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

[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?

[caveman2533]

It tells me if they were heating their stone that they had not discovered your mysterious process.

[Hopewell point]

Mullet, you offering Florida rock? How much and what is it like?

[soy]

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

Yep

[JackCrafty]

Hopewell,
Florida chert is rated pretty low by most knappers unless it's heat treated... maybe a 2 on a scale of 1-10.  And you need a lot of heat (600^oF). More heat than a turkey roaster can generate.

With heat, Florida chert becomes about a 6 on a scale of 1-10.

[Hopewell point]

Thanks Jackcrafty for the info!

[mullet]

Ocala Chert and Brooksville Chert will heat treat around 450-500dgs. Quiet a few chert points found down here have been heat treated. Most high quality Marion, Newnan and Hillsborough's that are real thin have been heat treated.

[AncientTech]

On Clovis thermal alteration, at the Ready Site

http://www.clt.astate.edu/jmorrow/clovis.pdf

Morrow: 

"Due to the proximity of the Ready site to residual or lag deposits of high- quality Burlington chert, it is not surprising that this material dominates the entire site assemblage. Considering only the fluted points and preforms from the site, some 86.5 percent are made of Burlington chert. An occasional fluted biface in the collection will exhibit traces of incidental (unintentional) exposure to heat but NOT ONE OF THESE ARTIFACTS APPEARS TO HAVE BEEN INTENTIONALLY HEAT TREATED. Judging from the large numbers of unfluted, but potentially Early Paleoindian stage 2 and 3 bifaces recovered from the site, all stages of point production are well represented in this local RAW MATERIAL."

A total of 694 artifacts were collected at the Ready Site, and preserved in six collections.  This includes HUNDREDS of Paleoindian artifacts (PAGE 3).  This also includes 224 fluted biface rejects (PAGE 4).  Approximately 86.5% of the fluted bifaces, and preforms, were made from local Burlington chert (PAGE 4).  What is the incidence of intentional thermal alteration of the Burlinton material?  Apparently, the incidence of intentional thermal alteration is 0%, in spite of the fact that there are HUNDREDS of Burlington bifaces, and preforms (PAGE 4).

MODERN KNAPPER'S EMPLOYMENT OF THERMAL ALTERATION VERSUS CLOVIS PEOPLE:

Burlington 600-650 DEGREES - On the following materials, dry out at 200° for 8 hours,then ramp up 50° per hour temp indicated. HOLD FOR 8 HOURS. Ramp back down at 30-50° per hour.

http://orerockon.com/Heat_treating.htm

Burlington Chert - 650-675 DEGREES - HOLD MAXIMUM TEMPERATURE FOR 72 HOURS.

http://www.pugetsoundknappers.com/how_to/Heat%20Treating%20Guide%20with%20Table.html

CLOVIS PEOPLES - NO THERMAL ALTERATION OF BURLINGTON CHERT.

So, why would Clovis people not employ thermal alteration at all, while working materials such as raw Burlington chert, even while they occasionally used thermal on other materials such as hard jaspers, and possibly even agatized coral?

It is my opinion that there are at least three possible answers to this question:

A.  The Clovis knappers were employing flaking processes that generated much greater power than can be generated via hand-swung batons.  Thus, making the stone any weaker simply was not necessary.

B.  The Clovis knappers employed thermal alteration primarily as a means of altering grain, while conserving tensile strength as much as possible.  As a result, some types of naturally blocky materials, would have become more easy to work, possibly with the conservative employment thermal alteration.  In essence, the focus would be to improve grain, but not necessarily overly weaken inherent tensile strength. 

C.  Possibly, both ideas - A and B - could have been involved in the choices once made by the Clovis knappers.

Here is an example of a high power flaking operation, involving a common deer tine, and raw Colha chert, that other knappers may cook for up to fifteen hours, in a commercial kiln - this experimental example is raw.  Perhaps the Clovis knappers used some similar process:





(Disclaimer:  These photos are not intended to represent a "how to" tutorial on flaking.  They are simply intended to illustrate the effects of high powered flaking processes, on raw materials, similar to what can be noted in the archaeological record.)

   

[AncientTech]

It tells me if they were heating their stone that they had not discovered your mysterious process.

So, do you think that the stone tool needs of Paleoindian era big game butcherers was the same as the stone tool needs of archaic era small game hunters?  If not, then of the two parties which party would have needed high tensile strength edges, that did not quickly wear out? 

[mullet]

I've watched Claude Van Order percussion reduce/ flake, raw coral and Florida cherts using copper boppers, antler billets and hammer stones. He consistently reduces these hard rocks using these different tools. But he prefers copper. I have a Clovis made from Argilite that he freehand fluted with a copper billet while talking to a group of people just to show it could be done with copper.

[Dalton Knapper]

Jeff Gatwood in NE Arkansas makes excellent Clovis and Dalton points from unaltered Lafayette Formation (Crowley's Ridge) materials and that stuff is hard as the dickens. Julie Morrow (the author referenced a couple of posts up, and station archeologist at ASU in Jonesboro, AR) has even made comments to me about Jeff's ability to tackle the raw material. I am pretty sure Jeff uses billets and likely copper.