This isn’t about flutes, but bagpipes get plenty of moisture. My friends the Seivanes of Galicia, Spain are masters of using boxwood for their gaitas. Their wood storage room is amazing!
Check out this link and scroll down to the “Process” section - https://seivane.es/en/nuestras_gaitas/index1.html
I am wondering if the placement of where a billet is from makes much difference. Boxwoods are not large trunked trees (?) and so possibly the billets are an asymmetric mixture of heartwood and sap wood ?
I have read of a flute called “The hygrometer” which changes shape according to moistute, and if so that would suggest that really we would be looking at expansion coefficients along or across the flute, which long seasoning might not overcome.
Though I have not made many flutes, the only one which shows minor movement so far is a renaissance flute which has vastly different wall thickness across its diameter. So that is forgiven for being greater pressure available on one side. All the flutes I make are from the center of a trunk or branch.
I don’t think hardness is too important, but it matters for certain choices of sound. For example (and with the limited experience I have), I find fruit wood will give clear high tones of any note, but it balances them with a more gentle or mellow background, whereas harder woods give a stronger contrast and discernible bass.
Just recently I have been trialing the layout for a cylindrical flute, and decided to use an iron tube for that because it was handy, had the right bore size, and is solid enough to hack around with. So the sound from that is good and clean but quite expressionless (I think we are supposed to pity metal boehm flute players at this point ?). I then pick up the rudall style flute, and for a moment I think it is like balsa wood. It is a heavy enough flute, the renaissance flute is very light by comparison, but after holding and playing an iron flute for half an hour, it felt like balsa. For a moment I thought it would not play for being so light, even though I knew it did. So playing it then, the contrasts of how the sounds of the instrument are made really stood out, going from 10k/m3 to 1k/m3 . Iron was very even, to the point it could almost be called dull. Wood, instead of that evenness, transformed it to a sound that was distinct and alive.
Personally I could not say the difference between harder hardwoods and softer hardwoods is in favour of one or the other.
They are subtly different is all.
The boxwood that I have certainly appears to be unmixed (heart or sapwood),and given the source (it is considered Grade A instrument wood) I expect that wasn’t a factor. But your speculation about where in the tree the billet is cut from is pertinent, I think. I feel certain that there are different degrees of grain tension in woods, depending upon what portion of the tree it came from, but I expect one has to really know their timbers to be able to use that knowledge effectively. But I’ve been able to observe that billets that have “grain run-out” (where the grain direction does not run straight throughout the billet) are far more inclined to bend.
I would be very interested to know how a blindfolded listener would respond to hearing an iron flute played alongside a wooden flute (presuming the design was identical). According to Coltman’s study on materials, it shouldn’t matter to the listener, but I am certain that the “under the ear” experience of the player would be really different!
All the flutes I have made so far follow the centerline of a branch, and all I know is that that (so far) has been ok. Given that these are branches I am using, it would be expected that they were under tension of some kind originally.
These links explain tension wood in boxwood better maybe
https://filipfrydrysiak.com/about-wood/
https://ebts.org/uk/2019/boxwood-as-timber/
https://arbtalk.co.uk/forums/topic/62463-compressive-amp-tension-reaction-wood/
https://agritrop.cirad.fr/391164/1/document_391164.pdf
The first link says some wood never settles, and for stable wood you would probably have to make sure the billet was taken from a non stressed part of a tree. It would be expected that makers using box frequently would have a good enough idea of choice of suitable wood that doesn’t warp ? I am just thinking that if close enough to centerline, the wood there has become quite consistent, even on tension side.
I was going to mention that study (I think it is that one) at some point, where different metals were used ? The trouble with it though is the difference in density between different metals is very small, especially when we consider they are interacting with air. It is possible the player hears any difference and audience not, and so even that would affect how they play, and so what the audience thinks of the whole recital ?
