Chapter 9: Firearms, Tool Marks, and Other Impressions

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Welcome back to The Deep Dive, where we take complex evidence, often presented as dry facts, and reveal the fascinating science that underpins it.

Today, we are undertaking a fundamental forensic deep dive.

We're looking at how inanimate objects, things like firearms, tools, even your shoes, can leave behind a physical signature.

A unique fingerprint, really, that can undeniably connect them to a crime.

Our focus is on this essential skill of individualization.

Exactly.

We're moving beyond just simple identification, like knowing a shoe is a size 10 Nike that's just a class.

We're looking for the accidental microscopic flaws, the nicks, the where the prove this specific object and no other was at the scene.

And to set the stage for just how dramatic that leap is from a general factory -made item to a unique individual signature, we have to start with a crime that, well, dominated the headlines.

The murder trial of former football star Aaron Hernandez.

The 2013 murder of Odin Lloyd.

It left investigators searching for hard physical evidence at the Industrial Park crime scene, and they found two really crucial pieces of impression evidence.

The first was a footwear impression.

Right.

And this impression was immediately informative on a basic level.

The forensic examiner could identify the pattern, the design, the size.

The class characteristics.

The class characteristics.

Exactly.

They narrowed the suspect's shoes down to a size 13 pair of Nike Air Jordan Retro 11 sneakers.

And the shoes themselves were never recovered.

Never.

But surveillance video did place Hernandez wearing similar sneakers shortly before and after the murder.

So that's good circumstantial evidence, but the individual characteristics, that's where the science becomes undeniable.

And that moment came with the tire impression evidence.

Prosecutors alleged Hernandez was driving a rented Nissan Altima.

So the tire examiner begins by matching the class characteristics of the tracks found at the scene.

The grooves, the sipes, the wear bars, all the standard stuff.

And initially they found that the front tires of the Altima were, well, they were eliminated.

Their designs simply didn't match the track, but the rear tires.

The rear tires were the right design.

They matched the class.

So that's where the investigation could have stopped, right?

A weak link, but a link.

It could have.

But the forensic examiner pushed for individualization.

The crucial discovery was made on the actual rear passenger side tire of the rented Nissan.

Okay.

Deeply wedged into the tread were four tiny distinct stones.

Four stones.

Four stones in four specific random positions.

So when the examiner compared the photograph of the crime scene impression to the actual tire, those four tiny indentations created by the stones pressing into the soft earth provided the unique random signature.

The individual characteristic.

The individual characteristic needed for an absolute match.

I mean, the probability of another car having four identical stones lodged in the exact same positions is forensically speaking zero.

Okay.

Let's unpack this.

That moment connecting a piece of evidence, not by its factory design, but by four random accidental stones.

That is the core principle we're diving into today.

It is.

It's the microscopic flaws, the scratches, the nicks and the wear that imparting unique identity, allowing a criminalist to definitively connect a specific bullet to a specific gun.

So we're transitioning now from those macroscopic impressions like tires and shoes into the microscopic world of ballistics.

Starting with the complex manufacturing process that gives every single firearm barrel its unique signature.

Before we talk about the marks left on a bullet, we probably need a quick primer on the structures that create those marks.

A little taxonomy, yeah.

We divide firearms into two broad categories.

Handguns or pistols and long guns.

And handguns are designed for single handed use, right?

Correct.

They come in three main types, single shot pistols, revolvers and semi -automatic pistols.

And then there's the action.

The distinction of action is vital.

Yes.

Single action requires you to manually pull back the hammer before you pull the trigger.

So two separate actions.

Right.

Whereas double action weapons do both the cocking of the hammer and the actual firing with one single continuous pull of the trigger.

Got it.

And revolvers are iconic.

They have that revolving cylinder with multiple chambers.

The most common one you see is the swing out revolver, where the cylinder just pivots out to the side for loading.

Then you have brake top revolvers, which hinge forward, and the older solid frame revolvers.

Where you load them one at a time through a little gate.

Exactly.

A much slower process.

