Any dust is bad for the lungs. Some react worse than others. I don't bother with a mask, but I have a dust extractor on the bandsaw and work in a well ventialted area.
On a bow of that poundage you can reduce the sapwood and violate the rings as long as it is done as a smooth transition, ideally with any violations showing as lines along the bow or at a feathered angle. 1/2" sap is ok at the grip, but by the time you get to the tip you will be running out of heartwood.
It is possible to follow a ring but it's a slow laborious process. That should be you 'aim' but you don't have to actually achieve it.
I've found the trick is to do it in stages. Rough down the sapwood keeping an eye on either edge of the limb as it's often not the same thickness each edge. Do it with a spokeshave and youll see the rings appear, you can try to keep it even with a dead flat back showing rings running along the length of the bow at the edges. This helps you to take it down fairly evenly and the lines help you see whats what.
The edges/corners will get rounded later.
Once the bow is roughed out and floor tillered, you can go over the back a bit more carefully. Doing it in the right lighting is key, sometimes strong light helps, sometimes the opposite light from obove or sometimes at a shallow angle... you have to fiddle about with it, but it makes a huge difference. I use an old rasp or coarse file held lightly and cutting across the grain, it will tear/crumble off the softer whiter layer (I think it looks like chicken meat) and expose the next layer of very slightly yellower stuff. It's a little and often, slow and steady. Ok you'll find you've done a load and suddenly you are a ring too high (or is it too low??? or maybe high...
), but just keep improving it and you'll end up with a decent compromise.
On a heavy warbow, the back needs perfection and the best ones I've done had virgin backs with just the sapwood removed. Od course this requires either thin sapwood or a lamination of sapwood which has been thinned down from the other side, leaving the bark side pristine.
Just done a search on my blog and there are some pics that give a bit of an idea.
http://bowyersdiary.blogspot.co.uk/search?q=following+a+ringDel
(BTW. I believe the Romans used to make a nice herbal tea from Yew needles when they wanted to commit suicide
Oddly the actuall flesh of the berries isn''t toxic, but they are not worth eating, I tasted one to see... a bit like seetish snot
)
