But, alas, time is scarce and so I am happy that I can shoot some B/W negatives every now and then. And here's the problem: the chemicals don't keep indefinitely. And they are rather expensive. For that reason, I'm trying out just how long each and every mixture will keep and yield "good" results.
Here's what I found out today:
Ilford ID-11 developer. It says something like "the stock solution keeps for a few days, the 1:1 version keeps only for hours and can only be used once". Ummm... talk about understatement. I keep correcting the developing time as instructed for the stock solution (2nd film +10%, 3rd +20% and so on) and easily got ten films out of the two liters that the 1l pack diluted 1:1 gives me... over two or three months even! Today I was a bit annoyed though: I found out that my last batch was from November 2013 - well over six months ago. But: perfect result... yay!
Ilford multigrade paper developer. It says something like "1:9 working solution keeps only days and the bottle only months after opeining". Hmm... tried the batch that I made up in September 2013... and no, the result was clearly underwhelming: barely any reaction at all: just a reddish mist and not even slightly gray. Had to throw that one out. But the fresh batch with the bottle that I broke open in 2011(?) still worked perfectly.
On a side note: I keep the liquids (the solutions) in folding bottles - i.e. without any air in them.
Why does the manual say otherwise? Two reasons: 1.: nobody (especially not the manufacturer or me) will give any warranty as to the success of any "spent chemicals" use. And 2.: Conspiracy theorists say that it forces you to buy more of the chemicals than what you really need.
Personally, I don't believe in the 2nd reason. But the first one is still valid: if you somehow DEPEND on the pictures turning out good, follow the instructions! If you (like me) use that stuff mostly for recreational purposes: save a few bucks and buy less but better chemicals for the same amount instead of cheap ones :P