Chapter 20: Solving a Case is Easier Said Than Done

In the past, fingerprint collection was generally categorized into two main types: physical methods and chemical methods.

In recent years, due to the increasingly widespread application of lasers, optical fingerprint collection has also become more common.

Detectives from Action Group Two typically use physical methods to collect fingerprints. It is not that they do not want to use more methods, but rather that they are not very proficient in the others.

Among the various physical collection methods, using transparent tape is the most convenient and rapid.

Of course, this quick method has significant drawbacks: the integrity of the collected fingerprint is low, and it is highly destructive to the print itself.

Furthermore, its application scenarios are limited; transparent tape can only collect fingerprints from hard, non-absorbent materials such as glass, floor tiles, and stainless steel.

As for fingerprints on materials like leather or paper, chemical methods must be used.

Naturally, in addition to transparent tape, Action Group Two also uses standard powder methods.

For instance, silver powder is widely used because of its strong adsorption capabilities.

Even a professional forensic detective like Lu Chuan would utilize silver powder.

However, at a fire scene, it is clearly impossible to use ordinary methods to collect fingerprints.

There are two reasons why fingers are able to leave fingerprints on a surface.

First, the surface of a finger has irregular ridges; the existence of these ridges reduces the friction between the finger and the object.

This differs from the general understanding; while it is common knowledge that a rough surface increases friction, for a finger, the skin itself is inherently rough and uneven.

The coefficient of friction is naturally quite high, so the bumpy fingerprints reduce the contact area between the skin and the object, thereby reducing friction.

This serves to protect the skin from being damaged by frictional forces.

Second, fingerprints contain numerous sweat glands and sebaceous glands. The mixture of these two substances left on an object's surface can be developed to reveal the fingerprint.

What Lu Chuan is doing now is a combination of physical and chemical methods to collect fingerprints.

Fingerprint collection at a fire scene has historically been a major difficulty in forensics.

First, after a major fire, the fingerprint substrate itself is often damaged; in such cases, even a deity could not collect a fingerprint.

Second, even if the substrate is not completely destroyed by the fire, the sweat that develops the fingerprint, as well as the proteins and amino acids in the oils secreted by the sebaceous glands, are all damaged by the high temperatures.

Physical methods collect fingerprints by utilizing the stickiness of the sweat and oils contained within the print.

But at a fire scene, where sweat has evaporated and oils have been destroyed, it is naturally impossible to use this method.

Most chemical methods rely on reacting chemical substances with the proteins and amino acids in the oils to develop the print.

This is clearly unachievable at a fire scene.

Therefore, theoretically speaking, the key to collecting fingerprints lies in identifying the substances within the fingerprint that cannot be destroyed by high temperatures.

And the prerequisite for all of this is discovering the fingerprint in the first place.

This is exactly the work Lu Chuan is performing now.

Following the area of the fire's origin, Lu Chuan meticulously swept his LV-brand brush across the surface.

He did not miss a single complete surface of any object.

Watching him, Ren Qiang nodded repeatedly.

The most important thing for a detective is not having superlative logical thinking or reasoning abilities.

What is truly needed is patience.

Just as Lu Chuan was doing now—following every possible direction, not missing a single detail, and working steadily, bit by bit.

Only in this way can one ensure full coverage of the perpetrator's movement path at the time.

This sweeping process lasted for three hours.

Effort pays off; Lu Chuan discovered five locations where fingerprints could potentially be extracted.

Afterward, Lu Chuan took out a bottle of solution and began the extraction process.

"Is this... silver nitrate?"

Having been a detective for so many years, Ren Qiang was familiar with some of the forensic toolkit.

Lu Chuan nodded. "In a burned fingerprint, only the sodium chloride in the sweat can survive high temperatures. Silver nitrate solution can be used to extract this substance."

As he operated, Lu Chuan explained, "However, once the silver nitrate reacts with the sodium chloride, it produces a black fingerprint."

At a fire scene, everything is pitch black; a black fingerprint is almost indistinguishable from the surroundings.

"So, next, copper powder must be used to develop the fingerprint. Only then can we extract the fingerprint we need."

"Why not use silver powder?"

Silver powder is cheaper and has stronger adsorption.

"Extracting these types of fingerprints isn't a matter of 'the stronger the adsorption, the better.' Fingerprints developed with silver nitrate are very fragile. Too much suction will cause the fingerprint to crack, creating unnecessary breaks."

Ren Qiang nodded as if realizing something, but the look in his eyes as he watched Lu Chuan began to change.

This morning in the forensic office, when Ren Qiang heard Lu Chuan say he had studied fingerprint collection, he hadn't actually held particularly high expectations.

It was simply that Lu Chuan had a precedent of merit, and being professionally trained meant he was at least better than people like themselves.

He had approached it with a "let's see" attitude.

But after watching Lu Chuan's entire process of on-site collection, Ren Qiang knew.

Lu Chuan didn't just have "ordinary research" into fingerprint collection; he was exceptionally skilled.

Having worked with Liu Guodong his entire career, he had naturally seen how the man collected fingerprints.

While Liu Guodong was leagues ahead of him, compared to Lu Chuan...

This Lu Chuan... is something else.

By the time everything was finished, it was already 5:30 PM.

"Only four sets of fingerprints were collected..."

It was a bit of a pity; the fifth set had failed. When Lu Chuan was sprinkling the copper powder, his hand had jerked, adding a few extra grams.

But those few extra grams of copper powder caused the fingerprint to fracture, making it completely unusable.

"Very good, very good! These four sets are enough!"

Looking at the golden, clear fingerprints in the photos, Ren Qiang felt there was no need to go to Liu Guodong and the others for comparison.

With fingerprints this clear, even the new college students in his group could identify them with the naked eye.

Lu Chuan nodded and began packing his forensic kit.

"Actually, in an arson scene like this, extracting footprints would be easier, but unfortunately, the ground traces have been too severely damaged."

Ren Qiang agreed. Since the perpetrator had set fire to the warehouse, they must have left footprints.

Furthermore, during the fire, those prints might have been covered by collapsing debris, thereby being preserved.

However, due to the rescue efforts and the trampling by members of Action Group Two, the ground was already a mess, making footprint collection impossible.

At least, it was impossible with Lu Chuan's current abilities.

The system had rewarded him with a basic footprint analysis skill, but it had not granted him footprint collection capabilities.

Once the work was done, Ren Qiang arranged for his vehicle to send Lu Chuan back to the Criminal Investigation Detachment.

The man had a dinner engagement tonight, so he naturally couldn't be late.

As for Action Group Two: "Everyone, come and compare these fingerprints! Whether we can catch the suspect depends on these four sets!"

Upon returning to the detachment, Lu Chuan put his forensic kit away, went to his office to change out of his police uniform, and put on civilian clothes.

"How is the arson case being handled?"

(End of Chapter)