Density of Stain vs. Estimated Grams of Thiosulfate Ion/sq. meter
Originally Posted by Photo Engineer
Well this is a yardstick that don't measure up to the job!
I'll explain that in simple terms.
A 35mm film is about 0,06 m2
Fixer is about 20% solution
In the film remains about 12 ml fixer absorbed in the emulsion, that is about 15 gram of solutiion.
Weight of fixer 15g x 0.20 equal to 3 gram fixer in the film.
Filling the tank from emty dilutes everything 1:20 and after a very short time equilibrium between concentration in film and water is reached (this time gets slightly longer as the concentration sinks)
Concentration before washing 3gram / 0.06 m2 = 50 gram per m2
1. tank 50 g/m2 : 20 = 2.5 g/m2
2. tank 2,5 g/m2 : 20 = 0.125 g/m2
3. tank 0.125 g/m2 : 20 = 0.00625 g/m2
Look at these numbers once again:
The test goes down to 0.01 g/m2. If that is archival quality, fine by me.
But the number show that 3 tanks after the Ilford method is already at a theoretical limit of 0.006 g/m2 in my book that is already past what you can HOPE to measure, by your indicated measuring method.
Now, I was under the impression that the Ilford method was not up to archival standards, I was assuming that we need to reach at least 0.001 or lower (from memory). That will be reached by more changes of water, easily and effortlessly.
But you cannot measure THAT by this method, you don't have maesuring RESOLUTION down to that level with this method.
Is that concept so damn hard to grasp?