I've been following this thread on an off for the last few days.
While I agree with you that the photochemical reaction taking place
is ONLY influencing the physical property of the gum itself through
crosslinking, is it not also true that the pigment itself influences
the wavelength of the light actually absorbed by the gum layer below
the surface. IOW, is not a yellow transparent pigment naturally
going to filter the light to some degree and pass more lower
frequency light to the gum than would a layer pigmented with say, a
transparent violet color? My sense is that pigment, while not
involved in the hardening reaction of the dichromated colloid per se,
is indirectly involved in the manner that it allows UV radiation to
be absorbed in the coated layer of gum/dichromate/pigment. Thoughts?
Clay
On Dec 1, 2005, at 7:58 AM, Katharine Thayer wrote:
>
> On Nov 30, 2005, at 9:46 PM, Judy Seigel wrote:
>
>>
>> I didn't understand much of the discussion about printing clear
>> gum arabic, partly because I didn't understand it, partly because
>> I didn't think I needed to understand it, and partly from
>> suffering from turkey poisoning. But I think one point needs to be
>> made -- or repeated: Results with plain gum arabic and dichromate
>> without pigment will NOT replicate results *with* the pigment. Or
>> let me correct that to say, I doubt that they would or could.
>
> Hi Judy,
> Of course; and I think I've made that point several times, but
> perhaps not clearly enough, in discussing all the different ways
> that pigment can affect the tonal scale. My point about the
> unpigmented gum was to emphasize that while the pigment does
> provide the tonal scale, it does not participate in the reactions
> which constitute the response to exposure, so unlike silver
> printing and many other photographic processes, with gum you cannot
> draw a curve relating exposure to *density of reaction product* to
> tonal scale. Hope that's clear as unpigmented gum,
>
> Katharine
>
Received on Thu Dec 1 08:45:06 2005
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