Hertzian Cone Fracture, thermal spalling?

I’ve pretty much come to the conclusion that blogging can be akin to the ole fishing boat saying about the two best days of fisherman’s life are buying the boat and selling the boat.  In short, I forgot I had created this blog.  I’ve been busy trying to write up my thesis from the Nolichucky excavation that was conducted at the Cane Notch site in the winter of 2015-2016.  We hope to provide a comprehensive overview of the dig during Tenneessee’s 30 days of archaeology, and perhaps I will blog more after this (or not)

Recently, after examining a great many of the partial vessels from the Cane Notch site, we began to notice some interesting fractures in 3 of the roughly 90-100 partial vessels.  Two of these fractures were  Jars from a recent excavation (winter 2016) of a house unit.  One of the fractures occurred on a jar  from the river bank assemblage.  The fractures appear to have been the result of a puncture, or possibly thermal spalling.  I had never really noticed this type of fracture before so I was curious to find out how common it is, and what kind of force it would take to create such a distinct cone fracture.  I should also note, that this is the first I’ve read of hertzian cone fractures in ceramics, so my knowledge base is really limited here (if I’ve royally misidentified this then …well disregard).  First off, a hertzian cone fracture is defined as:

“A Hertzian cone is the cone produced when an object passes through a solid, such as a bullet through glass. More technically, it is a cone of force that propagates through a brittle, amorphous or cryptocrystalline solid material from a point of impact. This force eventually removes a full or partial cone in the material.[1

Roshan Narayanan Manibharathi, Purdue University Dissertation

A good thinking comparison is the force of a bb when shot through  window pane glass…how it leaves that distinct cone fracture on the opposite side of the entry point.  I am not entirely sure how thermal spalling would produce such distinct (directional?) fracturing.

Here are the two examples of the fractures from two distinctly different contexts at the Cane Notch site:

herrzi

Above: Bank Vessel puncture occurring from the exterior through the interior

Below: House Vessel Punctures or indenture occurred from the interior exiting the exterior

herzti2.pngnoliarchea

nnana

 

 

So we have three examples, two of which appears to have been punctured from the exterior and the other that appears to have been punctured from the interior.  A relatively small number of this type of fracture/breakage is represented at the site which appears to parallel studies of ethno-archaeology studies, but they may occur more often then we realize due to the small sherds normally recovered on archaeological projects.  Next I began to wonder if this type of vessel breakage was noted in other assemblages (still pending).  I haven’t noticed this type of fracture discussed frequently in archaeological reports but if you have some examples I would be curious to know.  How such punctures occurred I haven’t a clue, but one can imagine vessels bumping into rocks, particularly during cleaning episodes at the rivers edge, or a myriad of mishaps ie vessels falling over onto sharp objects, kids throwing rocks…a simple saying applies here: schmit happens.

RickSschmit

Above: Ric Schmidt (Indiana Pacers 92-05)

But I do find it interesting that it appears to be a low frequency thing, and yet we have two examples from the 17 or so whole vessels from a burned structure at that site.  If these examples are clear cases of cone fractures by puncturing, then that might indicate some possible intentionality, or that the two vessels were punctured during the collapse of the burning house.

As an aside, another oddity is involved with the first vessel photographed.  It appears that after breaking this vessel, the Native Americans made a pottery disc from one of the salvaged sherds (perhaps more remarkable is that they stayed together in the same context after the abandonment of the site).

At times I am amazed at the layering of analysis that is possible with ceramic analysis.  I am sure there probably exists a mathematical equation for the fanning of the hertzian cone, which would probably give me an idea of the size dimensions of the object that punctured the vessels,  or perhaps the minimum amount of force needed to create such a fracture through a vessel wall of xyz thickness…

 

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