Northwind,
Sorry for the long post. Rough percussion flaking is my greatest areas of challenge, but here's what I can offer:
Grinding is actually one of the best tips I could give. When you break off a spall it leaves a sharp edge that you need to grind away before breaking off your next spall. If you don't grind it off, your blow will just shatter the edge into oblivion without running a decent chip. Grinding should be done even when you are pressure flaking.
Grinding the edge will also help you avoid hitting a "shelf," which is when the spall ends at a break-off point that looks like a small ridge running perpendicular to the spall across the face of your core. Once you get a shelf on your core all other spalls will end at that point unless you can remove it by breaking off a spall that runs the same direction as the shelf, thereby "lifting" the shelf off the core.
Also be sure you have a good platform to strike. If your core is too blocky you might want to break it in half just to get some good striking surfaces. Of course your spalls would then be smaller, but it would give you better control.
As far as the grain of the stone, the clearer the stone the less grain it will have. Mohogany obsidian is hard to chip smoothly because it is heavily grained. Source is also very important. I have some obsidian from the lava flows at Newberry Crater (collected before it became illegal). It is so full of bubbles and froth that I can't get a decent spall. Then several years ago I went out to Glass Butte and got a bunch of obsidian and it's like night and day from the Newberry Crater stuff. Glass Butte is about the best I've seen.
A good tip that I was given years ago is to try to run the chips between your fingers. For example, if you have a piece of obsidian resting in your hand (on leather, of course), try to position your fingers to either side of where you want the chip to come off. This will encourage the energy of the strike/pressure to run through the stone instead of absorbing into your hand. I wouldn't say that this is the key to success, but it has helped me quite a bit.
Above all, practice, practice, practice. I sometimes sit on a stump or rock and just spend time breaking stone. Even if I don't make anything I'm still getting to know the breakage patterns and learning how to read the material, and I'll usually walk away with some great spalls to chip down later on. Of course I also walk away leaving a huge pile of obsidian debris behind, so don't do this in an area where small children will be wandering about. Or put down a tarp so that you can pick up your tailings when you're done.
cv