And then the semi -automatic pistols, which are sort of the modern standard.

They are.

They use a removable magazine in the grip.

After the first shot is loaded manually, the gas from the fired round automatically cycles the gun, ejects the spent casing, cocks it, and loads the next round.

But the key is you still have to pull the trigger for every single shot.

It's not automatic fire.

Precisely.

One pull, one shot.

Okay.

Moving to long guns.

These are designed for use against the shoulder.

So we have rifles and shotguns.

The main difference is the ammunition in the barrel.

Rifles fire a single projectile, a bullet, and their barrels are rifled.

And shotguns.

Shotguns fire shells that usually contain multiple ball -shaped projectiles or pellets, and their barrels are smooth.

And shotguns have that specific feature near the muzzle called the choke.

Right.

You won't find that on rifles.

The choke is a slight narrowing, a constriction, designed to control the spread of the shot pattern.

A tighter choke keeps the pellets clustered together longer.

So it increases the effective range?

Increases the range and the concentration.

Now let's get to the true forensic signature.

The process of rifling the barrel.

Every gun barrel starts as a solid steel bar that's drilled out, creating the central tube, which we call the bore.

And even that initial drilling leaves marks, right?

Oh yeah.

Crucially, even that first drilling leaves microscopic, random irregularities unique to that process.

But the rifling is what turns the barrel from a simple tube into an accurate delivery system.

So the rifling process shapes the inner surface with spiral grooves.

It's all about physics imparting a rapid, stabilizing spin to the bullet.

It's just like a quarterback throwing a spiral.

To prevent it from tumbling end over end.

Exactly.

Cumbling is the enemy of accuracy.

So when these spiraling cuts are made, you get two distinct parts of the structure.

Correct.

You get the lands, which are the raised portions of the bore that are left untouched, and the grooves, which are the low -lying cut portions.

If you were to look at a cross section, you'd see these alternating raised and depressed spiral segments.

And this whole structure is what defines the caliber.

It is.

The caliber is the diameter of the bore, measured between opposing lands.

It's usually expressed in hundredths of an inch, like .45 caliber, or in millimeters, like nine millimeter.

But it's not always an exact measurement.

That's a key point for any learner.

It's more of a designation or a category than an absolute measurement.

A so -called .38 caliber weapon, for instance, might have an actual bore diameter that varies slightly depending on the manufacturer.

So if the whole purpose of rifling is to mass produce accurate weapons, how do these mass produced items still end up being individually unique?

It's the imperfection that's just inherent in high -speed manufacturing.

There are three primary methods used today to create that rifling.

Okay, let's start with the first modern method, the broach cutter method.

So you have to imagine this massive tool, maybe four feet long, made up of a series of concentric steel rings, and each ring is slightly larger than the one before it.

Okay.

This tool is dragged through the barrel in a single pass.

As it moves, it simultaneously cuts all the grooves to the desired depth, and it's rotated to create that twist.

That sounds incredibly efficient.

It's very fast and efficient.

But where do the unique marks come from in that case?

Well, even though the broach cutter is highly precise, its surface will inevitably have minute flaws, chatter marks,

or tiny pieces of metallic debris.

As the cutter shaves the steel, these imperfections scratch fine random line striations onto the interior surface.

And those scratches are the individual characteristic.

That's the signature.

Okay, the second process is the button process, and this uses a completely different approach.

Right.

This method involves no cutting at all.

The best analogy is like a patterned cookie cutter.

A hardened steel plug, the button, is machined with the lands and grooves in reverse.

The negative impression.

The negative impression of the rifling pattern.

This button is then forced through the barrel under extremely high pressure.

So instead of cutting metal, it compresses and displaces the steel outward, forming the lands and grooves.

So the uniqueness comes from the imperfections on that cookie cutter tool itself.

Precisely.

Any tiny burr, scratch, or piece of form material embedded in the button gets impressed into the compressed metal of the barrel wall.

And the third major method is mandrel rifling or hammer forging.

This is probably the most high -tech version.

