FSC 240L(LAB)-Impression Evidence-USM

What does impression evidence refer to?

Marks or patterns made when an object (like a shoe, tool, or tire) contacts a surface and leaves identifiable characteristics.

How can inanimate objects be individualized?

Through small markings, scratches, nicks, and breaks that occur with use—similar to minutiae in fingerprints.

Why are new items (like new shoes or tires) only class evidence?

Because they are mass-produced and have nearly identical characteristics before wear.

What causes an object to become individualized over time?

Random wear patterns such as scratches, nicks, pitting, or breaks that develop through use.

What are the three categories of impressions?

Visible, plastic, and latent impressions.

What is a visible impression?

Occurs when an object touches a foreign substance (like dirt, blood, or paint) and transfers it to a clean surface—visible to the naked eye.

What is a plastic impression?

A three-dimensional impression left when an object presses into a soft, malleable surface (like mud, clay, or snow).

What is a latent impression?

An invisible impression requiring specialized lighting, powders, or chemical processing to be seen.

What are three main ways impression evidence is documented?

1️⃣ Photography — records the location and physical features.

2️⃣ Sketches — capture finer details and spatial relationships.

3️⃣ Casting — creates a 3D mold showing wear patterns and depth.

Why are casts important for impression evidence?

They provide a life-sized, three-dimensional replica that preserves small surface details not visible in photos.

What is the purpose of using sketches in impression analysis?

To record precise spatial relationships and annotate unique wear patterns.

What material is used for shoe impression casting?

Dental stone (2 lb / 906 g) mixed with 12 oz (354 mL) of water.

What consistency should the dental stone mixture have?

Similar to pancake batter—smooth and pourable.

Why should you pour the mixture from the side of the impression?

To prevent distortion of the pattern caused by direct pouring.

How long should the cast dry?

About 20–30 minutes.

What should be done after removing the dental stone cast?

Photograph and sketch the mold, annotating any unique tread-wear patterns.

What materials are used for tool mark impressions?

Mikrosil® or Accutrans® casting materials.

What types of objects should be used for this activity?

Small, detailed objects such as coins or keys (not valuable personal items).

How is Mikrosil prepared?

Mix equal 6-inch lines of white polymer and blue hardener until the color disappears.

How long should Mikrosil cure?

Approximately 10 minutes before removal.

What should be done after removing the Mikrosil cast?

Photograph and sketch, noting any unique tool mark details.

What is Accutrans?

A self-mixing polymer and hardener system used for fast, detailed impressions.

How is Accutrans applied?

Hold the object (e.g., a coin) with tweezers and apply Accutrans evenly over the surface.

What must be done once Accutrans dries?

Photograph the resulting impression for record.

What material is used to help record tire tread patterns?

Petroleum jelly (applied lightly to raised tread areas).

What surface is used to record the tire impression?

Butcher paper laid out in a long section (about 10–12 ft).

Why are strips of tape labeled around the tire?

To mark the manufacturer's rubber tread sections (1-12).

How is the tire impression made?

Apply petroleum jelly to tire. Roll the tire one full rotation across butcher paper while applying moderate downward pressure.

What is used to reveal the tread pattern?

Black magnetic powder and a magnetic wand to dust raised surfaces.

What should be done after developing the tread pattern?

Photograph and sketch a portion of the pattern, marking any unique wear areas.

How can impression evidence assist investigators?

By linking suspects to crime scenes, determining the number of perpetrators, and associating evidence types.

Why are 3D casts superior to photos for deep impressions?

They preserve sidewall and depth details that flat photos can't accurately capture.

What safety precautions should be taken during these labs?

Wear gloves, goggles, and lab coats—especially when handling dental stone or casting materials.