Only people/creatures are able to say what any combination of frequencies heard sound like, or for example are able to focus on certain ones in a passage. Recording equipment might just be able to separate out the different layers to a sound, but a computer would not have much idea how to interpret them in terms of what a listener might think or make of them. Most digitized sound is no more than a grosso modo aggregate of all the sound ploted to a single pressure point on a graph. Though it is understandable to listen to, it represents only a single perspective, and even stereo or dynamic sound are nowhere near an actual instrument as sound source. In my opinion.
That reminds me, this is a very clear explanation of how standing waves work …
https://www.acs.psu.edu/drussell/demos/standingwaves/standingwaves.html
That first article about boxwood is very interesting indeed! That explains a lot. And I have to say that the observations about the disproportionate cost of boxwood is true–it is really, really expensive for what it is. I love boxwood for a lot of reasons (mostly its fine-grained quality and how it feels when I play it) but I don’t have any illusions that it sounds different to a listener, any more than any other quality wood that I use. And I’ve found woods like holly and hawthorne to be equivalent to boxwood, and I like Castello boxwood as well. The list goes on. But if modern boxwood is inferior to what the old flute makers used, I could wish to find some ancient timber stash 
But what the author said about the bore is so true–it is far more important than the material. In fact, I go into detail on all of these aspects in one of my blogs: The Materials Argument https://www.ellisflutes.com/blog/the-materials-argument. And I have come to the same conclusion that you speculated upon, Greenwood. I do think that player experience has an effect on performance, which can change the audience experience. So even if the material doesn’t make a measurable sonic difference, it is still important, and the difference may be perceived but from a completely different source.
And of course how a material machines is hugely important in making consistent flutes.
“I could wish to find some ancient timber stash”
…well…in that first link the maker is using mammoth ivory … and Mr. Gumby posted a picture of bog wood earlier … which was a sort of seasoning method
https://sawmillcreek.org/archive/index.php/t-111553.html
..maybe he would allow you a billet, if he still has that wood ?
“I think it would be especially interesting to try a similar test using various metals that are common to Boehm flute players, such as silver, gold and platinum…”
https://www.researchgate.net/publication/228488924_Silver_gold_platinum-and_the_sound_of_the_flute
Or you knew that ? It was the one I had in mind, your study was new to me.
BTW https://www.rclarinetproducts.com/the-grenadilla-myth.html gives me an error message ?
Then there is always this story… still not sure if true
https://ask.metafilter.com/312245/Did-a-violinist-smash-his-instrument-during-Carnegie-Hall-concert
Anyway, I’m pigging out on the web and chiff today … long week, and a long trek yesterday to buy materials to make a wind powered, flute boring windmill… I figure I won’t make more than a flute a month and so why not … I count it will be able to make one bore in that time but not sure until I have it running … have a pile of wood and the idea of hand drilling each one sort of encourages delay. If it works I will post on it.
That aside, concensus is often something to be wary of
Looks like Ridenour Clarinets just changed a little something in the URL listing. Thanks for pointing it out (I’ve updated my blog with a good link). Here is it:
Back in 2007 I came across this article and was really intrigued, so I ordered a Bb clarinet from Ridenour. It was a very disappointing instrument: yeah, it did sound like a clarinet (with a proper mouthpiece, not the one that was included) but the bell note was unexplainably flat and the keywork was junk and felt really cheap. I resold it after 2 weeks and went back to my Buffet-Crampon E13, which was a far better instrument.
The take-home point for me was that the material the instrument is made of may not matter that much, but craftsmanship certainly does!
Amen!
The mystique of material is deeply ingrained, which is one of the reasons that I wrote the blog that I linked to above. Sort of like an extended FAQ because I run into questions and preconceptions about materials all the time. Most players are pretty balanced in their view of materials, falling somewhere in the middle of a spectrum (between “materials don’t matter” and “the right material will magically make my flute sound amazing”).
Every so often I’ll have to really talk someone down from the idea that some particular wood is going to take everything to the next level. In some cases it’s hopeless . Most people seem to be open to reason, however. But your point about craftsmanship is right on target: the design of the flute (its craftsmanship) and the player’s ability totally eclipse the question of what the flute is made from, so long as it’s made from something appropriate (not meringue, for example).