There is a simple fact in flintknapping, and that is you will snap points and bifaces in half from
time to time with the greater frequency being when you are a beginner.  I've witnessed master
knappers snap bifaces and points in half and I've done it myself (just this week as a matter of fact).
With that said I believe there are 3 principle reasons why knappers snap bifaces in half:

#1     The most frequent cause for snapping bifaces is that a knapper has struck the biface on one
end or the other (tip or base) without proper support.  If you have a typically shaped biface
happening, you should have a rounded basal orientation on one end with a narrower tip orientation
on the opposite end.  In earlier biface stages, there may only be a slight difference in shape between
the basal end and the tip end.  If you attempt to take a chip from the basal end and do not deaden
the shock wave heading toward the tip end, the force will compound as it travels toward the tip and
ultimately seek to realease it's force by snapping the biface near the middle.  The same can happen
if you attempt to take a chip from the tip and fail to properly deaden the force that is traveling
toward the basal section on the opposite end.  To deaden the force, you must simple support the
opposite end with your hand or a finger as well as the middle(see below picture).  Jam the tip or the
base right into your hand or finger so that the the tip or base will not move when the billet takes off
the flake.  With that said, one should limit the number of flakes that taken off a tip or base.  Take
as many flakes from the side as you can to thin the base so that you only have 2 or 3 basal thinning
flakes.  I always leave a great deal of mass (thickness) in the middle of the biface if I have a lot of
basal flakes that need to come off.

         Follow this simple priciple and you will solve a most of your biface snapping problems:  Thin
the base, then the tip, then the middle.......pick around a bit to prepare striking platforms.....then
start again....thin the base, then the tip, then the middle.  A few times doing that and your piece will
be thin in no time at all.  If you have Jim Redfearn's video on making Calf Creeks, you will see this
seasoned master knapper snap his biface.  Watch that part a few times and you won't feel so bad!!

#2     Probably the next most frequent cause is failing to have a flake platform (in the middle of
your biface) far enough below center line.  
I've addressed the concept of the center line in previous
questions. This can happen a lot when you are a beginner, but a curious thing happens when you
knap for a begin to push the envelope.  It usually happens like this.  You are chipping
along and everything is falling right into place and you abraid the edge.....then that still small voice
in your head says "You better check to see if that's below center line".  You don't check because
you think it's close and it's all abraided so you strike it anyways and you have two
pieces.  About 3 years ago I started listening to that still small voice and it has paid it's dividends by
less breakage.  After you've knapped for many years, your mind knows better than your eyes rather
there is risk in the flake you will attempt to remove....listen to that voice and slow down?  If you
hear many voices in your head, you better go get that checked out by a professional or lay off the

#3      The last reason is one that is a phenomina I've picked up on through my own experiences
knapping and through teaching new knappers.  When you learn the basics and start wanting thinner
bifaces you begin working with steaper angles so that you can send those long thinning flakes that
travel past the center of the biface.  No longer are you satisfied with thick clunky points, so you push
yourself to the next level.  As you do this you will make the mistakes in #1 and in #2 as well as
putting too much force into your billet strikes in comparison to how thin your biface is.  When I see
this start to happen with a beginner I tell them that they are having the birth pains of moving from
beginner knapper to intermediate knapper.  The remedy is to just push through it.  This snapping
period will persist for a few months usually until you master #1, #2, and develope the skills of
knowing just how hard to hit the platform on the much thinner biface.  This is not a reason to get
discouraged, but incouraged!!  You are advancing and all at once you will move past this cruel
initiation into intermediate knapping skills and likely become obsessed with thinning your bifaces.  I
made it through and I've seen other push through it as well.  Then, of course, you have to know
when to say when with thinness!  Not all points are paper thin!!!!

Basal Flake-Tip Support
striking platform
Tip Flake-Basal Support