A hardened steel rod, the mandrel, is created with the desired rifling pattern on its surface.

This mandrel is inserted into an oversized barrel bore.

And then?

And then the barrel is compressed around the mandrel using tremendous pressure, often with rotary hammering.

The metal basically molds itself around the mandrel, taking on the negative impression of the rifling.

So it sounds like whichever method a manufacturer chooses, cutting, compression, or forging, that choice establishes the class characteristics.

Absolutely.

Class characteristics are the consistent features determined by the factory,

the number of lands and grooves, their width, and the direction of the twist, right or left.

And that lets examiners quickly eliminate certain guns.

Right.

If you find a bullet with five lands and grooves twisting right, you know it couldn't have come from a Colt .32 caliber, which usually has six lands and grooves twisting left.

But again, those are just exclusions.

The final, definitive match that rests entirely on the individual characteristics.

Which are those microscopic striations.

These fine lines are completely random, and the fundamental principle here is inviolable.

Even if two barrels were made one right after the other with the same tool, the process of wear and chipping ensures no two barrels will ever leave identical striation markings.

That is the forensic signature.

This principle leads us directly to evidence comparison.

So when a bullet is fired, its softer metal surface is engraved by the hard steel of the lands and grooves.

Recording those unique striations right onto the bullet's surface.

But the examiner can't just stick the evidence bullet next to the barrel's interior, can they?

No.

They need a pristine test sample.

The suspect weapon has to be fired into a recovery medium, either a water tank or a box filled with cotton.

Something to stop the bullet without damaging those crucial markings.

And the first step is always comparing the class characteristics.

If the number of grooves or the twist direction doesn't match the evidence bullet, the gun is immediately eliminated.

But if they do align, the real work begins.

That requires the cornerstone of modern firearms examination.

A comparison microscope.

Developed by Calvin Goddard in the 1920s.

This is the tool that really professionalized ballistics.

It allows the examiner to view the evidence bullet and the test fired bullet at the same time, through a single eyepiece.

The two images are split right down the middle.

So they mount both bullets and just rotate them until they find a match.

They mount them on rotating holders and carefully align them, rotating them until corresponding lands or grooves match up.

So the land and groove woods have to be identical, but the key is that line, the striations, must line up perfectly across that split view.

That moment when the striations merge and look like a single, continuous line running across both bullets, that is the moment of a definitive match.

Wait a minute.

Unlike fingerprints, where we have rules for minimum points of comparison, you said earlier that the conclusion in ballistics relies on the examiner's seasoned judgment.

If the technology just provides an image, but the match itself is purely subjective, doesn't that become the weakest link?

That is a fundamental concern and you've hit on a critical point.

While the microscope is powerful, that final leap from observing coincidence to declaring individualization is qualitative.

There's no universal rule for, say, 12 matching points.

So what's it based on?

The judgment has to rely on the rarity, the continuity, and the density of the matching striations.

And this really emphasizes the need for extremely rigorous training and peer review, because things like a mutilated bullet or rust inside the barrel can alter the markings.

So the expert has to distinguish between the original manufacturing signature and later damage.

Exactly.

That's where the seasoned judgment comes in.

That makes sense.

What if an investigator recovers a bullet, but the weapon is never found?

You can still get valuable information.

If the brillet hasn't lost mass, its weight can help estimate the caliber.

But most importantly, the class characteristics, the pattern of lands and grooves, are logged and searched.

Which brings us to the general rifling characteristics file.

A vital resource maintained by the FBI.

It lists the class characteristics for almost all known weapons manufactured globally.

This can quickly narrow the possibilities down to a few makes and models.

And you get unique cases, like micro -grooving.

Right.

Used by Marlin rifles.

It produces between 8 and 24 shallow grooves, making them very distinct from the standard 4, 5, or 6 groove patterns.

Let's shift away from rifled weapons to shotguns, where the evidence is totally different because the barrel is smooth.

Exactly.

Since the barrel is smooth, there's no rifling to impress marks onto the shot pellets.

So identification relies entirely on the ammunition components you recover.

