hardening from bottom (was: Re: Gum problem(s)

On Nov 18, 2005, at 6:27 AM, Christina Z. Anderson wrote:

>
> BTW, the idea that the dichromate may, in fact, expose the layer at
> the bottom (I wouldn't have a clue if it did or not) probably stems
> from the observation that when you first soak your paper in the water
> face up, all the dichromate leaches out immediately. This only takes
> about a minute to happen, and then you can lift the print out of the
> water and watch the dichromate stream off. The layer is still
> perfectly intact. I have no idea chemically why this is so--the
> chemists of the list can answer that one--but I have observed
> sometimes in coating a paper with magenta that in my brush strokes the
> yellow of the dichromate brushes at the paper base and the pigment
> seems to be on top, so perhaps the dichromate is of a different weight
> or whatever the term is chemists use...mole...blah blah..

This idea seems to suppose that the gum layer exists as a solid slab,
of which the lower part of the slab gets hardened and the upper part
remains soluble and runs off in the water. This picture is inconsistent
with how things actually behave in the world of molecules. In fact,
the colloidal matrix is like a net (is how it was explained to me by a
physical chemist) and the dichromate ions can easily slip through the
net, so unreacted dichromate can easily come out from within and from
below a layer of hardened gum. So the fact that the unreacted
dichromate runs off easily doesn't say anything one way or the other
about where hardening occurs.

No, the argument goes that while top-down hardening proceeds according
to the Beer-Lambert law in carbon printing tissues, the same law
doesn't apply to gum, because the dichromate ions are strongly absorbed
to the paper, therefore there is probably a higher concentration of
dichromate ions at the paper surface than higher in the emulsion.
This is all theoretical, but the person (who will remain nameless
since it was a private conversation) offered, as indirect support of
the theory, the "fact" that unreacted dichromate is very difficult to
remove from paper. Since most gum printers (the personage is not a gum
printer) know, as Chris observed, that unreacted dichromate runs out of
paper easily, that's another reason to question this theory. But as I
said, without some kind of sophisticated analysis, we'll never know for
sure where the hardening occurs. I personally think of it as an open
field where light packets encounter dichromate ions sort of at random,
at different depths, so that the hardening is occurring randomly at
various depths at the same time. I've been told that this too is a
wrong idea. But like I said, nobody really knows how it is.
Katharine
Received on Fri Nov 18 10:44:49 2005