Paint and soil analysis

Paint

This evidence is a powerful forensic tool, especially in forensic hit-and-run evidence.

• Layered structure → unique to each source

• Chemical composition → helps identify origin

• Can link suspects, vehicles, and crime scenes

• Each tiny fragment can provide strong associative evidence

Binder

Holds paint together and helps it stick to surface

Pigment

Provides the color and opacity of paint. Adds durability (e.g. corrosion resistance.

Solvent

Keeps paint liquid for application. Evaporates as paint dries.

Additives

Improves performance (control drying, flow, and UV stability) for paint.

Brushing

A manual paint application for small areas and detail work.

Rollers

A paint application for large, flat surfaces. Has fast coverage.

Spraying

A paint application for smooth, even coating. Good for complex shapes.

Dipping

A full immersion paint application for complete, uniform coverage.

Electrodeposition

A paint application that uses an electrical current for a durable, uniform coating (automotive).

True

True or false

Each paint layer serves a specific function and together they create a unique fingerprint.

Paint collection

Document:

• Photograph evidence in place

• Record location & context

Use proper tools:

• Clean tweezers or scalpel

• Avoid contamination

• Preserve layer structure

Collection sufficient sample:

• Gather enough material

• Keep layers intact

Package seperately:

• Use rigid containers

• Prevent damage or mixing

Color, texture, layer sequence, thickness, decorative elements, pigment analysis, defects

Label the paint parameters:

_____: Visual & instrumental comparison

_______: Surface finish & application patterns

_____ ________: Order of layers (substrate → topcoat)

_________: Individual & total layer thickness

________ ________: Metallic flakes, special pigments

______ _______: Particle size, shape, distribution

_______: Unique imperfections (individualizing features)

Pyrolysis GC

This analysis method is useful for determining binder composition in paint.

IR spectroscopy

This analysis method is useful for determining chemical identification of paint.

Microscopy

This analysis method is a visual screening which examines color, texture, and layer structure of paint.

SEM-EDS

This analysis method is useful for determining elemental composition and analyzes pigments & additives in paint.

Ford Model Pyrogram

This pyrogram shows characteristic peak patterns unique to Ford’s acrylic enamel formulation, with distinct thermal decomposition products.

Chrysler Model Pyrogram

This pyrogram displays peak distribution, demonstrating how manufacturers use proprietary binder formulations that can be distinguished forensically.

Paint collection

Documentation:

• Photograph in place (with scale)

• Record location & context

Rigid packaging:

• Use pillboxes or vials

• Prevent crushing, smearing, or contamination

• Keep questioned vs known apart

Precise labeling:

• Include case #, date, and location

• Add collector & description

Cross section

What is this type of paint cut?

Thin peel

What is this type of paint cut?

Wedge Cut

What is this type of paint cut?

Stair step

What is this type of paint cut?

Electrocoat, primer, basecoat, clearcoat

Label the different type of automotive paint types…

__________:

• Applied directly to metal

• Provides corrosion resistance

______:

• Smooths surface imperfections

• Prepared for color layers

________:

• Provides color & special effects

• Contains pigments (metallic flakes)

_________:

• Transparent top layer

• Adds gloss, UV protection, and scratch resistance

Alkyd resins, acrylic polymers, epoxy resins, polyurethanes

Label the different type of paint binders…

_____ ______:

• Common in household paints

• Traditional oil-based polymers

_______ ________:

• Common in modern automotive paints

• Good color retention & weather resistance

_____ ______:

• Used in industrial/marine coatings

• Strong adhesion & chemical resistance

_____________:

• High-performance coatings

• Durable, abrasion-resistant (often clearcoats_

Inorganic pigments

These pigments provide opacity & stability. Examples include titanium dioxide (white), iron oxides (yellow, brown), and chromium oxides (green).

Organic pigments

These pigments provide bright, vibrant colors. Examples include azo dyes, phthalocyanines (blue/green), quinacridones (red/violet).

