Re: (Gum) Tonal scale

On Dec 1, 2005, at 6:44 AM, Clay wrote:

> 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?

I thought I just made that point, in my clarification to Judy, below.
Throughout this discussion, I've made the point that the tonal scale
will be different depending on the nature of the pigment, among other
things, which is a more general way of saying what you've said above
more specifically. The problem is, we have no way of quantifying these
various effects. Remember that I was responding to Yves who wanted
tonal scale to be directly related to density of reaction product in
the same way that it works in silver, and for there to be an
easily-accessible answer to his question: what's the tonal scale that
can be printed in gum? To quote him:

  My understanding of "tonal scale" refers to the shape of the curve on a
> graph where the vertical axis is a measure (density most often) of the
> response to the relative amount of light received (horizontal axis).
> The
> tonal range would then be the difference between the maximum and
> minimum
> density values.

The point I made in response was to say that in gum, the response to
exposure is not DIRECTLY related to tonal scale. I didn't say that it
wasn't at all related, I only said that it wasn't directly,
quantitatively, related in a way that would allow one to draw a graph
showing the relationship between density of reaction product and tonal
scale. I did say, several times, that of course there is an indirect
relatiionship, but we can only infer that generally from what we
observe; we cannot measure those effects directly. To take this narrow
point I made in response to Yves' specific question and try to make of
it that I'm saying that pigment has no effect, is to miss the nuances
of the argument. I've said again and again, that pigment of course is
related to tonal scale, in fact pigment IS the tonal scale, but the
relationship between the pigment and exposure is indirect and (so far)
unmeasurable, so asking for a curve that gives the maximum and minimum
"density" values in terms of tonal scale, is to ask for the
impossible. That's all I was saying. To read it as if I were saying
that pigment has no effect on the tonal scale, is to overinterpret the
point. Thanks,
Katharine

>
>
> 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 09:22:35 2005