what causes hinge and step fractures?

[anasazi]

Tried making a biface or preform ( not quiet sure the difference between the two) today but i have a tendency to get quiet a few hinge fractures and some what i think are step fractures it kinda crumbles in some places more than makes any kind of flake. What could i be doing to cause this to happen? The only stone around here is obsidian.

[cowboy]

I haven't work a lot of obsidian but the biggest culprit for steps and hinges is striking or pressuring at too steep an angle. If its simply crumbling away sounds like you may to spend a little more time preparing your platforms.

[TRACY]

What cowboy said. Also, make sure that the path of the flake is convex and not concave. If not the flake will stop short of what you want.


Tracy

[Wolf Watcher]

Abrade Abrade  and prepare good solid platforms.  Joe

[Dalton Knapper]

What Tracy said is very important. After you successfully remove a good flake, because there is a "bulb of percussion" scar on your biface, there will often be a slight concavity there on that flake scar. If you strike there again, you will get a hinge. If you hit there again, you will start stacking up hinges.

The answer is to look at your biface each time you remove a flake (it doesn't take a second) and note whether or not there is a slight dip (concavity) or not. If there is, you need to dress the edge back by lightly hitting it straight down until you achieve a convex shape. In short, if the place you strike is not lens shaped (convex) your flake will stop and break off - a hinge. Hinges are caused when a flake has to dive deeper than the force and mass allow.

In all cases, you have to prepare the edge correctly, as many have said. Lots of time, grinding alone will accomplish creating a lens shape. Other times, you will need to dress it with light percussion taps and then abrade. If you abrade in line with the edge, try abrading perpendicular to the edge - more or less brush the edge with your grinder to remove enough small flakes to bring it into a lens shape.

There are other causes, but this is the major one. If this won't help - post again with more details about what is happening.

[bowtarist]

I've been hanging around the Glenn Black Lab on the IU campus lately and knapping with a guy there.  He told me all the stuff said above, but he also said something yesterday that has not been said yet.  If I have this wriong, please correct me.  He said to keep from getting steps and hinges, yes platform set up and abrading is needed, but also to run your flakes at an angle to the point, not straight in.  45 degrees or more to the point.  The ase can be run striaght in, but from the point down the sides, run the flakes at a 45 degree + angle.  It seemed to have helped me.  He told me that this was the way to make and "arrow head", not just a rock shapped like an arrow head.  If someone reads this and it is wrong, correct me please, if not, good luck! dpg

[Outbackbob48]

Dpg, I don't know if it is right or wrong but I have seen alot of early NA points that were done both ways Parallel collateral flaking an also Oblique/Angled flaking. I think both types of arrowheads produced excellent results for the anceint ones. I can  understand what your friend is saying especially on some materials angle flakes can create less finger nail type hinges but I am not so sure about one being an arrowhead an the other just a rock shaped like a head:( Bob