And shotgun barrel size is measured by gauge.

And the definition of gauge is surprisingly specific.

It's defined by the number of lead balls of that specific diameter that would weigh one pound.

So a 12 gauge weapon means 12 lead balls of that bore diameter weigh a pound.

Which means the higher the gauge number, the smaller the diameter.

The .410 gauge is the only exception where it's a direct measurement.

So what does the examiner look for with a smooth bore weapon?

They weigh and measure the recovered shot to determine the pellet size.

But the most valuable evidence is often the recovered wad, the component between the powder and the shot.

The wad's material, size, and shape are highly specific to the manufacturer and the gauge.

Now let's pivot from the projectile, the bullet, to the cartridge case, the spent shell that's ejected.

This also gets a highly distinct signature.

Oh yes.

When a gun is fired, the expanding gases push the bullet forward, but they also push the shell casing backward with tremendous force.

That casing slams into the breech face.

The rear surface of the barrel assembly.

Right.

And this impact is violent.

It creates a complex signature made up of four specific markings.

Starting with the firing pin impression.

The firing pin strikes the soft metal of the primer cup.

Minute, random irregularities on the tip of the firing pin are impressed into that soft primer, leaving a unique signature.

Second,

the breech face markings.

As the casing is slammed back, the machined surface of the breech face, which has its own random microscopic striations from manufacturing transfers, that unique pattern onto the casing.

These are one of the most reliable features.

And the last two markings come from the mechanisms that handle the spent casing.

Correct.

The extractor grips the lip of the casing to pull it out, and the ejector throws the casing out of the weapon.

Imperfections on both leave distinctive scratches or marks on the casing.

So you have four potential points of comparison.

Four highly distinctive signatures.

Pin, breech face, extractor, and ejector.

It makes the cartridge casing forensic gold.

The process of manually comparing these marks using a comparison microscope is, well, it's meticulous and time consuming.

It is.

And that manual process led to delays and questions in big historical cases, which drove the need for automation.

And the most famous case illustrating this need is the Sakura and Vanzetti trial.

A politically charged murder and robbery in 1920, right in the middle of the Red Scare.

The evidence hinged on whether a .32 caliber pistol, owned by Nikola Sacco, fired the crime scene bullets.

And the initial examination was flawed.

It was, leading to a guilty verdict and a death sentence.

But the controversy just persisted for years.

Until Calvin Goddard got involved.

It wasn't until 1927 that the authorities allowed the evidence to be reviewed by Goddard.

Using his revolutionary comparison microscope, he conclusively proved that a test bullet fired from Sacco's gun was an absolute match to one of the crime scene bullets.

So that confirmation, years later, highlighted the power of the science, but also the logistical nightmare of doing this manually.

Exactly.

And that's why, decades later, you saw the push for computerized systems.

Two competing systems emerged in the early 90s.

The FBI developed drug fire.

Which was initially focused almost exclusively on the unique markings on cartridge casings.

And the ATF developed the Integrated Ballistic Identification System, or IBIS.

And IBIS seems to have been more comprehensive from the start.

It was designed to process and compare digital images of features on both bullets and casings.

It used specialized modules, right?

Yes.

Bullet proof for the rifling striations on bullets,

and brass catcher for the breech face, and firing pin marks on casings.

So if we were to visualize this process, how does IBIS turn a physical casing into a searchable database entry?

Well, you'd place the spent casing on a stage under a high -powered microscope.

A video camera captures an image of the breech face mark.

That image is digitized, turned into data points, which the software analyzes for specific characteristics.

And that data is stored and compared against thousands of other images.

It's an incredibly complex image recognition process.

But having two separate federal systems, that seems inefficient.

It was.

It led to the necessary merger.

In 1999, the two systems were unified under the banner of the National Integrated Ballistic Information Network, or NIBN.

And the ATF and FBI split the duties.

The ATF manages the physical system sites, the hardware, while the FBI handles the overall communication network that allows data exchange nationwide.

And the impact has been staggering.

