PROBLEMS MATE! (Continued)
"The most common solution (for bleaches) of this type contains potassium ferricyanide as the oxidizing agent and potassium bromide as the rehalogenizer. The bleaching action is not very pH-independent but the solution is usually buffered in the range 5.0- 10.0. It is especially important to avoid lower than 5.0, which could decolourize the dyes and decompose the complex ferricyanide. It is therefore preferable to use bleach solution well buffered at a pH near 7.0.
Another pointer, this time from Vol. 2 "Photographic Chemistry" by Pierre Glafkides.
"If pH of the bleach is too low the magenta image may he reduced. A careful wash must precede bleaching or the developer present in the film may be oxidized giving a coloured fog".
And yet another comment regarding colour reversal film bleach in "The Focal Guide To Colour Film Processing".
Although this acidity or alkalinity is only slight it has a big influence on the final colour balance of the film and for this reason it is most important to use the bleach specified for the particular process you are using"
Now I know it is possible to use the ‘simple’ bleach, i.e. not containing buffer. I have ~used these simple bleaches without any undue effect, but suppose one of these days your tap-water contains an undue amount of impurities such as copper? It could well be one of the none-uniformity factors. After all, the authoritive designers of the photographic process have got to make sure that their designs work over a large range of conditioners, so why not give the experts the benefit of the doubt and use a well-established bleach such as those published in the British Journal of Photography formulae. The extra cost of using buffered bleach instead of a ‘simple’ bleach is only a matter of a pennies-worth of tar. What do you do when you come back from a far away hclidaiy to find that some of the best slides have a touch of the greens?
And why don’t I go the whole hog and use the. standard Kodak ferric EDTA (ethylene diamine tetra—acidic.-acid) bleach? My answer is because I am an amateur, The original Kodak E6 ferric bleach was designed for the large scale American processing houses. and the U.S.A. powers—that—be considered that EDTA was more easily treated by the sewage plants rather than using ferricyanide.
Actually not. fern, is a better proposition for the amateur, because unless a very careful control is kept on EDTA bleach activity another problem, called leuco dye raises it’s ugly head. It means that the normal cyan dye becomes less cyan and you will finish up with a red cast in the shadows in-the slides. You wont get any leuco dye formation trouble with pot. ferri, and if you are using a buffered pot. ferri. all the better for getting more constant results.
Some may say what is a good ferricyanide bleach recommended by the B J. Here is a couple:-
BLEACH (pH 6-7.0) 5mns 33-39deg C. BLEACH (pH 6-7—6.9) 7inns 33-39deg C.
Potassium Ferricyanide. 112.0g. Potassium ferricyanide 100.0g.
Potassium bromide 24.0g. Potassium bromide 35’0g.
Disodium phosphate 12H20 62.0g Disodium phosphate Anhy 20.0g
Monosodium phosphate Anhy 15 6g Water to 1000.ml
Sodiumthjocyanate 10.0g.
Water to 1000°ml.The first bleach is very successful and is the basis (with slight variations) for several good E6 brews such as the Arnold Noorhouse and the Graham Dixon processes.
I have not actually tried the second bleach, but as it was prepared by the well known photochemist Ernest—Ghanles Gehret, I imagine there cant be much wrong with it.
Alas you cannot check the matter with Gehret because his latest paper on the E6 process was finally corrected only two days before he met his death in a hang-gliding accident". Unquote.
Returning to my own lack of success starting on weighing & mixing. As I say, I am very happy with the bleach, fixer and chemical reversal baths I am using. So this would point to the two developers, which require concentrating on. Of course in this latest processing the fact that I was using the ‘untried’ Labachrome film probably.