Re: Could someone summarize that gum up or down discussion?

On Apr 13, 2006, at 5:52 PM, Joe Smigiel wrote:

> Could someone please *succinctly* summarize the gum hardening from the
> top down issue for me.

Top down rules, at least for printing on transparencies.

>
> I skimmed many of the posts but I missed seeing anything that would
> point to hardening from the bottom up.

Judy and Chris mentioned the fact that unreacted dichromate leaves
the print very rapidly, as an observation supporting bottom
hardening, but I don't see bottom hardening as a necessary conclusion
from the observation. The hardened gum matrix is a net rather than a
solid slab, and the dichromate can easily slip through the net, at
least that's the way it was explained to me by a physical chemist. So
the observation that the dichromate runs out easily is neither here
nor there as far as which way gum hardens, IMO.

> I have to admit my attention
> span isn't what it used to be and that I lost interest in reading the
> off-topic banter but I would be interested in any novel empirical
> results that would lead one to believe gum hardens from the bottom up.

No, you didn't miss anything; there's been nothing of that nature,
unless I missed it too.

>
> Marek's image on transparency helps convince that gum hardens from top
> down, but isn't that image problematic by also negating the widely
> held
> belief that more surface roughness (e.g., Pictorico ceramic layer vs
> smooth side) allows for a stronger or easier to print image in a
> single
> layer? (A post for perhaps another day...)

I'm puzzling about that too, but I think there's more to it than just
smooth vs. rough, because these special inkjet-prep coatings interact
in unexpected ways with the gum coating, as I showed with Pictorico
and Marek showed with the HP transparency.

>
> My take on this is has always been that gum hardens from the top down
> and that is evidenced by the often observed flaking of a too
> strongly-pigmented and underexposed emulsion layer. It comes off in
> chunks once the unexposed emulsion beneath it dissolves in water.
> This
> also seems confirmed by Marek's latest image.

Me too.

> What leads others to
> believe or postulate the opposite?

You'll have to look at Judy's posts for her reasons she believes the
opposite. The only observation I've made personally that makes me
wonder about top-down hardening is tonal inversion, such as the image
on my website where the gum layer was so underexposed that it ran off
the paper in development, and yet there was enough hardened gum left
behind in the paper to form a resist against stain.

http://www.pacifier.com/~kthayer/html/tonalinversion.html

I'm not arguing that this observation means hardening starts from the
bottom and goes up, but I don't think it can be explained by top-
down theory either. I don't know how it could happen if there
weren't some hardening at the bottom from the beginning.

Katharine
Received on Thu Apr 13 19:35:01 2006