noted the presence and position of breakaway spurs, and analyzed the cross-sectional surface of each cut. When I?d finished with Gagnon, I repeated the whole process for Trottier.
At some point Denis asked if he could lock something up, and I agreed, paying no attention to his question. I didn?t notice the lab grow quiet.
?What are you still doing here??
I almost dropped the vertebra I was removing from the microscope.
?Jesus Christ, Ryan! Don?t do that!?
?Don?t go bughouse, I just saw the light and thought I?d drop in to see if Denis was putting in overtime slicing up something entertaining.?
?What time is it?? I gathered the other cervical vertebrae and placed them in their bag.
Andrew Ryan looked at his watch. ?Five-forty.? He watched me lift the bags into the smaller cardboard box and set the cover on top.
?Find anything useful??
?Yup.?
I tapped the cover into place and picked up Isabelle Gagnon?s pelvic bones.
?Claudel doesn?t put much stock in this cut-mark business.?
It was precisely the wrong thing to say. I put the pelvic bones in the larger box.
?He thinks a saw?s a saw.?
I laid the two scapulae in the box and reached for the arm bones.
?What do you think??
?Shit, I don?t know.?
?You are of the carpentry and grout gender. What do you know about saws?? I continued laying bones in the box.
?They cut things.?
?Good. What things??
?Wood. Shrubbery. Metal.? He paused. ?Bone.?
?How??
?How??
?How.?
He thought a minute. ?With teeth. The teeth go back and forth and cut through the material.?
?What about radial saws??
?Oh well, they go around.?
?Do they slice through the material or chisel through it?
?What do you mean??
?Are the teeth sharp on the edge or flat? Do they cut the material or rip their way through it??
?Oh.?
?And do they cut when they go back or when they go forth??
?What do you mean??
?You said the teeth go back and forth. Do they cut on the back or on the forth? On the push stroke or on the pull stroke??
?Oh.?
?Are they designed to cut on the grain or across the grain??
?Does that matter??
?How far apart are the teeth? Are they evenly spaced? How many are on the blade? What?s their shape? How are they angled front to back? Is their edge pointed or squared off? How are they set relative to the plane of the blade? What kind of . . .?
?Okay, okay, I see. So, tell me about saws.?
As I spoke, I placed the last of Isabelle Gagnon?s bones in the box and tapped on the cover.
?There must be hundreds of different kinds of saws. Crosscut saws. Ripsaws. Pruning saws. Hacksaws. Keyhole saws. Kitchen and meat saws. Ryoba saws. Gigli and rod saws. Bone and metacarpal saws. And those are just the hand-powered ones. Some run on muscle, and some are powered by electricity or gas. Some move with a reciprocating action, some use continuous action, some move back and forth, some use a rotating blade. Saws are designed to cut different types of materials and to do different things as they cut. Even if we just stick to handsaws, which is what we?ve got here, they vary as to blade dimensions, and the size, spacing, and set of their teeth.?
I looked to see if he was still with me. He was, eyes as blue as the flame in a gas burner.
?What all this means is that saws leave characteristic marks in materials such as bone. The troughs they leave are of different widths and contain certain patterns in their walls and floors.?
?So if you?ve got a bone you can tell the specific saw that cut it??
?No. But you
He digested that. ?How do you know this is a handsaw??
?Power saws don?t depend on muscle, so they tend to leave more consistent cuts. The scratches in the cuts, the striae, are more evenly patterned. The direction of the cut is also more uniform; you don?t see a lot of directional changes like you do with a handsaw.? I thought for a minute. ?Since there isn?t a lot of human energy required, people using power saws often leave a lot of false starts. And deeper false starts. Also, because the saw is heavier, or sometimes because the person working it is putting pressure on the object being cut, power saws tend to leave larger breakaway spurs when the bone finally gives.?
?What if a really strong person is working a handsaw??
?Good point. Individual skill and strength can be factors. But power saws often leave scratches at the start of the cut, since the blade is already moving when it makes contact. Exit chipping is also more marked with a power saw.? I paused, but this time he waited me out. ?The greater transfer of energy with power saws can also leave a sort of polish on the cut surface. Handsaws don?t usually do that.?
I took a breath. He waited to be sure I was actually through.
?What?s a false start??
?When the blade first enters the bone it forms a trough, or kerf, with corners at the initial striking surface. As the saw moves deeper and deeper into the bone, the initial corners become walls and the kerf develops a distinct floor. Like a trench. If the blade jumps out, or is pulled out, before going all the way through the bone, the kerf that?s left is known as a false start. A false start contains all kinds of information. Its width is determined by the width of the saw blade and the set of its teeth. A false start will also have a characteristic shape in cross-section, and the teeth of the blade may leave marks on its walls.?
?What if the saw goes straight through the bone??
?If the cut progresses all the way through the bone, the kerf floor can still partially be seen in a breakaway spur. That?s a spike that?s left at the edge of the bone where it finally breaks. Also, individual tooth marks may be left on the cut surface.?
I dug Gagnon?s radius back out, found a partial false start on the breakaway spur, and angled the fiber-optic beam across it.
?Here, look at this.?
He leaned over and squinted into the eyepiece, fiddling with the focus knob.
?Yeah. I see it.?
?Look at the kerf floor. What do you see??
?It looks lumpy.?
?Right. Those lumps are bone islands. They mean that the teeth were set at alternating angles from the saw blade. That kind of set causes a phenomenon known as blade drift.?
He raised his head from the microscope and looked at me blankly. The eyepiece had left double rings, grooving his face like that of a swimmer with tight goggles.
?When the first tooth cuts into the bone it tries to align itself to the plane of the blade. It seeks the midline, and the blade goes along with that. When the next tooth enters the bone it tries to do the same thing, but it?s set in the opposite direction. The blade readjusts. This happens as each tooth comes along, so the forces acting on the blade change constantly. As a result, it sort of drifts back and forth in the kerf. The more set to the teeth, the
