Fixing salt prints: why two trays?

[CreationBear]

I’ve been happily re-reading Christina Anderson‘s book on salt printing lately, but one small detail left me confused: she recommends using two trays, presumably of the same dilution of fixer, during the printing process, That would make sense to me if fixer were “one shot,” say, like some gold toners, but it doesn’t seem to be the case that she means a tray of chemistry is exhausted by a single print.

Anyone able to help my confusion?

 

[fgorga]

It is all about serial dilution...

This is probably easiest to explain with some concrete but not real numbers.

Let's assume that your print contains 100 mg of silver ions that needs to be removed. We will also that you let prints soak in fixer a tray of fixer long enough to remove 90% of the silver ions from your print.

With one tray of fixer, you can remove 90% of your 100 units of silver. Thereby leaving 10 units of silver ions in the print.

If you now transfer that print to a second tray, you can remove another 90% of the remaining 10 mg of silver, leaving you with 1 mg silver in your print.

Again, the numbers in my example are not real. This however, does not affect the conclusion: that two trays of fixer are much more effective in reducing the residual silver than a single tray.