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

Quoting Mike Ware via Christina:
"According to the Beer-Lambert Law (see any photochemistry text) the
intensity of light, I, penetrating a homogeneous medium falls off
exponentially with depth, d........."

"...... the gumbi emulsion layer, when exposed, is no longer homogeneous,
and probably has a higher concentration of the photoactive
Cr(VI) species just near the paper surface - a distribution which will tend
to work contrary to the 'top-down hardening' phenomenon. The Beer-Lambert
Law no longer strictly applies in this system, because of this concentration
gradient. It's probable that relatively more light is absorbed at the paper
surface in consequence, and therefore relatively more hardening goes on down
there than would be predicted by a HOMOGENEOUS 'top down' model." (my caps)

To my mind, this explains why we have had the whole debate - gum hardens
from the top down, according to the laws of physics explained by Yves (and
then Mike Ware), but there is also an increased amount of chromium (VI) in
the gum that is in contact with the paper fibres, which allows THIS gum to
harden at least as thoroughly as the gum on the surface. These two variables
are sufficient to explain any anomalies in gum hardening - i.e. there are
two factors at play, and it's the combination of these that leads to the
anomalies.

So it's not a matter of top-down or bottom-up, but top-down PLUS bottom up,
caused by different factors. We can all go home and rest now.......

Kate

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