Thanks Bushboy for all your help, I'll do as you say and I've got a ripped out strip of the birch from right next to the blank I have. The wood looks really good. I'll dry it and try it.
My interest in black birch comes from the fact that practically every other tree on my land is a black birch -- most all with no branches for 20 or more feet and straight trunked with clean bark. I cut and split maybe 7 cords of wood a year, and it is a clean tough hard and heavy wood, very nice in general.
When I split the occasional yellow birch it's always knotty and twisty grain (propellor like), lots of defects and shreds.
Ash splits super easy and clean, but I just don't like it, except for maybe making handles. It rots very easy. Not the black birch, though.
I'd love it if the black birch worked out -- I've always wanted to find a use for it besides burning it. It just looks so good as wood.
The hop hornbeam -- I've only found two trees so far, and like the elm, if I cut them down, I'd probably have to wait a year for it to dry. As you can imagine just starting out like i am, waiting a whole year to begin making a bow seems like an eternity! A board bow from wood I have will help me not to go nutz after reading this forum every night! I don't expect it will turn out great.... it probably won't be too good by comparison with what I see here, I know.
I have done a little internet searching on woods, and I don't quite understand how published wood specs relate to bow making, but every time I look up black birch (sweet birch) it looks really high in strength specs. It's totally different than white (paper birch). I have some trees of that, and they are weak, soft, diseaase prone -- it's like night and day the difference between the two.
In the specs it seems a lot stronger than red oak or even elm:
Black (Sweet) Birch:
Betula lenta
Average Dried Weight: 46 lbs/ft3
Janka Hardness: 1,470 lbf
Modulus of Rupture: 16,900 lbf/in2
Elastic Modulus: 2,170,000 lbf/in2
Crushing Strength: 8,540 lbf/in2
American Elm:
Ulmus americana
Average Dried Weight: 35 lbs/ft3
Janka Hardness: 830 lbf (3,690 N)
Modulus of Rupture: 11,800 lbf/in2
Elastic Modulus: 1,340,000 lbf/in2
Crushing Strength: 5,520 lbf/in2
Red Oak
Quercus rubra
Average Dried Weight: 44 lbs/ft3
Janka Hardness: 1,220 lbf (5,430 N)
Modulus of Rupture: 14,380
Elastic Modulus: 1,761,000 lbf/in2
Crushing Strength: 6,780 lbf/in2
So I guess I'm just hopeful that it will make a better bow than is usually thought because maybe it has been confused with white, grey and river birch. It's not common everywhere (though it is right here). With specs like that above, I just wonder, how bad can it be?