Splitting igneous Basalt using wedges and shims.

["George Graham" <georgergraham@zzzzzzzzzzz>]

Sam,
I would add to what has been already been advised, that is very important to
have all the holes drilled on the same plane. With only 4 wedges , if one of
the holes is aimed off in another direction, the pressure from the wedges
will not come together.
Make sure the hole is deeper then the wedge is long, so the wedge cannot hit
the bottom.
If I read your information right, it sounds like the diameter of the wedges
is the same and the hole you drilled. If I'm assuming wrong, sorry. My point
is that the hole should be big enough to allow the wedge to fit between the
shims about a quarter to a third of the way down.
The stone you have is pretty thick, so I'd drill as deep as possible.
I've split a lot of India black granite, which I've been told is actually
basalt. I don't know for sure. Anyway, its always been in 10, 8, 6 or 4inch
slabs, so I had the advantage of consistent thickness and drilling on a flat
surface. The grain was always assumed to be perpendicular to the surface,
and always the same . In your case you might be working against the grain
instead of with it, and it does make a difference. When you do split the
piece look and see if the break continues in the direction of your drill
holes, or wanders off at an angle. The old rule is that if you are splitting
against the grain , drill all the way through the stone. Otherwise you won't
know where the break will come out.
Finally, its real important to give the pressure from the wedges time to
work into the stone. When you went back to hit the wedges again, were they
loose, so they went in when you hit them?
If they were, then you were on the right track. Its very likely that if you
had walked away after the 4th tightening up , and taken a break, a split
stone might have been waiting for you.
Go try it again, and be patient!

Good luck.
George Graham