From: Sandy King (sanking@clemson.edu)
Date: 10/16/01-10:58:29 PM Z
If the question is, can good gum prints be made with BL tubes, the
answer is definitely yes. There is more to the matter, however.
I am not an expert on gum printing but work extensively with carbon,
a dichromate colloid process that has similar spectral sensitivity
requirements to gum, per the research outlined in Kosar's Light
Sensitive Systems. Kosar's conclusions, which I have observed in
practice in carbon printing, is that dichromate colloid systems have
their greatest sensitivity, but lowest contrast, in the wavelengths
of 350 nm and below, and decrease in sensitivity but increase in
contrast as the wavelength increases.
Now, of the various artificial light sources in common usage BL tubes
peak at (or very near) the greatest sensitivity of dichromate colloid
processes. Other light sources such as mercury vapor, carbon arc,
metal halide, etc. emit most of their light at wavelengths of 400 nm
or higher, which give higher contrast, but less sensitivity. So, all
things being equal, which they rarely are, BL tubes will give the
lowest contrast of any of the commonly used artificial exposing units.
However, the fact that a particular actinic light source give images
of higher or lower contrast is not particularly significant since we
can adjust the contrast of most processes by various means. With
carbon I simply adjust the strength of the dichromate sensitizer to
adjust for the wave length of the light source. Gum printers have
their own special techniques.
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.
Sandy King
>Judy Seigel wrote:
>>
>>
>> This statement is ... incomplete.... as are many of Livick's
>> pronunciamentos. The "contrast" in gum printing isn't a factor of one
>> variable, but combo of exposure, mix, neg, paper, development, etc. The
>> easiest way to up contrast is just put in more pigment.
>
>
>I agree with Judy about contrast in gum. I can't speak empirically to
>the question of BL tubes, since I've never used them, but I've seen gum
>prints others have made with them that weren't flat and lacking in
>contrast, so I'm inclined to think that if flatness has been observed in
>occasional one-shot tests, it's more likely due to variables
>other than the light source per se. I rather suspect that
>just by adjusting pigment concentration, dichromate concentration, and
>exposure, any degree of contrast desired can be attained, whatever the
>light source.
>
>Katharine Thayer
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