Chris,
Very informative experiment. I do not think that effect is due to acidity. I would rather put out a hypothesis that the effect is due to the vitamin C (ascorbic acid) present in lemon juice. Vitamin C is a strong reducing agent that would react quickly with dichromate reducing it to Cr (III) and making gum insoluble. Kind of like a dark reaction in gum, no light needed. It appears as a stain, but it really is not. It is really a chemical fog.
ANyways to test acidity one would use solutions of pure acids.
Marek
> Date: Sat, 6 Sep 2008 15:49:18 -0600
> From: zphoto@montana.net
> Subject: Re: gum preservatves
> To: alt-photo-process-l@usask.ca
>
> Here it is:
>
> http://christinazanderson.com/Text_page.cfm?pID=2076
>
> Scroll down to the very bottom past the color charts.
>
> I'm not saying it proves anything, but acidity may account for some problems
> that people say are issues with gum, and it is only really meaningful in
> relation to the water control strips done at the same time with the same
> amounts of dilution. Otherwise a lot more tests would have to be done to be
> conclusive. If, as Ryuji and Demachy said, there is a reduction to chromic
> acid with the addition of lemon juice, which I don't know because I am not a
> chemist, I don't know if that is speedier or less speedy than dichromate.
> Chris
> __________________
>
> Christin
a Z. Anderson
> http://christinaZanderson.com/
> __________________
> ----- Original Message -----
> From: "Loris Medici" <mail@loris.medici.name>
> To: <alt-photo-process-l@usask.ca>
> Sent: Saturday, September 06, 2008 3:13 PM
> Subject: Re: gum preservatves
>
>
> Thank you!
>
> Now that you say that, I remember the section about it in the "Learn" part
> of your former website. (Adding lemon juice and staining... I'm not making
> it up right?)
>
> Judy's note on sizes was interesting and real food for thought, BTW.
>
> Ryuji's notes also were interesting -> I mean the probability of citric
> acid (or any other organic acids) interacting with dichromate in an
> unwanted manner. That powered my original position which was increasing
> the acidity by not adding alien compounds... (Still don't know if that can
> work or not -&
gt; I may do some tests in the future if the exposure times
> become unbearable to me and/or I can't do nice casein prints...)
>
> Regards,
> Loris.
>
>
> 5 Eylül 2008, Cuma, 1:12 am tarihinde, zphoto@montana.net yazmış:
> >
> > ...
> >
> > Have at it, Loris. I found that the more lemon juice drops
> > I added to the mix (with drops of water added to the control
> > group in the same proportion) that I got lots of staining of
> > the highlights and lower contrast, but with paper negs this
> > might be helpful to you--I mean, the lower contrast part.
> > Step wedge steps were not too differentiated.
> > Chris
> >
> > ----- Original Message Follows -----
> > From: Loris Medici <mail@loris.medici.name>
> > To: alt-photo-process-l@usask.ca
> > Subject: Re: gum preservatves
> > Date: Thu,
04 Sep 2008 21:37:54 +0300 (EEST)
> >
> >>David & Chris, that also arrived to my mind just after had
> >>sent my last message... Even if I refrain to introduce
> >>another alien compound into consideration - as a first
> >>impression -, it sounds interesting / promising. You can
> >>bet I will try this (with citric acid)as soon as possible!
> >>Of course there's also the staining issue... I'll see.
> >>Thanks!
> >>
> >>Chris, do you know how much lemon juice was Demachy adding
> >>to his gum?
> >>
> >>Regards,
> >>Loris.
>
>
>
>
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