Re: Tanning theory of dichromated colloids (was gelatin

"But I have the same question about this as about the statement in
Duncalf & Dunn. If you put the EDTA with Cr(VI) and it keeps the
crosslinking from happening, how do you know where the inhibition
occurs?"

Good question, I don't know the answer.

"The other observation, which shows that EDTA can dissolve
hardened PVA (...)."

Could it be the relatively high pH (EDTA being fairly basic)? Cr(III) is
said to require an acidic environment.

 "And this dye sensitization I think is the same thing that is referred to
in Kosar as "optical sensitization" which is what I asked someone to
explain to me a while back and got no answer. Can you explain quickly
how this works?"

I ignore what Kosar says about it. I do know however that as early
as 1892 Lippmann used a couple of dyes in connection with dichromated
cellulose and
dichromated albumin. I don't have much details about that though. Neuhauss,
another practitioner of Lippmann photography, made tests with dichromated
gelatin. He stated having accomplished good color balance by adding
erythrosin, glycinred and
pinachrome.

More recently, Jeff Blyth (Methylene blue sensitized dichromated gelatin
holograms: a new electron donor for improved photosensitivity, Applied
Optics, Vol 30, pp. 1598-1602, 1991) introduced a greatly improved
red-sensitive DCG system. The mechanism involves:
Red light is absorbed by the dye (methylene blue). The excited dye oxidizes
the electron donor (TMG). At the same time that electron donor recovers its
electrons from the dichromate which thus gets reduced to Cr(III).

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, May 17, 2004 6:38 PM
Subject: Re: Tanning theory of dichromated colloids (was gelatin

> MARTINM wrote:
> >
> > "So in this case they knew they already had Cr(III) but the EDTA
inhibited
> > the crosslinking?"
> >
> > I am not sure if this applies to Cr(III) specifically but certainly to
> > "dichromate". If the quantity of EDTA (actually the dichromate/EDTA
ratio)
> > reaches a certain threshold, it will impair any crosslinking to take
place.
> > In former dye sensitized DCG systems EDTA used to be added as an
electron
> > donor for the dye.
> >
>
> But I have the same question about this as about the statement in
> Duncalf & Dunn. If you put the EDTA with Cr(VI) and it keeps the
> crosslinking from happening, how do you know where the inhibition
> occurs? The other observation, which shows that EDTA can dissolve
> hardened PVA, is more compelling to me, but is still indirect.
>
> And this dye sensitization I think is the same thing that is referred to
> in Kosar as "optical sensitization" which is what I asked someone to
> explain to me a while back and got no answer. Can you explain quickly
> how this works? Thanks,
> Katharine
Received on Tue May 18 11:38:27 2004