The capacity of FX40 fixer is given as 10 fibre base prints
/ 40 RC prints per 1 litre of 'working solution' without
indicating dilution used. Tom.
There is no doubt you have a rapid fix concentrate;
ammonium rather than sodium thiosulfate. The limit
stated, 10 FB 8x10s, is the same 1:4 or 1:9.
Likely Fotospeed has adopted Ilford's HIGHEST standard
for capacity. If the concentrate is of the usual strength and
processing for less than the Greatest longevity Ilford's 40
8x10s FB limit at 1:4 is the limit. At 1:9 the limit is 20.
The 10 print limit is not based upon chemical capacity.
1:4 or 1:9 there is more than enough chemical capacity
for 10 8x10s. THE LIMIT, 1:4 OR 1:9, 10 prints OR 40
prints is the ACCUMULATED SILVER CONTENT of
the fixer.
Ilford sets the limits at 2 grams silver per liter of fixer
for usual processing and sets the limit at 0.5 grams for
optimal permanence processing. That's the difference
twixt 10 and 40 FB prints.
Have I made all that clear? Dan