Kathy Reichs - Deadly Descisions

Tick.

"Shape. Since the ratio of a stain's width and length accurately reflects its angle of impact, regardless of striking surface, we look carefully at the shape of the stains.

Tick.

"Spatter size. Smaller or slower-moving forces produce large spatters, while larger or faster-moving forces produce smaller spatters.

He stopped, thumb pressed to his fourth finger

"Still with me?"

"We talk about low-, medium-, and high-velocity-impact spattering, although these terms are really relative."

"Give me examples."

"I'll do better than that. Come with me.

He led me down the hall to a stainless steel refrigerator and withdrew a one-liter bottle labeled Sang dtt boeuf

"Beef blood," he explained.

I followed him along a narrow side corridor to an unmarked door, and we entered a windowless room where large sheets of white paper were taped to most surfaces.

The small chamber looked like a massacre site. Blood was pooled along one baseboard, streaked and splattered on the walls, and dripping from various-sized spots at knee level in the far corner Above each stain I could see pencil notations.

"This is our blood-spatter experiment room," said Gilbert, placing the bottle on the floor "Watch"

He removed the cap, dipped a wooden rod into the blood, then allowed it to drop onto the paper under his feet.

"Low-velocity-impact spatters are associated with drops passively falling onto a surface. Dripping blood, for example. The characteristic spatter size is greater than three millimeters in diameter In these situations the blood is moving slowly, from normal gravitational pull to up to five feet per second."

I examined the small round stains he'd created.

"Medium-velocity blood spatter results from activities such as beatings, blunt trauma iniuries, or stabbings. The blood is moving faster, with a force velocity between five feet and twenty-five feet per second."

As he said this he poured a small amount of blood into a dish, signaled me to step back, then swung the rod into it. The blood flew up and struck the wall. Gilbert gestured me over and pointed to several of the stains. They were smaller than the ones at his feet,

"See these spatters? The size range for medium-velocity spatter is typically less, averaging between one and four millimeters in diameter."

He lay down the rod.

"But the spots aren't as fine as with high-impact spatters. Come look at this."

We moved to the far wall, where he indicated an area that looked as if it had been spray-painted.

"High-velocity-impact spattering means a force velocity greater than one hundred feet per second and results from gunshots, explosions, and mechanical accidents. It's more like a mist, with individual spatters averaging less than one millimeter in diameter

"But don't get me wrong. Not every spatter falls neatly into one of these categories. Blood that is splashed, cast off, or projected can really complicate a picture."

"How so?"

"These are actually forms of low- to medium-velocity spatter, but they differ from the ones I just described. For example, splashed blood results from someone stepping into already-pooled blood. This leaves long, narrow spatters surrounding a central stain, with very few round stains present.

"Projected blood results from someone running through, stamping into, or slapping a pool of blood. Or from arterial gushing, or beating a head on the floor Again there are long, spiny spattcrs radiating from a central stain. But in this case the borders of the central stain are also distorted.

"Blood cast off from a weapon leaves yet another pattern. Let me show you.

He went back to the rod, dipped, and swung it in an arc. Blood flew from the tip and struck the wall to his right. I drew close and studied the stain.

"Cast-off drops are smaller than those in typical low-velocity spatter, and the greater the force the smaller the drops. Also, since the blood is being thrown from a moving object, cast-off spatter occurs in straight or slightly curved trails, and the drops are fairly uniform throughout."

"So you can determine the nature of an assault based on the size and shape of the spatter?"

"Yes, And in most cases we can pinpoint where the attack took place. Let's go back to my office and I'll show you something else."

When we were once again in front of the computer he placed his hands on the keyboard and entered a command.

"You saw us taking videos of the bloodstains in the victim's apartment, right?"

"Yes."

"We used a simple video camera, but you can also use digital. We recorded each area of spatter using a scale and a plumb line."

"Why a plumb line?"

"The program uses that to determine the vertical direction of the stain.

Gilbert hit a key and a cluster of elliptical brown forms came up on the monitor

"The images on the videotape are entered into the computer and can be played back on the monitor Single frames are grabbed and recorded on the hard disk as bitmaps. A program then displays the image of each stain so we can take measurements. The measurements are used to calculate two angles: the angle of directionality and the angle of impact."

More keystrokes, and an oval-shaped white outline appeared superimposed over the stain at center screen. Gilbert pointed at it.

"The direction of the main axis of the ellipse with respect to the plumb line defines the directionality angle, or gamma of a stain. That can range from zero to three-sixty.

"The impact angle, or alpha, can range from zero to ninety degrees. That's calculated from the shape of the ellipse."

"Why is that?"

"Remember, when a drop of blood travels through space it's spherical. But when it strikes a target it flattens and leaves a trail. That's because the bottom of the drop is actually wiping across the surface."

He made a swiping gesture with his hand.

"The trail is small at first as the drop strikes, then widens, with the widest point of the trail corresponding to the center, or widest part of the drop. The trail then narrows and eventually tapers off. See this one here?"

He pointed to an elongated oval with a small dot at one end. It looked like many I'd seen in the blood-spatter room.

"It looks like an exclamation point."

"That's exactly what it's called. Sometimes a small dot of blood detaches from the original drop and hops to the head of the trail. So looking at a spatter from above, it resembles either a tadpole or an exclamation point, depending on whether the far end is merely elongated, or a small portion detached completely. In either case the direction o.f travel is clear."

"The dot points toward the direction the drop was moving."

"Exactly. The program produces a file containing the values of the angles for each stain analyzed. It's from that data that point of origin is calculated. And believe me, using the computer is a lot quicker than the old string method."

"Back up.

"Sorry. With the string method one end of a string is fastened to the surface at the position of the stain, then stretched in the estimated direction of motion. This is repeated for a number of bloodstains around the scene. The result is a pattern of strings extending away from the spatter toward the source of the blood. Home plate is the point where all the strings converge. The procedure is time consuming and leaves a lot of room for error Instead of doing it by hand the computer draws virtual strings computed from the data."

His fingers flew over the keys and a new image appeared. X and Y coordinates ran down the left side and across the bottom of the screen. A dozen lines formed an X-shaped pattern, crossing each other in a geometric bow.

"This is a bird's-eye view of a set of virtual strings based on twelve spatters. It's hard to get this point of view with real strings, yet it's the most useful one."