Re: GUM PRINTING QUESTION

> Katharine Thayer wrote:
> >
> > Chris,
> > Well, I guess I'm confused, because I thought that in the issue of sun
> > bleaching dichromate stain, you were talking about a brown (reduced)
> > dichromate stain, not a yellow (unreduced) dichromate stain.
>
> Because if whatever it is that causes that rare bright yellow stain
> (I've only seen it once or twice, when I was testing a lot of different
> paper-sizing combinations) somehow arrests hexavalent chromium in the
> hexavalent state and renders it stable (or at least unreducable) in that
> state, that would be a rather remarkable finding.

Curious, I tried to create a bright yellow dichromate stain (I've had an
idea about what might cause this) but was able to create only a soft
butter-yellow stain, not the bright yellow characteristic of hexavalent
chromium (but still definitely yellow, not the tan or brown stain that
is usually seen in gum practice). This yellow stain, after being
developed half an hour and dried, did turn brown when placed outdoors
half covered; the yellow was completely replaced with tan/brown in less
than an hour. While there was no direct sun, there was enough UV to
create a brown dichromate stain within two minutes on a (separate) sheet
of paper coated with dichromated gum, that I placed beside the other one
for a rough test of the available UV.

The brown stain that resulted from the yellow stain being placed outside
on a cloudy day didn't bleach within the four hours of remaining
daylight.

One test never proves anything, of course, but on the basis of this
observation I wouldn't recommend that people assume that a yellow
dichromate stain will stay yellow, or will lighten, when exposed to UV
over time. My observation would suggest that yellow chromium can still
be reduced to another form in the presence of UV, even when it is
trapped within the sizing/paper as a stain, and even if the UV isn't in
the form of direct sun.

 
Katharine
Received on Thu Aug 11 09:14:16 2005