What are the advantages of performing the C41 process with separate bleach and fixer baths rather than a single blix bath from the commonly available Unicolor/CineStill/etc kits? If there is an advantage, does it amount to a perceptible difference in the resulting negatives?
Commenters on several websites make reference to arguments from Ron, an ex-Kodak photo engineer who insists the common blix baths are second-rate. Quoting him, "C41 relies on DIR and DiAR couplers for color control. The material released is Phenyl Mercapto Tetrazole, among others and it is hard to remove from silver metal. Thus, bleaches are stronger than blixes to remove this residue and the silver. If you do not remove it properly, then you can get granier pictures with desaturated colors due to residual silver."– Christian ChapmanOct 12, 2020 at 23:13
Above quote taken from the thread here: photrio.com/forum/threads/… . The critical part I am wondering is if it actually makes a practical difference. I suspect it does not, since I have never seen this discussed.– Christian ChapmanOct 12, 2020 at 23:17
1Critical differences pertain to photofinishing labs that used automated enlarger printers. These printers used a lamphouse with trimming filters and cyan, magenta, yellow shutter filters. setting the color exposure custom for each negative, based on a scan. The make-over rate was held down by keeping the film processing to specifications. All this is beyond a home darkroom’s instrumentation. For the home lab, each negative is likely all over the map and the reprint rate is likely over 300%. When you digitally scan and print, modern pre-view software takes much of this variability out.– Alan MarcusOct 13, 2020 at 2:48
The C-41 and E-6 processes and their cine counterparts evolved in an era where giant regional processing labs were the norm. I was technical manager for 7 such labs in the Southeast United States. Each was sized to handle 20,000 rolls of color film a day. Such labs as these were duplicated in countless locals worldwide. Regardless of what you think, the vast majority outputted a top quality product, The standards were plus/minus 1/6 of an f-stop. Additionally, the color balance of outputted prints was topnotch.
That being said, when you process at this volume, the chemistry must have an indefinite life. Now we are talking, automatic temperature control, both heating and refrigeration. Test films are run interlaced with customer film. Test strips are measured and graphed with high precision.
Chemical tanks are filled with a tank formula and replenished, based on the graphs with a specialized replenished formula. The bleach must not only be replenished, it must be aeriated. The EDTA (red) bleach is iron based and its operation depends on the level of iron oxidation. The fixer accumulates silver complexes. As this fluid works to remove silver salts and metallic silver, it’s silver level must be controlled. This is accomplished by on-line electrolytic devices.
Besides all the problems of keeping the solutions at specification, the lab must sewer or haul-away to an licensed waste facility. In other words, fluids are treated, re-used and then finally discarded safely.
It is not economically feasible to mass process without keeping the bleach and fix as separate chemical baths. The color paper developing process did combine the bleach and the fix. This works out OK but separate would be better. This is true because paper prints can be reprinted if something goes wrong, film if ruined is usually not salvageable.
Separating bleach and fix is not a requirement is a home darkroom setting.
Modern color films contain miniature light-sensitive crystals. These are compounds of silver combined with a halogen (Swedish for salt maker). These halogens are iodine, chlorine, and bromine. They are imbedded in gelatin to form an emulsion. Color films consist of multiple emulsion coats, each doctored so they become light-sensitive to one of the three light primary colors, which are red, green, and blue.
The C-41 film, when exposed in the camera, receives a latent (Latin for hidden) image. This latent image is proportional to scene brightness and coloration.
As the film develops, developing agents seek out exposed crystals and reduce them to metallic silver and a halogen. The halogen is dissolved away in the waters of the developer. Remaining is a tuft of opaque metallic silver. This silver creates the black and white image. As the tufts of silver materialize, they will be acted upon by oxygen dissolved in the waters of the developer. The tufts of silver are caused to tarnish.
Dispersed in each emulsion are incomplete oily globules of dye -- cyan dye in the red sensitive emulsion, magenta in the green emulsion, and yellow in the blue emulsion. Being incomplete, they are said to be in a leuco state, (Greek for white). All three are missing the same ingredient which is present in the color developer. Those dye globules adjacent to tarnished tuffs of metallic silver are caused to unite with the missing dye ingredient. They now blossom into cyan, magenta and yellow dyes based upon their emulsion layer.
At this stage the film contains unexposed and thus undeveloped silver salts. The film also contains tufts of metallic silver forming a black & white image in each emulsion. The film also contains a cyan image, a magenta image and a yellow image.
The yellow image is fine-looking, the magenta image is fair, and the cyan image is actually substandard. To correct these deficiencies, the leuco magenta and the leuco cyan are tinted. This forms the orange mask of the C-41 film. Its purpose is to bolster the magenta and yellow dye image. The mask is not uniform as to its density. The mask forms two positive colored images superimposed on the three negative colored images. This makes of total of 5 colored images.
As the C-41 film emerges from the color developer, the colors are veiled by the metallic silver images, and the film has residual unexposed and thus undeveloped silver crystals.
A bleach to the rescue! The word bleach is deceiving. This solution seeks metallic silver and changes it back to a silver halide. This act sets up the film for fixing (to render permanent). The fixer is a solvent for silver halogens. The fixer removes most all traces of silver from the film.
Now the dyes have blossomed and the film is long-lasting. The dyes used are organic. Organic means they are made from the chemicals of life. Such things exist in a narrow environment. We are talking temperature, humidity, contamination and pH and the like. If these parameters are exceeded, the dye modifies. That is the reason that unpredictable results occur when something goes wrong either in the processing steps or in storage.
The edit makes this less clear, adding a bunch of irrelevance before getting to the good stuff. All bleach does is reconvert developed silver back to halide (not "halogen") so the fixer can remove both the developed image and the undeveloped, unexposed halide. Oct 13, 2020 at 16:21
Thanks for the clarification, it is appreciated, A tip of the hat from Alan Marcus Oct 13, 2020 at 16:23
For home processing, there are two advantages of separate bleach and fix over blix.
First, bleach and fix have antagonistic longevity requirements -- that is, bleach likes oxygen, thiosulfate (the core ingredient of all commercial fixers) does not. Therefore blix cannot last well. For kits, with a limited developer life, this isn't a huge problem, hence why many kits use blix, but if you do enough volume of processing to make replenishment of the developer feasible, longevity of the bleach and fix become advantageous.
Second, having bleach and fix separate allows tailoring how much bleaching action is given -- from normal bleach to completion, to full bleach bypass (produces a look like the films Saving Private Ryan and Band of Brothers), or anywhere in between. This is handy if you like to develop for special effects, or if, like me, you want to sometimes get a "free push" with XP-2 Super (bleach bypass lets me shoot at a higher EI with no increase in grain that I've noticed and no "pushed" contrasty look).
For me, running Flexicolor chemistry and experimenting with replenishment, and occasionally wanting to bypass bleach, separate bleach and fix are an advantage (though in fairness, if you're using a kit and want to bypass bleach, you can use a B&W rapid fixer in place of the blix). If you're content processing C-41 from chemical kits, it really doesn't make a lot of difference.