It allows law enforcement to connect crimes that would otherwise look like isolated incidents.

It's compiled over 800 ,000 images,

and has resulted in over 28 ,000 verifiable linkages between previously unconnected crimes.

Like the Houston security guard case.

A perfect example.

A bullet and a casing were recovered, and entered into NIBN.

The system immediately linked those unique marks to a prior double homicide of two store clerks and a separate robbery that happened just two weeks before.

So it linked three separate violent incidents to one specific .40 caliber Smith & Wesson before the police even had a suspect.

That's a transformative power.

Once a suspect was eventually detained and their firearm seized, a test fire confirmed the match for all three incidents using a comparison microscope.

But we have to reinforce the crucial limitation here.

NIBN is not the final answer.

Absolutely not.

NIBN is merely a screening tool.

It sifts through hundreds of thousands of samples, and produces a list of potential matches or candidates.

But the computer's correlation isn't definitive proof.

No.

The final, legal conclusion that two pieces of evidence came from the same weapon must always be made by a trained forensic examiner, physically verifying the match with a traditional comparison microscope.

The computer searches, the expert confirms.

And looking ahead, there's this intense debate around ballistic fingerprinting.

This proposes that manufacturers test fire every single new handgun or rifle, sold and store its ballistic fingerprint in a centralized database.

The potential to trace new firearms used in crimes is huge.

But the logistical hurdles are massive.

Who manages a database with millions of images?

Who pays for it?

How do you standardize it?

It's a compelling idea, but the technological illegal infrastructure required is just daunting.

Okay, so let's move from the gun itself to the result of the discharge.

We're talking about distance determination.

This is incredibly important.

Knowing the distance between the muzzle and the target can corroborate or refute a self -defense claim, or help determine if a death was suicide versus homicide.

And the mechanism relies entirely on the physics of the discharge.

Exactly.

When modern smokeless powder is ignited, it rapidly generates expanding gases.

But the powder is never completely consumed.

So the discharge includes unburned or partially burned particles, along with smoke and vaporized lead, all propelled toward the target.

And to make an exact determination, you have to have the suspect weapon and the suspect ammunition.

That's the gold standard.

The examiner has to perform numerous test firings at known distances, 6 inches, 12 inches, 18 inches, onto material comparable to the victim's clothing.

And then they compare the residue pattern on the evidence to their test patterns.

To give a precise opinion on the firing distance.

If the weapon isn't recovered, the examiner has to rely on general visual observations.

Let's walk through those critical distance intervals.

At contact distance, meaning the muzzle is touching or held less than an inch from the target, the effect is explosive.

You see a heavy concentration of smoke -like vaporous lead, scorching, and critically,

the stellate tear pattern.

A star -shaped tear.

A star -shaped tear caused by the gas blowing back and ripping the fabric outward around the hole.

And if the distance is increased slightly to say 12 to 18 inches, the stellate tear disappears, but you still see a distinct halo of vaporous lead or smoke deposited around the hole.

Okay, so scorching and tearing at contact and a smoke halo up to 18 inches.

What happens as the distance increases further?

When we move out to about 25 to 36 inches, you find scattered specks of unburned or partially burned powder grains.

But the smoke, or soot, is almost entirely absent.

Why does the smoke drop out first but the grains travel farther?

The smoke particles are much lighter and lose momentum rapidly.

The heavier unburned powder grains, especially dense ball powder, retain their momentum longer and can travel up to six or even eight feet.

So if you see grains but no soot, you know you're in that mid to long range.

And finally, if the weapon was fired more than three feet away.

At that range, you usually get no powder residue.

The only indication is a dark ring right around the entrance hole known as bullet wipe.

And what exactly is bullet wipe?

It's a composite ring of carbon, dirt,

lubricant, and lead residue stripped from the bullet surface as it passes through the cloth.

And again, shotguns are different.

Shotgun distance determination is all about measuring the spread of the discharge shot pattern.

At very close range, it's a single hole.

As the distance increases, the pellets separate.

