Hard Hammer Percussion
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Third in a series on flint knapping, this article explains one type.
Using a hammerstone large flakes are struck off of a core. This is the
initial stage in the reduction process. Hard hammer percussion is used
to remove the cortex from the core and the flakes which will eventually
be knapped into tools. Some terms need to be defined at this point. The
core is the chunk of rock that flakes are knocked off of. The core itself
may be turned into a tool, but more likely the flakes become the tool.
Cortex is the outer 'skin' or 'rind' of the raw material.
To begin removing flakes with a hammerstone you will have to be familiar
with these three rules:
1. Conchoidal Fracture
The stones used for
flintknap-ping, (flints, cherts, obsidian, etc) all exhibit a characteristic
fracture when broken – called a conchoidal fracture. When you strike the
surface of the stone, the force spreads out from the point of impact like
ripples spreading out in a pond. The force is also directed into the stone,
so as the force spreads outward it also travels inward. What is produced
is a cone, with the pinnacle at the point of impact and the widest part
somewhere inside the rock. This is called a Hertzian cone or the cone of
force. 
If you
do this in the middle of the stone, unless you hit it VERY hard the force
will dissipate without detaching a flake. The trick is to angle the blow
and apply it near the edge of the stone so that only part of the cone passes
through the core. Luckily for us, these cones always tend to be about the
same shape so the angles don’t change and once you learn how to make the
cone work for you you’ve got it made! The angle that the flake detaches
is about 130 degrees to the direction of the strike.
2. Near 90 degree platforms
The platform is the part of the core you strike to remove a flake. To find
a potential platform on a core you want to look for a place where two faces
meet at an angle of 90 degrees or less. It can not be greater than 90 degrees
because of the way you are going to use the cone of force to work for you.
It's not that angles greater than 90 degrees behave differently, it's just
that it's impossible to hit them at the correct angle to get the edge of
the cone to cut through them and remove a flake. In the Spot the Platform
diagram, the platform is shown horizontal and the striking direction is
angled. In practice, it is easier to find a near 90 degree angle, and then
tilt the core so that your striking angle is vertical, as is shown in the
Make the Cone Do the Work illustration.
3. Flakes follow ridges
This rule is essential to controlling where your flakes travel and in planning
how to remove them. When a flake is removed from a flat surface it will
fan out. The resulting flake will be short and wide (see flake B in the
diagram), and quite often will end in a hinge or step fracture (not good).
A flake struck at the same angle and with the same force above a ridge
left by previous flake removals will travel through the area of most mass,
the ridge, and will tend to be longer and narrower (see flake A). Knowledge
of this rule will be crucial to controlling the flakes you strike off.
Flake C curves and forks as it follows first one ridge and then a second.
Selecting a good hammerstone can be just as important as selecting
good stone to flake. Most knappers have several different hammerstones
of different sizes and hardnesses, depending on the hardness and size of
the core which is being worked. For example, you want a soft hammerstone
when working obsidian. However, a soft hammerstone might not work at all
on harder materials like cherts or jasper. Judging the hardness of your
hammerstone will come with practice.
Hammerstones are usually round to egg shaped and range in size from
a chicken egg to larger than your fist. One of the most important qualities
of a hammerstone is that it fit comfortably in your hand. I prefer egg-shaped
hammerstones, using the little end as the striking end. You can find suitable
stones in most creeks and rivers. Alternatively, look along railroad tracks.
The grade of gravel used along railroads (in my part of the world, anyway)
is often exactly what you are looking for.
--Tim Rath, Knappers
Anonymous. Used with permission.
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