Re: Anderson's "gum-pigment ratio test" (fwd)

FotoDave@aol.com
Tue, 09 Jun 1998 14:23:19 -0400 (EDT)

In a message dated 98-06-09 10:26:39 EDT, FotoDave@aol.com writes:

<< Same thing when we say adding more dichromate. We think of the dichromate
as
*one* variable, but it is not an *independent* variable. In particular, when
related to gum printing, adding the dichromate increases the amount of
dichromate ions, which affect the hardening of gum, but it also adds water,
which affects (or can affect) the viscosity of the mix and the thickness of
the coating.
>>

Maybe I will use this example to again illustrate the difficulty of drawing
any conclusion with a "variable" that affects more than one things. Then I
will quit before I, too, bore the list to death. :)

With the addition of dichromate solution, we have more dichromate ions in the
emulsion mix, so logic says that it should allow for longer scale. Maybe it
does when you add just a little more. But when you add more, it dilute the
whole mix so coating is thinner, so while it is capable of hardening more gum,
in actuality less gum is available on the surface of the paper, so you start
to see shorter scale. When you dilute even more, you start to break to
suspension capability because the gum is too diluted, then you might see
different effect.

I have seen at least one contradictory writings about whether the addition of
dichromate gives longer or shorter scale. It was interesting. I don't think
there was a typo. I don't think either one is wrong. I actually think both are
right. It is just the cross-dependent effect that they are seeing (depending
on how much they add).

Dave