And we have that standard rule of thumb for 12 gauge weapons.

Typically, the pattern will spread about one inch for every yard of distance.

So a 15 inch spread suggests a distance of about 15 yards.

But this is just an estimate.

It's significantly influenced by barrel length, pellet size, and most importantly,

the degree of choke.

Okay, once garments with bullet holes are collected, how does the lie proceed if the residue is obscured by blood or on dark clothing?

The first step is often infrared photography.

It enhances contrast and can visualize deposits hidden beneath dark colors or blood stains.

If that isn't enough, we turn to chemistry.

The classic chemical test for uncombusted powder is the grease test.

The grease test is designed to detect nitrites, which are a specific chemical byproduct of incompletely burned nitrocellulose.

Think of nitrites as a type of chemical smoke that stains.

And how is the test performed?

The examiner takes a sheet of chemically treated gelatin -coated photographic paper.

They place it over the area and press a hot iron against the paper.

The heat causes the nitrate particles to transfer.

And then they treat the paper.

Right.

They treat the paper with specific chemicals, and the transferred nitrites produce an orange color, revealing a clear visible pattern of the powder grains.

Is there a separate test for the vaporous lead residue?

Yes.

That test uses the chemical called sodium rhodizinate.

The target is sprayed with it.

If lead is present, it turns pink.

Then it's over -spread with a weak acid, which causes the lead to rapidly shift from pink to a distinct blue violet.

And that color change is the definitive confirmation.

Confirmation of lead residue, which corroborates a close -range shot.

The explosive discharge doesn't just go forward.

It also blows back toward the shooter, depositing traces of primer and gunpowder onto the firing hand.

Concentrating in the thumb web and the back of the hand.

Detecting this is critical to placing the weapon in the suspect's hand.

Historically, they tried various tests, including the failed dermal nitrate test.

Right, which used hot paraffin wax to lift residues.

And why was that test discredited?

A severe lack of specificity.

Nitrates are everywhere in fertilizers, cosmetics, tobacco, even urine.

The false positives made it scientifically meaningless in court.

So modern science shifted its focus to the unique chemical components in the primer mixture.

Exactly.

Modern primers usually contain a signature blend of three elements.

Lead stefinate, barium nitrate, and antimony sulfide.

When the weapon is discharged, those three elements, lead, barium, and antimony, are blown back.

So detecting that specific elemental triplet is the key.

That's the key to identifying gunshot residue, or GSR.

And how is this microscopic residue collected?

The two most common methods are using adhesive tape, or sticky stubs, to lift particles from the hand.

Or swabbing the hand separately with cotton moistened with weak nitric acid.

But the challenge is that this evidence is incredibly fleeting.

It is.

It's easily dislodged.

Studies show it's difficult to detect after as little as two hours.

Many labs impose a strict six -hour limit for collecting GSR from living subjects.

But it's often more successful in suicide cases.

If the hands are immediately protected with paper bags, yes, it prevents accidental dislodgment.

To overcome the issues with older tests, modern labs rely on the ultimate tool for detection, scanning electron microscope, or SEM analysis.

The SEM is the highest standard.

The adhesive stub is placed inside the microscope, which bombards it with electrons for extremely high magnification.

The particles themselves are located and analyzed.

They have characteristic size and unique morphology or shape.

They often appear as spherical particles.

Right.

And the SEM is linked to an X -ray analyzer, which provides that second level of confirmation.

Like a high -powered microscope combined with a tiny metal detector.

That's a perfect analogy.

The X -ray analyzer confirms the elemental composition.

The definitive finding of GSR is the simultaneous presence of that crucial combination.

Lead, barium, and antimony.

But even this advanced method isn't foolproof.

You mentioned contaminants.

That's the persistent complication.

Particles with those three elements have been found in things like brake linings and fireworks.

The examiner has to carefully consider the context.

Plus, the manual search time is still very slow.

Finally, before we leave firearms, we have to talk about serial number restoration.

How is it possible to recover a number that's been deliberately ground out?

It comes down to metallurgy.

