One of my recent posts has gone missing - lost during problems with the server? Here it is again, more or less.
Some information from Agfa that sheds some light on the issue:
When processing black-&-white reversal films and in micrographics and the graphics sector, the developed silver is bleached with bleaching baths which contain a strongly acidic potassium dichromate solution containing approximately 7.5 g of K.sub.2 Cr.sub.2 O.sub.7 /l as the oxidising agent. A soluble silver salt, for example silver sulphate (Ag.sub.2 SO.sub.4), is formed during the bleaching operation, and is rapidly washed out of the photographic layer.
It has become important on environmental grounds to reduce the quantity of dichromate or, preferably, entirely to avoid bleaching baths containing dichromate.
Substances having a similarly high electrochemical oxidation potential as dichromate, such as for example salts of tetravalent cerium, heptavalent manganese, trivalent iron and persulphate solutions are not suitable because, even at elevated concentration, they oxidise metallic silver only slowly (bleaching times of more than 10 minutes) or because, as in the case of heptavalent manganese, while they are indeed sufficiently active, they are so unstable in solution that they decompose after a short time and deposit manganese dioxide on the tank walls, in the hoses, on all equipment components and on the belts.
Combining the stated oxidising agents with known bleaching accelerators, for example thioglycerol, is also unsuccessful because bleaching times still remain above 10 minutes. Moreover, this method requires an additional bath (prebath) because sulphur compounds such as thioglycerol are immediately oxidised by the oxidising agent in a combined bath and thus rendered ineffective.
The object of the invention was accordingly to provide a bleaching bath for black-&-white silver halide materials which allows bleaching in a reasonable time (<10 minutes), is stable, may be used in continuous regeneration operation and contains at most 3.4 mmol of dichromate/l. The bleaching bath should preferably contain no dichromate.
This object is surprisingly achieved by the combination of at least 2 oxidising agents having an oxidising potential of .gtoreq.500 mV, optionally together with a metal salt having a stabilising action of a metal of subgroups 1 or 2 of the periodic system of elements, wherein, in the event that dichromate is used as one of the oxidising agents, it is used in a quantity of at most 3.4 mmol/l.
Suitable oxidising agents are, for example, persulphates and metal salts in elevated valence states.
Suitable persulphates are, for example Na.sub.2 S.sub.2 O.sub.8, K.sub.2 S.sub.2 O.sub.8 and (NH.sub.4).sub.2 S.sub.2 O.sub.8.
Metal salts having an elevated valence state are, for example, salts of tetravalent cerium, heptavalent manganese and trivalent iron.
Metal salts of subgroups 1 and 2 are, for example, AgNO.sub.3, CuSO.sub.4, ZnSO.sub.4 and CdSO.sub.4. Surprisingly, these metal salts stabilise permanganate solutions, for example, so suppressing the formation of manganese dioxide.
The persulphates are in particular used in a quantity of 0.001 to 0.5 mol/l, preferably of 0.01 to 0.1 mol/l.
The metal salts having an elevated valence state are in particular used in a quantity of 0.001 to 0.2 mol/l, preferably of 0.01 to 0.1 mol/l, wherein, as stated, the quantity of dichromate does not exceed 3.4 mmol/l.
The metal salts of subgroups 1 and 2 are in particular used in a quantity of 10.sup.-1 to 10.sup.-4 mol/l, preferably of 5.multidot.10.sup.-2 to 5.multidot.10.sup.-4 mol/l.
The bleaching bath according to the invention in particular has a pH value of 0 to 2, preferably of 0.1 to 1.5.
In order to shorten the bleaching time, an upstream bath with a bleaching accelerator, for example thioglycerol or 3-mercapto-1,2,4-triazole, may be used. This bath preferably contains the compound containing sulphur in a quantity of 10.sup.-4 to 10.sup.-1 mol/l.
It's from US patent nr 5,716,767 and the full text can be found by searching on the patent number at http://www.uspto.gov/patft/index.html
You might also like to do some searches like
(an/agfa-gevaert or an/agfa) and ((abst/black and abst/white) or abst/reversal)
which means applied for by Agfa or Agfa-Gevaert and containing 'black' and 'white' or 'reversal' in the abstract. I think.
Anyway, reading that got me thinking - why was I not having problems? I'm guessing that the answer is down to using inversion processing in small tanks and using fresh solutions for the bleach and the clearing baths. Less carry-over. Constant, brisk agitation.
Later edit: If you look at the full text of the patent, or better, the image of the patent, you will see the results of tests that Agfa carried out with various formulae on developed APX100. The formula, time and final density are given.
Patent 6,350,563 also gives quite complete details of a process (need not be the
process) for 'A commercially available black-and-white reversal film, e.g. that of film speed 200 supplied by Agfa-Gevaert AG'. While one wouldn't want to copy these, there is useful background information.