Re: gum preservatves
You might want to be a bit careful here. Lemon juice is not just acid, but it has some reducing properties that can interact with dichromate. Although less reducing power, citric acid may also be oxidized by dichromate more easily than gum molecules. Some other acids, especially mineral acids, have less reducing, but rather more oxidizing properties. Also, citric acid and many organic acids have affinity to form chromium (III) complex ions. I would not attribute your observation (and also what's described in your quote from Demachy) to acidity (that is, the pH dependency) until sufficient evidence is gathered to support such an idea. Also, meanwhile, modifying your practice based on the assumption that what's described is an acid effect may lead to a confusing or inconclusive result. -- Ryuji Suzuki "Strange how people who suffer together have stronger connections than people who are most content." (Bob Dylan, Brownsville Girl, 1986) From: "zphoto@montana.net" <zphoto@montana.net> Subject: Re: gum preservatves Date: Thu, 04 Sep 2008 16:12:08 -0600 > Demachy says: “A strong proportion of citric acid or > a smaller proportion of more active acid added to the > mixture of gum and bichromate will utterly destroy its > sensitive properties; that is, will render it insoluble > without exposure to light to such an extent that repeated > friction with a sponge and hot water will scarcely affect > it. We may, therefore, take advantage of this property to > counteract the excessive solubility of freshly prepared > paper and lessen exposure, giving more stability to the > half-tones. A small quantity of a weak (say 5%) solution of > citric acid, or simply a few drops of lemon juice added to > the sensitive mixture, will start insolubilization and allow > of slower and surer development. It is better, however, to > master the ordinary process before resorting to this > expedient.” 39 > > 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. > > > > > >4 Eylül 2008, Perşembe, 8:28 pm tarihinde, > >> zphoto@montana.net yazmış: Lemon juice does the trick > and > >> was Demachy's method in 1897. Chris > >> > >> ----- Original Message Follows ----- > >> From: davidhatton@totalise.co.uk > >> To: alt-photo-process-l@usask.ca > >> Subject: Re: gum preservatves > >> Date: Thu, 04 Sep 2008 16:18:47 +0100 > >> > >>>Hi Loris, > >>> > >>>Is there any reason one can't add some acid to the mix, > >>>Oxalic or citric say, to give an increase in printing > >>>speed? You could have a non-smelling acidic mix then.. > > > > > > Assistant Professor of Photography > Photography Option Coordinator > Montana State University > College of Arts and Architecture > Department of Media and Theatre Arts, Room 220 > P.O. Box 173350 > Bozeman, MT 59717-3350 > Tel (406) 994 6219 > CZAphotography.com >
|