Saturday, December 7, 2013

Usable Edge Loss: Obsidian Knife Performance (slightly bloody)

Unused obsidian knife
Hi all,

Images contained in this post are a little bloody. 

It's been a little while.  As it turns out, dissertation manuscripts take up a little of one's time for other things.  I've managed to slip in some time for experiments here and there though.  I was wondering the other day  about use wear patterns and usable edge loss, specifically how do different kinds of damage occur during a single activity.  Cleaning game takes more than just slicing through meat so i was interested to see how the processes stacks different types of use wear.  Questions that came to mind were things like: What types of damage occurs throughout the process and in what order might damage compile?  Does damage come in the form of dulling only, or is there actual loss of material?  The method of butchering likely predicates the variability in tool damage, and one test can't illustrate all possibilities, but I figured it would be informative to check it out.

I procured a rabbit while out in the piney woods of East Texas for some stew, a favorite winter food for me and my brother.  I processed the rabbit from start to finish with an obsidian knife.  The red and black banded obsidian comes from the Glass Buttes source complex in Oregon (Thanks Tim D and friends!).

Ribs presented no problem

I took total usable edge for both top and bottom cutting edges before and after to see edge loss and total damage.  I used a section of dental floss and a millimeter ruler to achieve accurate measurements along the curved blade edge.  I did not use butchering techniques that would help save the integrity of the blade.  If I wanted to keep the blade sharp for as long as possible I could choose to not use it for cuts through vertebrae or the pelvis.  When it comes down to it, all one really has to to is remove the organs which can be done with one incision.  The hide can be peeled off and requires no blade.  I chose to remove the head and cut the pelvis to thoroughly clean the critter.  I consider loss to be damage incurred such that cutting through tissue is no longer possible.
The pelvis created a decent amount of edge loss


The total usable cutting edge breakdown:
initial top: 60mm
initial bottom: 65mm
resultant top: 5mm loss (8.33%)
resultant bottom: 42mm loss (64.61%)
TOTAL: 47mm of 125mm loss (37.6%)

Final edge form after use


A quick look at the patterning in the damage illustrates that I damaged the bottom (curved section) of the blade most.  This is no surprise as I used this bottom edge for cutting through bony sections.  The top was used primarily for cutting through hide while the bottom was used for cutting the rib cage, cervical vertebrae and separating the pelvis.  Usable edge loss occurred in minor amounts with ribs, rapidly during interaction with the cervical vertebrae, and less so with the pelvis.  Edge dulling occurred rapidly upon initial hide and tissue cutting and continued gradually after the initial sharp edge was lost.  The large section of step fracture damage on the bottom blade edge came last and removed signals of earlier dulling as I removed the head and separated the pelvis. 


The blade worked extremely well all said.  Different performance needs throughout the process created a compound history of edge use and damage within a single butchering event.  I was surprised at the bone cutting ability of the blade, regardless of edge damage incurred.  The blade could be used again in the current state, yet would require reliance on the top of the blade for most of the work.  While the blade worked well for all challenges presented, cutting bone, etc. a strategy of edge conservation would be a better approach.  Perhaps two tools, a sharp obsidian blade and a heavy utility edge of basalt or chert for cutting bone would combine to save the use-life of the formal tools used.


Practicing those fun channel flakes for upcoming tests

Working on overshot thinning for upcoming tests

Happy Hunting!