Re: again on color absorption

[Katharine Thayer <kthayer@pacifier.com>]

On May 12, 2009, at 7:34 AM, Katharine Thayer wrote:

> Ah, that makes more sense  than the explanation for a one-time only  
> appearance of this phenomenom in gum (on an HSL array) which was  
> advanced on the hybridphoto site, and which has been advanced on  
> this list from time to time, that the tonal inversion is a function  
> of underexposure, or because the "negative" had too long a density  
> range for the process.  Both those explanations were easily  
> refuted; this one has some merit in your case, especially if it  
> happens every time.  Mine, as I said, happened only once in many  
> printings of the same HSL array with the same light source and  
> correct exposure time,   so this explanation wouldn't work for what  
> happened in my case any better than the other ones did.
>
> Katharine
Loris wrote:

> Katharine, the only case I can remember that you have brought this  
> issue
> (gum tonal reversal, that is) into consideration was your 21-step  
> tablet
> test prints on glass - not HSL array... So nothing is refuted (as  
> well as
> nothing is proven) in my view -> since these two are completely  
> irrelevant
> cases.

Loris, apparently your memory is faulty, since I've brought a number  
of examples into the discussion here, many of which are included in  
my web page on tonal reversal in gum.  But the example I referred to  
specifically was one of an HSL array I printed in gum that showed  
this reversal, that came up in the midst of a discussion on  
hybridpthoto about choosing a good blocking color.  I didn't bring it  
up here to revisit that discussion ( discussions about tonal reversal  
in gum are more aversive to me than I can say), and in my brief  
mention of it I wasn't claiming to have refuted anything in general   
about gum tonal reversal, only to have refuted explanations for that  
particular example of it.  If you're interested in the discussion,  
it's in a long thread on hybridphoto; the relevant portion starts on  
page 7 of the thread:

http://www.hybridphoto.com/forums/showthread.php?t=347&page=7

I only brought it up here as an illustration of the  wide variety of   
opinions about the cause of tonal reversal in gum. When noticing the  
reversal on Alberto's HSB print,  I was just curious if the causes of  
this are more straightforward for metal processes than for gum.
Katharine Thayer





> On May 12, 2009, at 2:09 AM, Alberto Novo wrote:
>
>
> > > Katharine, IIRC, the portion below the dashed line (pure color)  
> > > would be
> > > less colored ink(s) + more black ink (due to less lightness/ 
> > > brightness -
> > > depending which test pattern is used) as you progress more to  
> > > bottom.
> > Now I am undestanding...
> >
> >
> > > So,
> > > my understanding of Alberto's test would be that black ink isn't  
> > > as opaque
> > > to the specific light which that specific process requires as  
> > > some other
> > > colored inks, that's why there's reintroduction of tone. This is  
> > > something
> > > I experienced with Epson 890/1290 OEM dye-based inks and HP9180
> > > pigment-based inks.
> > You are right, the mixing of colour with black decreases the  
> > amount of both. The absorption of pure black is in the last row,  
> > which is a little denser (absorbing less) than pure green.
> > Alberto