Ok, you never know what you are going to learn each day on APUG. I now know why my images and negatives don't match up with Ansel and Edward's.
My negatives and prints are all made in dirty air! ;-)
And who can forget this famous apug discussion on the topic:
Where did you think those gray cards came from?? :D
Which brings to mind: Why did George Eastman choose Rochester New York to establish Kodak?
Because Rochester New York is the world's largest natural darkroom!
I had the pleasure of seeing a lot of Adam's early interpretations of some of his famous works a few years ago. I was struck by the much more normal contrast range of Adam's early prints. I think most of the black skies in the later prints are the result of darkroom manipulations.
Of course they were "manipulations". To quote the man himself, "the negative is the score; the print is the performance." During the course of a long career, Adams' opinions about how his prints should look changed. Any of us might discover this; if I was to go and reprint a neg from 25+ years ago it's likely I'd print it differently. A key difference in this is, of course, that Adams had (I should say made) a market for his older images, unlike myself...
Remember also that the Zone system is not about the literal transcription of light values- but being able to control them to achieve an emotional result. Which Adams did.
As I know Ansel was very smart guy (photographer). He knew limits, or where to stop, in darkroom work very well. It is possible that he added some light in darkroom but sure not that much so anyone set a question. So it is a filter (he used Red one a lot).