When a serial number is stamped into metal, the dye doesn't just mark the surface.

It exerts tremendous force.

This causes a permanent crystalline strain in the metal that extends beneath the original number.

So the metal itself has a memory.

In a way, yes.

Even when the surface is ground off, that underlying strain remains.

And the restoration uses chemistry to explain that difference.

The obliterated surface is polished to a mirror finish.

Then a suitable etching agent is applied.

The critical principle is that the strained metal beneath the original number dissolves faster than the surrounding unstrained metal.

This uneven etching eventually reveals the pattern of the original numbers.

Unless the grinding was too deep.

If you remove the entire zone of strain,

restoration is usually impossible.

The collection and preservation of firearm evidence is highly technical.

It demands absolute caution.

Safety is the paramount concern.

The investigator must prevent accidental discharge and they must meticulously record the position of the hammer, the safety, and all remaining ammunition.

And we have to caution against that classic mistake from the movies.

The Hollywood image of picking up a weapon by putting a pencil in the barrel.

It's forensically disastrous.

It can disturb powder deposits, rust, anything crucial for later comparison.

So how should they handle it?

Carefully, by the edge of the trigger guard or the checkered grip.

Areas that rarely hold clear prints.

And there's a specific crucial protocol for documenting revolvers.

Since the casings remain in the cylinder,

the investigator has to document their position.

They mark the chamber aligned with the barrel, then diagram the contents of every single chamber.

So they can reconstruct the firing sequence.

Absolutely vital for reconstruction.

And we mentioned water recovery.

Why the special rule about keeping a recovered firearm submerged in the same water?

It's all about chemistry.

When a steel gun is submerged, it's in a low oxygen environment.

If you pull it out into the air, the massive influx of oxygen triggers rapid oxidation or flash rusting.

Which would destroy the microscopic striations.

Instantly.

By transporting it submerged in the same water, you slow that chemical reaction down, giving the lab time to analyze it before it deteriorates.

And protecting the fired ammunition is all about protecting those striations.

Extreme caution.

You have to carefully break away the surrounding material, never prying a bullet loose with a sharp instrument, and you must never scratch the bullet directly for identification.

How should they be packaged?

Wrapped carefully in tissue paper or cotton, then placed into a labeled pill box or sturdy envelope.

And for casings, the exact location where they were found must be precisely documented.

And finally, handling clothing with gunpowder residues.

The area around the bullet hole is critical.

Avoid cutting or tearing it.

Air dry wet clothing out of direct sunlight.

And place each item in a separate paper bag to prevent cross contamination.

We've spent a lot of time on firearms.

Now we shift our focus to other inanimate objects that leave forensic signatures, starting with tool marks.

A tool mark is broadly defined as any impression, cut, gouge, or abrasion caused by a tool coming into forceful contact with another object.

Very common in burglaries.

And just like ballistics, tool marks are divided into two fundamental types.

First, you have indented impressions.

This is when a tool is pressed into a softer surface without sliding, like a crowbar against a window frame.

These usually only show class characteristics, size and shape.

And second, and more forensically valuable, are abrasion marks.

Or striated marks.

These are left when a tool slides or cuts across a surface.

Just like a gun barrel, the working edges of tools have random microscopic irregularities, nicks, and wear marks.

And those become the individual characteristic.

Exactly.

When that edge scrapes across a surface, it transfers those unique striated lines.

The examiner uses the same methodology as ballistics.

They make test marks in soft lead and compare them to the cry mark under a comparison microscope.

And given the risk of damaging the mark, what is the critical rule for collecting tool mark evidence?

The absolute rule is, never attempt to fit the suspect tool into the tool mark.

Any contact risks destroying the individual characteristics.

So you submit the whole object if you can?

If possible, yes.

If not, you photograph it to scale and create a precise caft using liquid silicone.

And the tool and the cast must always be packaged separately.

Moving on to other impressions, focusing on footwear and tire marks.

What are the essential preservation steps?

Photography is always the first non -destructive step.

You photograph the impression with a ruler directly overhead.

