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

[Alberto Novo]

Loris, 

> Wow, thanks for the tip Alberto. Clearing in 1 minute? That's
> NICEEE... (Of course, it's much better not overexpose, therefore, not
> have to use a clearing bath...)

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
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