Re: GUM PRINTING QUESTION

Katharine Thayer wrote:
> >
> 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.
> >
>
> It's worth, I think, making a special note of the fact that the
> dichromate that's trapped in this yellow stain doesn't work as fast as
> active dichromate in a gum-dichromate coating, as shown in the above
> observation that it took an hour for the yellow dichromate stain to turn
> brown in diffuse UV but only a minute or two for the gum-dichromate
> coating to turn brown.

Which probably explains the otherwise puzzling fact that this stain
remains untouched by the UV in a normal gum printing exposure; that is
it keeps its bright yellow color through a usual printing exposure,
even though the dichromate that remains free in the coating (not trapped
as stain) behaves normally and creates a completely normal print on
top of the yellow- stained sizing. Somehow the trapped dichromate
reacts much more slowly than the dichromate that's not part of the
stain; I certainly don't have the time, the equipment, the knowhow, or
the inclination to find out why that might be the case, but it's an
interesting question.

Katharine
Received on Wed Aug 17 10:52:17 2005