And crucially, side lighting is essential to reveal the subtle details.

And once photographed, how are impressions lifted if they are in dust on a floor?

For dry dust, the specialized highly effective technique is electrostatic lifting.

That sounds very high tech.

How does it work?

It uses high voltage.

A sheet of Mylar film is laid over the dust print.

The device charges the film, creating a static charge differential that causes the dust particles of the impression to jump up and adhere to the film.

It's remarkably effective.

And chemical enhancements are also used.

Yes, especially for prints in blood.

Chemicals like luminal can visualize faint blood stains.

And a critical point is that these chemicals do not interfere with subsequent SDR DNA analysis.

But the most robust method for preserving deep impressions in soft earth is casting.

Casting is still the preferred method.

Specifically using class I dental stone, a high quality gypsum.

And the technique requires precision.

It does.

After photographing, you carefully remove loose debris,

then apply a fixative -like hairspray from about 18 inches away to stabilize the soil.

Then the dental stone.

It's mixed with water to the consistency of thin pancake batter.

You pour it carefully beside the impression,

allowing it to flow gently into the track.

A direct pour can collapse the delicate details.

And for impressions in snow.

Snow poses unique thermal challenges.

Before casting, the impression has to be treated with aerosol snow impression wax.

You apply three light coats to seal the snow and prevent the heat from the curing dental stone from melting the details.

And finally, we return to comparing these impressions.

Again, it all hinges on those individual characteristics.

Always.

Class characteristics, size, design, can only say the impression could have been made by the suspect's shoe.

But it's the individual characteristics, the random wear patterns, the unique cuts, the gouges, or even four small stones, that provide the individuality for a definitive match.

And just like Nibbin for firearms, there are automated systems for footwear.

The SICAR system, shoe print, image capture, and retrieval searches images of crime scene impressions against expensive databases.

It accelerates the initial investigation.

But just like Nibbin, the final match must be confirmed by a forensic examiner.

And we can't overlook bite marks.

Right.

Bite marks are evaluated by forensic odontologists.

They compare test marks, dental models from a suspect, to the evidence mark.

The comparison relies on the unique characteristics of an individual's teeth.

Spacing, chips, misalignment.

This entire principle, using rare class characteristics to narrow the field, then individual characteristics for the definitive match was played out spectacularly in the O .J.

Simpson trial.

The bloody shoe impressions were critical.

FBI examiner William J.

Boziak analyzed the print and identified the shoe as an extremely rare, expensive Italian -made Bruno Mogli Lorenzo style, size 12.

Remarkable detective work, relying just on the sole pattern and size.

It showed the power of rare class characteristics.

The investigation found only 299 pairs of that exact size and style had been sold in the United States.

And while Simpson denied owning them, photographs taken months before the murders later surfaced, proving he had worn the exact style.

This narrowed the field to just 29 pairs sold in the entire country, providing powerful circumstantial evidence that was crucial in the subsequent civil trial.

That case perfectly encapsulates the journey we took today.

It solidifies the principle that even a common object, if examined closely enough, possesses an undeniable forensic signature.

We have tracked the path of evidence today.

From the macroscopic scale of those four stones in the Nissan tire to the microscopic scale of the fine striation lines inside a gun barrel.

Every single scratch and wear mark contributes to the narrative.

So what does this all mean for the future?

The foundational principles of forensic science, connecting an inanimate object to a person, rely entirely on the idea that manufacturing defects and wear create a unique signature.

And that signature is fleeting.

The lifespan of this evidence is often measured in hours, not days.

Now you have to consider the impact of environmental and technological changes.

If manufacturers continue phasing out lead in primers to reduce contamination,

how will the highly specific SEM analysis, which relies on that lead barium anthimony triplet, adapt to maintain its specificity?

And at the same time, how quickly must systems like Niven expand their databases to keep up with the pace of crime?

The technology must constantly race to work faster than friction and time itself.

Food for thought indeed.

Thank you for joining us for this deep dive into the fingerprint of inanimate objects.