Chapter 14 - Fingerprints

Why Fingerprints Matter

Fingerprints are:

  • Unique

  • Persistent (don’t change over time)

  • Widely accepted in court

2. Types of Fingerprints

Latent Prints

  • Invisible

  • Made by sweat and oils

  • Need development

Patent Prints

  • Visible

  • Blood, ink, dirt

Plastic Prints

  • 3D impressions

  • Wax, clay, soap

3. Fingerprint Patterns

Three main types:

  • Loops (most common)

  • Whorls

  • Arches (least common)

4. Minutiae (Individual Characteristics)

Examples:

  • Ridge endings

  • Bifurcations

  • Dots

Matches are based on:

  • Quality

  • Quantity

  • Position of minutiae

⚠ No universal number required for a match.

5. Fingerprint Development Methods

Physical Methods

  • Powder dusting

Chemical Methods

  • Ninhydrin (paper)

  • Cyanoacrylate (superglue fuming)

Choice depends on:

  • Surface type

  • Age of print

CHAPTER 14 TAKEAWAYS

  • Not all prints are visible

  • Pattern + minutiae matter

  • Development methods depend on surface

  • Examiner judgment plays a role

PART 1: WHAT FINGERPRINTS ARE MADE OF (Fingerprint Matrix)

When we talk about a latent fingerprint, we are really talking about the stuff left behind by your skin when you touch something.

A fingerprint is not just “oil” or “sweat” — it is a mixture, and we never know the exact mix.

That’s why fingerprint development is not automatic — it’s a best-choice decision based on what might be there.

What can be in a fingerprint?

A fingerprint can contain:

  1. Natural secretions from the body

  2. Foreign materials picked up from objects

  3. Or a combination of both

There are two main types of glands involved in fingerprints.

1. Eccrine Sweat Glands

These are:

  • Found all over the body

  • Especially dense on palms and soles

  • Controlled by the autonomic nervous system

    • You cannot control them consciously

What eccrine sweat is made of

  • 98.5–99.5% water

  • 0.5–1.5% solids

That means:
👉 Most sweat fingerprints fade quickly because water evaporates.

What are the “solids” in sweat?

They are:

  • Inorganic salts

  • Organic substances

Inorganic salts (simple version)

Mostly:

  • Sodium chloride (salt)

Salt levels vary depending on:

  • How often you sweat

  • Diet

  • Age and sex

  • Weather

📌 Cold weather = fewer sweating events but higher concentration
📌 Hot weather = frequent sweating but more diluted

This matters because:
👉 Higher concentration = better fingerprint development

Organic substances (VERY IMPORTANT)

These include:

  • Amino acids

  • Fatty acids

  • Ammonia

  • Urea

Most are water-soluble.

Amino Acids (EXAM FAVORITE)

Amino acids are huge in fingerprint science.

Key idea:
👉 Stress increases amino acid secretion

Why stress matters

When someone is nervous:

  • Heart rate increases

  • Breathing increases

  • Amino acids increase

Example:
A person committing cheque fraud is internally stressed even if they look calm.

They touch paper →
Amino acids transfer →
Ninhydrin reacts →
Fingerprint appears

📌 This explains why:

  • Exam papers

  • Crime documents
    often have excellent fingerprints

Fatty Acids (Lipids)

These are:

  • Natural oils

  • Part of the lipid group

Important concept:
👉 They don’t evaporate like water
👉 They slowly “cure” or oxidize

That means:

  • Lipid-heavy prints last longer

  • Especially if protected from air

Ammonia and Urea

  • Present in sweat

  • Not commonly used directly in fingerprint development

  • May play a role in ninhydrin reactions

2. Sebaceous Glands (Oily Secretions)

Sebum is:

  • Thick oils

  • Waxes

  • Squalene

Important facts:

  • No sebaceous glands on palms or soles

  • Sebum gets onto hands by:

    • Touching face

    • Hair

    • Skin

Why sebum matters:

  • Powder sticks to it

  • Iodine fuming reacts with it

  • Physical developer works well with it

📌 Sebum lasts longer than sweat but still degrades over time

🧼 PART 3: FOREIGN MATERIALS IN FINGERPRINTS

Foreign materials are anything picked up before touching a surface.

Examples:

  • Motor oil

  • Food grease

  • Hair products

  • Dirt

  • Bodily fluids

Key idea:
👉 Foreign materials can make fingerprints last much longer

Example:
Motor oil does not dry out like sweat → fingerprints can last almost indefinitely

🧠 SUMMARY OF FINGERPRINT COMPOSITION

Key takeaways:

  • You cannot control fingerprint composition

  • Stress increases amino acids

  • Sweat evaporates quickly

  • Oils last longer

  • Investigators never know exactly what’s in a print

⏳ PART 4: LIFE SPAN OF A FINGERPRINT

This section answers:
“How long can a fingerprint last?”

Answer:
👉 It depends. A LOT.

What affects fingerprint lifespan?

1. Composition

  • Mostly sweat → short-lived

  • More sebum → lasts longer

  • Foreign materials → can last very long

📌 Chemical methods can develop:

  • 40-year-old prints (ninhydrin)

  • 9-year-old prints (physical developer)

2. Environmental Conditions

❄ Freezing

  • Secretions harden

  • Powder sticks everywhere → no ridges

Solution:

  • Warm object

  • Wait for condensation to evaporate

  • Then powder

☀ Hot temperatures

  • Secretions “bake” onto surface

  • Lose tackiness

Solution:

  • Cool the object

  • Allow moisture absorption

  • Then powder

“Huffing” Method

  • Breathing moist air onto print

  • Works sometimes

  • Fails on hot surfaces outdoors

Freeze-Thaw Method

  • Freeze object

  • Let it warm

  • Moisture restores tackiness

📌 Often used for:

  • Old prints

  • Dried-out impressions

🌧 High Humidity

  • Powder sticks everywhere

  • Smears easily

Solution:

  • Move object to dry, air-conditioned space

  • Let it acclimatize

  • Test low-risk areas first

🕰 PART 5: CAN YOU TELL THE AGE OF A FINGERPRINT?

Short answer (exam answer):
👉 No — almost never

Long answer:
There are too many unknown variables.

Why fingerprint aging is unreliable

Variables from the person:

  • Type of secretion

  • Pressure

  • Length of contact

  • Stress level

Variables from the surface:

  • Porosity

  • Cleanliness

  • Temperature

Environmental variables:

  • Humidity

  • Airflow

  • Weather

📌 Even “fresh-looking” prints may be years old

Case examples (WHY YOU CAN’T ASSUME)

  • Washed milk bottle → fingerprints still present

  • Fingerprint between window panes → years old

  • Fingerprint on hot lamp chimney → survived heat

👉 Common sense assumptions were wrong every time

Final rule (VERY IMPORTANT)

⚠ Do not offer opinions on fingerprint age
⚠ Courts should not expect this
⚠ Experts must remain objective

Only rare exceptions:

  • Fresh paint

  • Recently manufactured surfaces

🧠 FINAL EXAM TAKEAWAYS

  • Fingerprints are chemical mixtures

  • Stress improves fingerprint quality

  • Oils last longer than sweat

  • Environment drastically affects development

  • You generally cannot date fingerprints

  • Never assume based on appearance