Deja Dead Page 0,56
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 can determine the most likely class of saw that made the cuts.”
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.