Color negative developer near to C41
Here we go again… :)
With some curiosity I checked different websites for colour negative developer formulations and some time ago I’ve posted (a bit cheeky) variations of developing solutions (which aren’t very well…). Very most public formulations are suspicious similar to formulations can be found in various patents by Kodak and Fuji concerning stability / speed / amount of concentrates of colour developers solutions. These formulations include typically CD4 0,015mol /Litre (4,38g/L) 1,3g NBR, 12mg KJ and a Ph of 10.00 (of course Potash ~30-37,5g, HAS ~2g, Sulphite ~4g, and chelating agents are present too)
Developer of that kind is (in my eyes) only for research and meant as a comparative to set a stable basis. They do work, but not as a genuine C-41 developer should work. Even if the densitometer’s readings for speed and contrast for blue and green are partway OK, the red channel suffers badly. Taking some time to burrow through these scripts and search for changes in D.min/max under different formulations will show that too.
Problems and Pitfalls:
The bottom red sensitive, cyan coupling layer(s) do need more developing agent as usually listed.
The middle green sensitive, magenta coupling layer(s) behave quite well but:
The top blue sensitive, yellow coupling layers tend to be underdeveloped.
Increasing the amount of CD4 in increments till 5,9 g/Litre (max. 6.2g) leads to an overall rise of density and contrast in an undesired manner. (You may remember, that’s the “photo techniques” formulation, all other found formulations are wrong as well (4.4/ 4.5/ 4,7/ 4,8 and 5g CD4))
Than the spec. density of the solutions came into my mind, the usual 37.5 till 38 g /ltr. Potash leads (in my eyes) to an excessive diffusion pressure of alkali into deeper layers.
The ph, with it’s most magnitude of influence has to carefully adjusted, usually at about 10.00 +/- 0.05 at 25°C (in the research /comparative formulations). For non chemists, equipped only with a quite simple pH meter, this is not a fun!!
Forgetting the factor of maybe insufficient measuring equipment, I suspect this pH of 10.00 is too low.
The amount of bromide is very important too, it does affect the first layer(s) more than the middle or bottom layers, which stay under influence of bromide, released by developing the surrounded layers. Bromide concentrations usually are given in mol/Litre. Be aware that KBR has to multiply by 1.16 due the different molar weight, if NBR is requested in a formulation!!
Iodide (KJ) is till now 12mg/ltr. From former experiments with E-6 homebrew I know that blue-yellow axis can be controlled (within limits) via the iodide concentration in the FD, as well as in the CD. Maybe that’s typical only for E-6, and having read in PE’s valuable posts that the interimage effect and as a result sharpness is affected by iodide too, I’m till now a bit reluctant modifying the KJ dose.
By the way, I’m doing tests with dry prewarm and wet prewarm /prewash. Since ever I’ve done a prewash, goanna stop this now. Diffusion speed for developing agent / alkali is increased for the cyan and magenta layers, R-G show an observable shift.
So this is a funny game between pushing and pulling components, diffusion speed, and pH. At least the temperature stay’s constant… ;)
My last tests (No.19 – 22) were encouraging, the colour curves match well, exception still a slight but quite constant decrease of red. If using a prewash the results are very close to a former probe, developed with Flexicolor (prewashed too) but still different from my “reference minilab”. Maybe this is mainly the difference between rotary tube and leader card / or continuous developing machines, never have checked that before.
… a week later…
Anyway, after some nights in the dark, some thinking, spreading sheets of paper all over the flat, I’ve found a formulation that works VERY WELL.
Very well means he colour curves shift not more than 0.09 from the green base. This is for the white pattern (HD) from the “Jobo test Chart” that one with 6 grey levels, RGB and CMY Patterns. This nearly reaches my minilab DIY reference.
Density of HD is good, film speed either.
I’m goanna try to load a scan of my densitometry values, but be aware, this is no conventional C-41 plot, I do it the way I do it for chromes too… values in braces are absolute, the other RGB values for field 1 till 6 are already subtracted from Dmin…
This is the VERY BEST result so far, works much better than all other formulations I’ve ever tested. I call it C-25, for test run 25 ;)
Here the formulation so far. In order to avoid the subsequent pH correction with sulphuric acid or KOH (which is a must-do in all formulations) I will try to include a carbonate/bicarbonate buffer system at the next run, but this may take some time...
800ml Tab water
1 ml Orwo A905 (wetting agent, can be omitted)
2 g Calgon
29g Potash (28 till 34 g are OK)
4g sodium sulphite
1.5 g potassium bromide
12mg Potassium iodide
Up to 1 Litre
pH adjusted with KOH or sulphuric acid to 10.11 at 25°C
Spec. gravity reads 1.035
As you may see it would be make sense to increase CD4 in a very small fraction (maybe 0.1 till 0.15 grams, than maybe reducing the ph carefully to maintain HD at about 1.00
If you are in some homebrewing, try this, you will not regret it.
Regards from Germany,