B&W film fixer lifespan.

[Cluster]

Hi, I am presently using some ADOX B&W film fixer. It's been in my garage for around 10yrs (with my Rodinal and Stop concentrate).
The instructions suggest 1:4 mix for use with film. I think it also suggested that it is discarded after use.

As fixer seems to be quite expensive for the amount that I might get through, I thought that I would try to see if it would last for successive uses.
At 1:4 it clears a scrap of Fomapan100 in seconds, maybe 10s. That seems fast! so I tried with it watered down a little more, now it's somewhere around 1:6 and still clears pretty quickly.
I made a mix of 500ml and have reused it on 5 or 6 135 rolls. Each time I use it, I have checked that it clears film. It is presently clearing scraps in under 20s.

I guess that it will get slower and slower to clear the film scraps. At what "clearing" time should I regard it as done? Will 30s be reasonable?

BTW, I'm happy with the results on film.

 

[dmb]

Most fixer instructions normally recommend fixing for twice clearing time and that when clearing time is double what it was when fresh then it is time to move on,

 

[Cluster]

Thanks.... 30s will be about double the time at the concentration I'm using.

 

[Peter B]

Just to expand on what David said, here's the info from the Ilford website https://www.ilfordphoto.com/amfile/file/download/file/1833/product/711/

Film clearing time
In order to avoid the risk of insufficient fixing. film
should remain in the fixer for twice the time it
takes the emulsion to clear. Fixer should be
discarded when the clearing time in used fixer
exceeds twice the clearing time in fresh fixer.

The clearing time of a film and fixer combination
can be found by the following method. It can be
carried out in normal lighting.

Take a piece of scrap unprocessed film and place
a drop of the working strength fixer on to a small
part of the emulsion side. Leave it until the
emulsion under the drop is a clear spot, this
should take around 30 to 60 seconds. Immerse
the piece of film in the fixer bath and using a stop
clock time how long it takes for the rest of the film
to clear. Clearing can be judged by comparing
the surrounding film area with the clear central
spot. The time taken for the rest of the film to clear
is the clearing time. The fixing time needed is
double the clearing time.