Re: Gum hardening: top down experiment

Martin,

you said it yourself this is an anomaly and further it is a chemical
inhibition preventing hardening in presence of oxygen but this doesn't
prevent the photons to hit first and in larger numbers the atoms closer to
the surface then those closer to the bottom. Nothing will change that, this
is based on physical laws that had plenty of time to be challenged.

Can this happen in our case here, probably not, at least there seems to be
plenty of evidence it doesn't but again are we checking for the right
things. I have absolutely no doubts in my mind that in the strict physical
sence photons go through from top to the bottom and any of them (and there
are) bouncing from the paper will out harden the hardening from the top is
statistically extremely improbable if not inexistant. Now thanks to you I'll
have to add, except in cases where somes non physical factor(s) interfere
with the photo-mechanical actions of the photons, I stand corrected.

Regards
Yves

PS You got me...

----- Original Message -----
From: "martinm" <martinm@gawab.com>
To: <alt-photo-process-L@usask.ca>
Sent: Friday, April 14, 2006 7:49 AM
Subject: Re: Gum hardening: top down experiment

> ----- Original Message -----
> From: "Yves Gauvreau" <gauvreau-yves@sympatico.ca>
> To: <alt-photo-process-l@usask.ca>
> Sent: Thursday, April 13, 2006 1:49 PM
> Subject: Re: Gum hardening: top down experiment
>
> > The point is this, when a photon comes in to meet with this emulsion it
> will
> > either strike an atom on its path through all these molecules or it will
> go
> > through, it could even go through the substrate if it's not completely
> > opaque (obviously). This is definitely top-down or to be more accurate
> > photon always hit atom from the side they come from or if you prefer
> > "hardening always happens in the same direction as the path of the
> photons".
> >
> > Now, why in the world would someone be led to believe that it could be
the
> > other way around?
>
> In certain photopolymer systems you might just find that kind of
anomalies.
> To a large extent oxygen inhibts radical polymerization. Hence, you might
> have full curing in the depth of the layer whereas the surface remains
soft
> and tacky. One way to get around this, is to "sandwich" the photosensitive
> solution between two plates/films that protect from atmospheric oxygen
> (glass, PET, PVDC etc.). Incidentally, paper is certainly not a
particularly
> efficient oxygen barrier...
> Having said that, I've no idea whether atmospheric oxygen does play a
> significant role in dichromated systems though.
>
> > There is a
> > very low probability if any that enough UVC and UVB photons will go
> through
> > the emultion, hit an atom in the substrate and be emitted as UVB or UVA
> such
> > that they could hit an atom in the emultion and cause sufficient
hardening
> > from the back to outweight the combine effect of all UVs coming directly
> > from the source.
>
> I guess at the high levels of dichromate used for gum printing it becomes
> unlikely that radiation shorter than say, 450nm does penetrate the layer.
>
> Martin
Received on Fri Apr 14 07:15:21 2006