[alt-photo] Re: Gum...again!

[Loris Medici]

Dear Alberto,

Thanks much. I have concentrated sulfuric acid, if I ever need to use
a clearing bath I'll do by your method #2 quoted below.

About the remaining (light, IME) greenish cast; that' due
chromium(III) I presume...

Regards,
Loris.


2010/5/16 Alberto Novo <alt.list at albertonovo.it>:
> ...
> it is really a long time I don't make a gum bichromate, so I gave those
> amounts by memory.
> Seaching the list archives I have found what I wrote on 04/09/02:
> "I have used in the past pot. bisulphite 5%, or sulphuric acid 1%, or
> sulphuric acid 0,5%+sodium sulphite 0,5% (this last clears in a few
> seconds), but if the stain is really heavy the chemical methods leave a
> greenish or bluish tone.
> So I prefer plain water and, if it reveals to be not sufficient, only at
> that poin I use the chemicals (the third one)."
> Resuming:
> 1) prepare a stock solution of 5% w/v sulphuric acid (26 cc/L of 98%
> sulphuric acid) or a solution of equivalent strength, for example 31 cc/L of
> 37% hydrochloric acid. EVER add te acid to the water and not the contrary!
> 2) prepare a solution of 5 g/L of sodium sulphite
> 3) mix an equal amount of stock acid solution diluted 1+9
> This mixed solution will smell of sulphur dioxide; if it bothers you, cover
> the tray with a transparent sheet (glass, plastic) or an other tray.
> 4) rinse well the paper with tap water and give a last bath of water with a
> pinch of sodium bicarbonate in order to neutralize the excess of acid that
> may be still in the paper.
> As you wrote, it is better to not overexpose. I was used to see some stain
> after three or four layers, and in this case an overnight soak in still
> water was able to eliminate almost all. I made the sulphite+acid teatment at
> the end of many (6-10) gum layers.
> Alberto
> www.grupponamias.com
> www.alternativephotography.com/articles/art102.html
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