Plasticizers, UV stabilizers, flow agents, anti-settling agents

Label the functional additives (paint)…

_________: flexibility

__ _________: prevent degradation

____ ______: smoother application

____-_______ _____: keep pigments mixed

Issues in paint interpretation

Environmental weathering:

• Sunlight, moisture, pollutants alter paint

• Causes fading, chalking, oxidation

• Must compare aged vs fresh samples carefully

Repair and repaint complexity:

• Adds non-original layers

• May not match factory finish

• Can complication or help comparison

Contamination concerns:

• Dirt, oil, rust can obscure color/layers

• May interfere with analysis

• Cleaning must preserve original structure

Paint data query (PDQ)

This database contains over 70,000 automotive paint samples, helping investigators narrow vehicle possibilites based on layer structure and composition.

Soil

This evidence is common at crime scenes and easily transferred between people, objects, and locations. It can be distinguished by color, texture, and composition. Even simple side-by-side comparison can differentiate from different locations. Provides valuable trace evidence linking suspects to scenes.

Points of comparison for soil

• Minerals (type and abundance)

• Organic material (plant remains, microbes)

• Grain size and texture

• Color & layering

• Artificial debris (glass, fibers, pollutants)

Sandy, clay, silt, loam

Label the soil types…

_____ soil:

• Large particles, gritty texture

• High drainage, low nutrient retention

____ soil:

• Very fine particles, smooth texture

• Poor drainage, high nutrient retention

____ soil:

• Medium particles, smooth feel

• Moderate drainage, fertile

____ soils:

• Mix of sand, silt, and clay

• Balanced properties (ideal soil)

Granular

This soil structure…

• Small rounded pieces

• Common at surface

• Good for plant growth

• High permeability

Blocky

This soil structure…

• Irregular blocks

• Found in subsoil

• Moderate drainage

Prismatic

This soil structure…

• Vertical columns

• Clay-rich soils

• Water moves slowly

• Moderate permeability

Platy

This soil structure…

• Thin, flat layers

• Compacted soils

• Poor water movement

NIST guidelines for soil analysis

Documentation:

• Record how & where soil was collected

• Maintain chain of custody

Analysis protocols:

• Use standardized methods & instruments

• Examples: microscopy, chemical tests

Comparison standards:

• Compare to known/reference samples

• Use statistics to support conclusions

Inorganic minerals

This part of soil gives it its structure. Ex… sand, quartz, feldspar

Organic matter

This part of soil affects colors & nutrients. Ex… decaying plants & animals

Microorganisms

This part of soil can link it to a location. Ex… bacteria, fungi

Human (anthropogenic) debris

This part of soil contains strong forensic clues. Ex… glass, paint, fibers

Soil reference samples

• Collect from multiple locations

• Include the exact site + surrounding area (~100ft)

• Used for comparison late

• DO NOT remove soil from items

• Package items as is

Munsell Color System

This is a standardized way to describe soil color. It removes subjective descriptions “brownish”.

Hue:

• Red, yellow, brown

• EX: 10YR, 5R

Value:

• How light or dark?

• Scale: 0=black → 10=white

Chroma:

• How intense?

• Dull → bright color

• Low = grayish, high = vivid

Stereomicroscopy (soil)

• 10-50x magnification

• Examines color differences, particle size, and visible debris (glass, paint)

Polarized light microscopy (PLM) (soil)

• 100-400x magnification

• Identified minerals

• Sees optical properties

Moisture content

Affects color, clumping, and weight. Must be standardized (drying methods). Can show recent environmental exposure and helps preserve transfer evidence.

pH measurement

• Acidic: 3-4

• Neutral: 7

• Basic: 8-9

• Reflects environment & geology

• Helps link soil to a specific location

Natural factors, human activity

What changes soil over time?

______ _______:

• Weathering & erosion

• Wind, water, temperature

• Changes soil composition naturally

_____ ________:

• Farming, landscaping

• Construction

• Adds or disturbs materials