Bulb of percussion

[tomf]

I'm interested in the bulb of percussion. It's a constant feature of conchoidal fracture and appears, to some extent, on nearly every spall and flake.

With California artifacts, it's invariably removed. Sometimes the reason seems obvious (like to reduce and even a scraping surface), but more often, it seems removal serves no purpose to the design and seems to be done out of habit or convention.

Even the smallest flake tools will have a scar on their reverse which is a gesture toward bulb removal, sometimes even where no bulb exists -but where it might, ordinarily.

Could their be a technical reason? Maybe something to do with the transfer of energy?

Is there a cultural meaning? Is it just habit?

An while I'm on the subject does anybody have artifacts that have purposely incorporated a percussion bulb into their design?

Here are two examples of bulb removal on small utilized flakes.

Neither seem to serve purpose. Or do they?

[SurfaceHunter]

I know you get a bulb on chert but idk if you get the same on obsidian cause I don't find it. Good question

[Ron Kelley]

Hey Tom, A lot of thumb scrapers have just a little work on the unifaced side to remove the bulb. I always assumed that was to make the bottom surface flat. The material of the percussion tool will make a difference in the size of the bulb. From largest bulb to smallest: Stone, Copper, Hardwood, and Antler.

[Broken Arrow]

I find that angle of strike, speed of strike, platform size, and surface area of the strike heavily influence the size of the bulb also, somewhat regardless of the percussion material.

[Ron Kelley]

The bulb will vary from lithic to lithic too. Here is an Arkose flake with a big bulb.

[PointHound]

I assume you mean the reverse side on the stem? The obverse I would guess that is a flute?

[Ron Kelley]

The bulb will be the thickest part of the flake so it would need some thinning in that area.

[PointHound]

I think I see now. It would have been where that big flake scar is on the center of that piece on the reverse face?

[Kyflintguy]

The bulb will be the thickest part of the flake so it would need some thinning in that area.

I agree with Ron, that's why your utilized flakes were likely thinned in that area, to make them more wieldy.

[tomf]

In some cases that makes sense, but often there seems no clear purpose and, with thin flakes, in might even weaken them.

[gregszybala]

From a post in 2012 , photo by another but Hoss's notes

[tomf]

Nice explainer.

[gregszybala]

My “bible” on this subject is “Lithics: Macroscopic Approaches to Anaylsis (2nd Edition)” by William Andrefsky Jr - one of the excellent Cambridge Manuals in Archaeology series - Dr. Andrefsky is Prof. of Anthropology at Washington State University. He has also published a companion volume, “Lithic Debitage (2001)”.
At 300 pages, the book is a long way from being light reading and definitely not for beginners, but is a highly authoritative and helpful reference for recognising intentional work. It also has fewer illustrations than a beginner needs, but those that are given are excellent and of a line-art nature (ie they’re very clear, if a bit small).
I offer this illustration, with apologies for the quality (it’s not from the above book) but it’s the most complete and helpful “anatomy of an artefact” I have seen.
The bulb of percussion represents a portion of a Hertzian cone of force – not the cone itself. Normally, when detaching a flake from a core or preform, the bulb is detached with the flake. It’s a bulge - rounded and raised above the surrounding surface. The core or perform will have a corresponding inverse of this – a depression that sits below the surrounding surface. Most people also refer to this as a bulb of percussion.
Which way round this happens depends on the orientation of the material when struck and the relative sizes of the core and the detached flake. This in turn depends to some extent on whether the removed flake is intended to be the artefact or is just waste. The smaller piece tends to have the raised bulb (but not always). The bulb occurs where the (normally dorsal) surface meets the striking platform and the position also normally determines which is the proximal end.
The bulb can be diffuse, moderate or distinct. Generally, the harder the material used to strike the blow and the heavier the blow, the more distinct the bulb. It may, or may not have ripple marks (normally not). These are more usually strongly evident on the area below the bulb, known as the percussion scar or “erailleur”. Scarring also occurs in positive and negative, but because it is a series of sharp undulations, it normally looks the same in both positive and negative. There’s a simple explanation here:

http://www.stoneagetools.co.uk/making_tools.htm

Frequently people latch on to the term “bulb of percussion” when what they are really seeing is an “erailleur”. It’s also important to recognise that concentric radiating shock waves are not bulbs of percussion. They are just conchoidal fractures that are characteristic of glassy materials. You’ll find them as a result of human handiwork (including pressure flaking) but you’ll also find them when the material breaks naturally… because that’s how it breaks. By contrast, there is no geologic process that creates a bulb of percussion and so it’s a truly diagnostic feature of an artefact.



