Re: tonal inversion and pigment loads

On Jan 26, 2006, at 6:40 PM, Katharine Thayer wrote:

>
>> Tom wrote:
>>
>> By the way, I am not of the opinion that gum has a limit in the
>> quantity of pigment that it can hold, and that staining is the
>> result of the pigment 'having to go somewhere'. Solutions have a
>> saturation point but gels do not. Any tube of watercolour is the
>> proof that gum can hold more pigment than we ever use in gum
>> dichromate.
>>
>
> I've thought about that, and I know what you're saying, but I think
> we're talking about different things, and maybe I should use a
> different vocabulary for what I mean. The point is that if you took
> that pigment/gum mixture straight from the tube and tried to print
> with it, it would stain like crazy, and you need a certain amount
> of more gum to keep the pigment from depositing on the substrate.
> Demachy's quote on my page says what I mean, and my experience and
> observations coincide with Demachy's, whatever you want to call it.

For example, why do you think Chris saw increasing inversions as she
added lemon juice to the coating mix? To me, the most reasonable
potential explanation is that the acid changed the gum somehow so
that more pigment deposited as stain. The fact that she added more
water to another sample of gum, to control for it simply being more
liquid that made the difference, and that the gum with water added
did not deposit pigment, suggests that this may be a reasonable
explanation.

It's entirely possible that pigment stain (either kind) doesn't
account for every example of inversion that could happen, but so far
it provides a sufficent explanation for all but one of the inversions
I've seen, either with my own eyes or vicariously, and that other
observation, as I described the other day, was problematic on several
counts. I prefer a more general taxonomy, with allowance for
exceptions, whenever possible. And if and when I see something that
looks different, then I'll change my taxonomy. That's how science works.

You're right; Demachy is an old guy, and like you, I know that the
old guys (Eder for example) didn't always know what they were talking
about, or to be more respectful, didn't have the advantage of knowing
what we know today about the photochemistry of the process, say. But
on this particular point, all my experience and observations say that
Demachy was right. People tend to be more respectful to Demachy than
they are to me, so I hauled him in for reinforcement.
Katharine
Received on Sat Jan 28 09:29:43 2006