U of S | Mailing List Archive | alt-photo-process-l | dichromate stain and chemistry question

dichromate stain and chemistry question



Good morning, all,
I sit here, in the dark in my hotel room in Toronto, sipping my coffee, waiting for grouchy to wake up so we can go for breakfast. He already yelled at me for clinking my coffee cup on the table one too many times....can't imagine what he would've said if HE had received Karen Quinn's emails....

Nice to be able to visit with Michael Sinclair yesterday, another alt lister whom we had the benefit of being with in China for 3 weeks last summer. Michael took us to some wonderful galleries around town.

Today I visit an old high school friend of mine who is, by coincidence, a photographer, and represented by a Toronto gallery we visited--platinum prints (Elizabeth Siegfried). What a hoot to see her work there, as we have not seen each other in many years.

Away from the travelogue and back to dichromate stain. As I said in a previous post, I do not clear. I don't use full strength am di (only 15%), I don't overexpose, and I usually develop prints for an hour so I never have a problem with stain. Sil Horowitz delivered me from clearing hell.

To the chemists of the list, does this chemical explanation sound plausible--it's Greek to me:

"Light acts the same as heat, and in the presence of organic matter sets free the ammonia from the double salt, which then becomes acid by the formation of chromic acid, which is reduced by the organic matter to chromate or chromic oxide. If the washing is continued this chromate is reduced to chromium oxyhydrate which is a pale greenish color."

Way back when I talked of putting a gum print out in the sun which will get rid of stain, too--it was scoffed at, as is usual, but try it--sun fades the stain. By making a gum print with no pigment, you can demonstrate all this stuff. In fact, the reason gum printing with no pigment never made the grade with its originator Mungo Ponton was that the brownprint faded to pale green with time and light.

However, way back when on the list, I remember Peter Frederick or Peter Marshall talking about the green stain being annoying, and we talked about Luis Nadeau's method of getting rid of that: "Greenish stain could PROBABLY be removed by .5 to 1% hydrochloric or sulfuric acid, but it is chromium oxide and is a permanent pigment and should be of no concern." 40.Nadeau

I just came across another method of getting rid of the greenish stain:

Feb 1904 no 2 Dechromating gum-bichromate prints. P. 59-60 H. C. Kosel recommens, in Apollo of Oct 8 p. 237 powdered alum, 50g; water 1000cc, strong hydrochloric acid 15cc, acid sulphite solution 10cc. which is a saturated aqueous solution of sodium bisulfite, through which sulphurous acid gas is passed until no more is absorbed; but if you can't do this use double the quantity of hydrochloric acid and then add 15g crystallized bisulphate of soda. Print remains in this a few minutes and then is washed thoroughly. He says, however, that prints in various ocher or earth colors are likely to suffer considerably, but in carbon black it is fine.

I share this for historical interest only, and again reiterate that I do none of these, as whatever pale green may be left in the shadows doesn't affect the look of the print as far as my eye can tell, and is not in the highlights. Plus, who has sulfuric or hydrochloric acid lying around these days? But I suppose back in the day the greenish, or yellowish, tone affected the one coat black prints they made in a muddy way, perhaps.
Chris