From a different discussion in 2012, comments and example by Painshill

[tomf]

Hey Tom, A lot of thumb scrapers have just a little work on the unifaced side to remove the bulb. I always assumed that was to make the bottom surface flat. The material of the percussion tool will make a difference in the size of the bulb. From largest bulb to smallest: Stone, Copper, Hardwood, and Antler.

I had suspected that force and location of strike determines the nature of the bulb created.

Interesting that the material of the striking object has a role too.

Each material you list is increasingly soft, which makes sense when you think about it.


I see why removing the a small bulb might improve a scraping surface, but why do it on expedient knives and small flakes?

[Ron Kelley]

Hey Tom, On a tool that is hafted the thinning would make the haft better. On a tool that is hand held, removing the bulb could make it more comfortable to hold. If I take a good sized thinning flake off then I can get a good grip with my thumb in that area.

[tomf]

My “bible” on this subject is “Lithics: Macroscopic Approaches to Anaylsis (2nd Edition)” by William Andrefsky Jr - one of the excellent Cambridge Manuals in Archaeology series - Dr. Andrefsky is Prof. of Anthropology at Washington State University. He has also published a companion volume, “Lithic Debitage (2001)”.
At 300 pages, the book is a long way from being light reading and definitely not for beginners, but is a highly authoritative and helpful reference for recognising intentional work. It also has fewer illustrations than a beginner needs, but those that are given are excellent and of a line-art nature (ie they’re very clear, if a bit small).
I offer this illustration, with apologies for the quality (it’s not from the above book) but it’s the most complete and helpful “anatomy of an artefact” I have seen.
The bulb of percussion represents a portion of a Hertzian cone of force – not the cone itself. Normally, when detaching a flake from a core or preform, the bulb is detached with the flake. It’s a bulge - rounded and raised above the surrounding surface. The core or perform will have a corresponding inverse of this – a depression that sits below the surrounding surface. Most people also refer to this as a bulb of percussion.
Which way round this happens depends on the orientation of the material when struck and the relative sizes of the core and the detached flake. This in turn depends to some extent on whether the removed flake is intended to be the artefact or is just waste. The smaller piece tends to have the raised bulb (but not always). The bulb occurs where the (normally dorsal) surface meets the striking platform and the position also normally determines which is the proximal end.
The bulb can be diffuse, moderate or distinct. Generally, the harder the material used to strike the blow and the heavier the blow, the more distinct the bulb. It may, or may not have ripple marks (normally not). These are more usually strongly evident on the area below the bulb, known as the percussion scar or “erailleur”. Scarring also occurs in positive and negative, but because it is a series of sharp undulations, it normally looks the same in both positive and negative. There’s a simple explanation here:

http://www.stoneagetools.co.uk/making_tools.htm

Frequently people latch on to the term “bulb of percussion” when what they are really seeing is an “erailleur”. It’s also important to recognise that concentric radiating shock waves are not bulbs of percussion. They are just conchoidal fractures that are characteristic of glassy materials. You’ll find them as a result of human handiwork (including pressure flaking) but you’ll also find them when the material breaks naturally… because that’s how it breaks. By contrast, there is no geologic process that creates a bulb of percussion and so it’s a truly diagnostic feature of an artefact.



From a different discussion in 2012, comments and example by Painshill

This is all great information. Thank you for taking the time to explained it so well.

Obsidian is a material that can really show the phenomenon clearly. I have seen very heavy ripples and even striations that fan out from point of impact.

Certainly true that the bulb is nearly always situated at basel end of artifacts but why is it so often removed (see percussion scar on your diagram)?