From: Sandy King (sanking@CLEMSON.EDU)
Date: 11/12/01-08:23:05 AM Z
Judy Seigel wrote:
>On Mon, 12 Nov 2001 FotoDave@aol.com wrote:
>> Hi Judy,
>>
>> ....it is hard to apply the "standard" sensitometry tests even
>> though gum printing does obey photomechanical laws.
>
>Hi Dave,
>
>Like we used to say in the olden days: you said a mouthful-- about
>applying "standard" sensitometric tests... especially since each color
>exposes differently even before you consider emulsion thickness.
>
>But what photomechanical laws are we talking about? Do I know them?
>
>J.
One obvious photochemical law that applies to gum printing is that
the colloid we know as gum is most sensitive to radiation (and tans
or hardens most) in a fairly narrow arrow band that appears to lie
between 300-450 nanometers (and perhaps below 300 nanometers as well
if we can use it). Regardless of other variables (how thick or thin
you make the emulsion, or the pigment used, or the strength of the
dichromate), it will be very difficult to make a gum print by
exposing with a 50 watt bug-light. This law can be demonstrated
readily by sensitomery, either low tech ( 21-step guide) or high tech
(log density readings with densitometer).
Sandy
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