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1971: "Investigations concerning the thermal alteration of silica minerals"

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AncientTech:
https://archive.org/details/investigationsco00purd  (Purdy, 1971)

JackCrafty:
Very interesting read.  A surprising finding was the idea that the removal of water from the stone makes it easier to chip.

A desirable change does occur when Florida cherts
are thermally altered resulting in a stone that is easier to
flake than its unheated counterpart. No structural change
occurs in that the size, shape, and orientation of the
individual microcrystals remain the same, but through the
removal of interstitial water, the microcrystals are fitted
closer together when certain materials other than SiO2 serve
as fluxes. When the flaw is introduced which is preliminary
to and necessary for fracture to occur, the heated rock
responds more like glass than a rock aggregate. In other
words, crystal boundaries are no longer interfering with
the removal of flakes. These statements have been substantiated
throughout this dissertation by rock mechanics
tests, scanning electron microscope illustrations, analyses
demonstrating a reduction in surface area of heated materials,
as well as intuitive observations and experiments.


Pages 89 and 90.

caveman2533:
is an interesting read,  I found it interesting that compressive strength increased and tensile decreased.

 

AncientTech:
"The unheated Ocala chert withstood forces of 2700 psi.  This represents a reduction in force of 45% needed to break the material."..."The amount of force used to induce failure of the material by point tensile stress is ESSENTIALLY THE SAME as the strength needed to induce fracture when manufacturing lithic tools BY EITHER PERCUSSION OR PRESSURE METHODS."

What this study does not address is two aspects of failure - initial breakage, and the carrying of the flake to completion.  Which requires more energy?  The initial break?  Or the completion of the break, after it starts? 

So, they accurately point out that a reduction in tensile strength corresponds to easier material failure, in terms of creating a fracture.  For this reason, the advantages in either percussion flaking, or pressure flaking, are cited.  But, what if a tool flaking process actually generates greater force, with less shock, and over longer periods of time?  Then, what does that say about the kinds of flakes that can be initiated, and carried to completion?  And, what does that say about the materials that can be worked?

If they knew that they were going to employ greater flaking forces, with less shock, then would they necessarily have to weaken the tensile strength, via thermal alteration, simply to reduce the stone?   

mullet:
This is really nothing new for people that chip Florida stone. Anybody want some?

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