus should react in a similar way to
different light sources), which is really the only point I wanted to
make in comparing gum and carbon.

>UVB is "ultra
>violet blue"?

Ultraviolet light by convention is often divided into three areas,
UVA, in the 320-400 nm, UVB, 280-320 nm, and UVC, 200-280 nm.

>Did I mention, by the way, that different gums produce
>different contrasts? Did I mention that different proportions make
>different contrasts? Did I cite andsoforth?

This does not surprise me because it is also true in carbon printing.
Color also makes a big difference with speed. A blue tissue prints
much faster, and with lower contrast, than a red one.

Sandy

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