What is "Knapping" in the Creation of Arrowheads or Spearheads?
The term "knap" is of Germanic origin, "knop" meaning '(to) strike.'
"Knapping" in our context is the process of striking and pressure-flaking (also called lithic reduction) stone or rock to create hand tools. Other features of knapping include flat or semi-flat 'chiseled' faced rock for building blocks/bricks made from suitable naturally-occurring rock and require dedicated machines with specialized chiseling edges to produce regular and predictable results.
For 'flint-knapping' handtools and arrow heads, it is easiest to find rocks that potentially can be knapped (shaped, using lithic reduction of the base material to create the tool) in washed-out riverbeds, places where exposed rock outcroppings occur and anywhere large surfaces of angular rock face is exposed and readily accessible.
What Makes a Good Knapped Rock?
Virtually any rock can be knapped with varying degree of success, but some lend themselves much more suitably to the art and the craft of knapping. A good flake for a spearhead or arrow is longer than it is wide, perhaps slightly bulged along its length and shaped more-or-less like a pumpkin seed. Typically a good knapped shard for the purposes of hand-tools has very sharp leading edges.
Further knapping and pressure-flaking produces its final intended shape. Mounting points where it can be attached to a stick or branch can be pressure-flaked using the edge or point of a harder stone, if so desired. Serrated edges turn a knapped shard edge into a crude hand saw for cutting of cold/frozen meat, vines or small branches.
A good-sized obsidian or other suitable rock can produce dozen or hundreds of flakes from its larger core. Once such a large mass of suitable material is found, is it retained by the knapper for future sourcing needs. -A veritable arrowhead factory!
Volcanic glass, such as obsidian, works best as it fractures cleanly into super-sharp shards when precisely pressed and/or hammered with a percussive strike. Other stones such as flint are also preferred, as is most chert. What you are looking for in a suitable rock is one that when struck produces a conchoidal fracture, -basically, a thin-edged flake.
The rocks cited produce good conchoidal fractures without prior heat-treating, but other rocks unsuitable for knapping need to be super-heated in a fire-pit and allowed to slowly cool before being knapped. The heating and slow-cooling process makes them more brittle and glass-like, improving their conchoidal fracturing properties. The process is said to make the material more knappable.
Heat-Treating Rocks for Knapping
If the errant traveler needs an ad-hoc tool for digging roots, a simple knapped rock will suffice. A knapped stone can also be used for cutting rope, stripping/hammering bark from dead/down trees for kindling & fire fuel, for weather-proofing a lean-to structure, cutting vine or other purposes in a survival situation.
The easiest way to heat-treat rocks is to use campfire rocks, rocks used to line the edges of a campfire. The intense heat and subsequent cooling improves knappability of many stones and rocks. The best way to determine what rocks work is to experiment with differing types of rocks found in your search area.
Identifying Rocks for Knapping
It is difficult to visually identify most rocks for suitability for knapping by just examining their exterior as natural weathering makes them look rather featureless. The best way examine the interior, is to crack open the rock. If the rock shatters into sherds (shards, but with irregular angular and unsharpened edges, such as smashing a clay flowerpot might produce,) then the rock is unsuitable.
I have found that even certain sedimentary (mudstone) and accreted (layer deposited) stone material can be knapped into crude but useful basic survival tools. The crude tools produced are for short-term use. They tend to wear-out out and lose their sharpness, or pieces break off rather easily so you make another on demand.
The hardness of the rock isn't really of importance when selecting a short-term source for knapping tools. A ring of campfire rocks will provide you enough working material to create additional tools on demand.
Practice Knapping, Improving your Knapping Skill
Practice is required knapping stone into tools and weapons. A good way to learn is to attend a knap-fest, -a social event whereby knappers gather to hone their skills, buy/sell their produced items and trade suitable rock material for use. At a knap-fest, people gather to share their knowledge, trade their wares, and demonstrate the tools and techniques used in the creation of knapped stone tools.
Often, a working knap-fest features a work-group session which features a collection of discarded toilet bowls which are broken into smaller pieces and the attendees gather around the pile of debris to work, engage in conversation and hone their skills.
Toilet bowls are a mixture of several types of clay to produce what is called vitreaous (amorphous glass-like) china, making them suitable for simple knapping.
This material has a degree of conchoidal fracturing when struck and pressure-flaked. Kiln Fired (heat-baked) toilet bowl material is fairly soft and is a readily available yet disposable material for these events. A recreational activity for beginners to experts to experiment different techniques with, thus saving their prized natural stone for the actual build when their skill-set is ready for the task.