Re: (Gum) Tonal scale

A couple of side comments have made me wonder about another possible
misreading of what I'm saying here. No one has commented directly about
this onlist, but just the same I think perhaps futher clariification is
needed, still. It was a simple, specific answer to a simple, specific
question; it's quite amazing to me how much is being read into it.

My point was simply that for gum, tonal scale cannot be read directly
from density of reaction products, because unlike silver, for gum the
reaction products are separate from the material that produces the
tonal scale. So you can't say for gum, as you might for silver or some
other processes, that x amount of whatever-compound-results-from-
exposure, will result in x tone on an absolute scale, or that the
maximum tonal scale that can be produced by GUM in one printing will be
around x units on an absolute tonal scale, as a result of the minimum
and maximum amount of the compound that can be produced in a printing.
Instead, the absolute tone of the darkest and lightest values produced
by gum, and the resultant tonal scale, is a function of added pigment,
and is thereby largely a function of the pigment and the pigment
concentration. So rather than there being one universal curve for gum
that relates the density of reaction products to the tone, there are
a million different curves, one for every gum exposure, and we don't
know how the reaction product relates to tonal scale, so we only worry
about how tonal scale relates to exposure, for any given coating mix
within a given context of variables.

But that's ALL I was saying. I was simply disagreeing with the
statement that there must be a universal curve for gum from which one
could read absolute tone directly from density of reaction products, a
curve that would apply to every gum exposure. This, of course, is an
impossibility. It doesn't seem like this should be even
controversial.

But none of my comments should be taken as criticism of the use of step
wedges or of the PDN system or any of that, and it surprises me that
anyone would take them that way. Heavens, I've used Dan's system for
years. My argument was against the idea that there would be one
universal curve for gum that relates exposure to density of reaction
product where said density can be read directly as tone. But my
argument was not against the idea that for any given specific coating
mix, one might usefully create a curve that would relate exposure to
tonal scale, setting the whole question of how tonal scale relates to
reaction products aside as an unknowable and irrelevant question.

Certainly for any specific pigment/gum/dichromate mix, one can make
judgments about optimal exposure and development, even about pigment
concentration, by using these tools. While I find that my own
intuitive sense of gum printing often works better for me for judging
how a mix is going to print an actual image than step wedges do, I do
use them sometimes, and have nothing against other people using them.
It may well be that those who calibrate their mixes and curves exactly
can relate tone, measured absolutely, to exposure in a direct and
reliable way, for that particular coating mix in that exact set of
fixed variables and that exact environment. I don't find it surprising
that that may be the case. Gum isn't THAT unpredictable, that given
exact conditions and protocol, it won't behave pretty much the same
every time. . But like I said, that's a completely different issue than
the question I was debating, about the relationship of reaction
products (crosslinked matrix) to tonal scale in gum. Thanks for
listening,
Katharine

On Dec 1, 2005, at 7:21 AM, Katharine Thayer wrote:

>
> 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 21:55:39 2005