The collection of fingerprints in the past generally involved two major categories: physical methods and chemical methods.
In recent years, due to the widespread application of lasers, optical fingerprint collection has also become more common.
Detectives from Action Team No. 2 typically use physical methods to collect fingerprints, not because they don't want to use more methods, but because they're not very familiar with the others.
Among the various physical methods for fingerprint collection, using transparent adhesive tape is the most convenient and quick.
Of course, this quick fingerprint collection method has major drawbacks, as it doesn't collect fingerprints in their entirety and is highly destructive to them.
Additionally, the application scenarios are limited. Transparent adhesive can only collect fingerprints from hard, non-absorbent surfaces like glass, tiles, and stainless steel.
For fingerprints on materials like leather or paper, chemical methods must be used.
Besides using transparent tape, Action Team No. 2 also generally employs the powder method for fingerprint collection.
For instance, common silver powder is widely used due to its strong adhesion.
Even a professional crime scene investigator like Lu Chuan would use silver powder.
However, at a fire scene, ordinary methods cannot obviously be used to collect fingerprints.
Fingerprints are left on surfaces by the fingers for two reasons.
First, the surface of the finger has irregular patterns, the presence of which reduces friction between the fingers and the object.
This differs from general understanding; it's common knowledge that a rough surface increases friction, but for fingers, the skin is already rough and uneven.
The coefficient of friction is naturally high, and the uneven surface of fingerprints reduces the contact area between the skin and objects, thereby reducing friction.
This is to protect the skin from damage by friction.
Second, there are numerous sweat glands and sebaceous glands in fingerprints, and the mixture of these two substances left on the surface of objects can reveal fingerprints through developing these substances.
Lu Chuan is now working on collecting fingerprints by combining physical and chemical methods.
Fingerprint collection at fire scenes has always been challenging in crime scene investigations.
Firstly, the fingerprint carrier itself may be damaged after the fire, making it impossible to collect fingerprints even for a miracle worker.
Secondly, even if the fingerprint carrier isn't completely destroyed by the fire, the sweat residues that reveal fingerprints evaporate, and the proteins and amino acids in the sebum are damaged by high heat.
Physical methods involve using the adhesion of sweat and oil in fingerprints to collect them.
But at the fire scene, with sweat evaporated and oil destroyed, it is naturally impossible to collect fingerprints using this method.
Most chemical methods involve using chemical substances to react with the proteins and amino acids in the oil to reveal fingerprints.
At a fire scene, this is clearly also impossible.
Therefore, theoretically, materials in fingerprints that cannot be damaged by high heat are key to fingerprint collection.
And the premise of all this is to discover the fingerprints.
Lu Chuan is currently undertaking that task.
Following the ignition point, Lu Chuan carefully brushes with an LV-brand brush.
He doesn't overlook any intact surface.
Ren Qiang frequently nods as he watches.
The most important thing for a detective is not having super logical thinking or deductive ability.
What's truly needed is patience.
Just like what Lu Chuan is doing now, meticulously following every possible direction, leaving no detail unchecked, doing everything step by step.
Only in this way can every part of the criminal's path be covered.
This one sweep takes three hours.
Patience rewards the diligent, and Lu Chuan discovers five locations where fingerprints may be extracted.
Then Lu Chuan takes out a bottle of solution and starts extracting the fingerprints.
"Is this... silver nitrate?"
Having worked as a detective for so many years, Ren Qiang is familiar with some crime scene investigation tools.
Lu Chuan nods, "Fingerprint sweat chloride can survive high temperatures in a burned scene, and silver nitrate solution can extract this substance."
Lu Chuan explains while working, "However, after reacting with silver nitrate, the fingerprints appear black."
At a burnt scene, everything is blackened, making black fingerprints almost indistinguishable.
"Then we must use copper powder to develop the fingerprints, so we can extract the ones we need."
"Why not use silver powder?"
Silver powder is cheaper and has stronger adhesion.
"For extracting these fingerprints, stronger adhesion isn't necessarily better. The fingerprints developed by silver nitrate are quite fragile, and too much adhesion can cause them to crack, creating unnecessary breaks."
Ren Qiang nods in understanding, but his gaze towards Lu Chuan gradually changes.
In the morning at the crime scene office, Ren Qiang heard Lu Chuan say he had research on fingerprint collection, but he didn't have high expectations.
Lu Chuan had previous achievements, and since he was professionally trained, he was surely better than the rest.
With a try-it-and-see attitude.
But after watching Lu Chuan's fingerprint collection on-site, Ren Qiang knew.
Lu Chuan is not just ordinarily knowledgeable about fingerprint collection; he's exceptionally skilled.
Having worked with Liu Guodong for a lifetime, he's naturally seen him collect fingerprints.
Even though Liu was much better than himself, comparing him to Lu Chuan...
This Lu Chuan is remarkable.
Finishing everything, it's already five-thirty in the afternoon.
"Only four sets of fingerprints were collected..."
It's a pity the fifth set failed due to a shaky hand while sprinkling the copper powder.
Just a few extra grams of copper powder caused the fingerprints to crack and become unusable.
"Excellent, excellent, these four sets are sufficient!"
Looking at the golden, clear fingerprints in the photos, Ren Qiang feels there's no need to compare with Liu Guodong's team.
Such clear fingerprints, even the new university graduates in his team could match them with the naked eye.
Lu Chuan nods and begins to pack up his crime scene investigation kit.
"Actually, at an arson scene, footprint collection is easier. It's a pity the ground traces are too severely damaged."
Ren Qiang agrees; since the criminal must've left footprints while setting the fire in the warehouse.
During the fire, footprints may well be concealed by collapses and preserved.
However, given the rescue operations and the trampling by Action Team No. 2, the ground is too chaotic for footprint collection.
At least, not with Lu Chuan's current capability.
The system rewarded him with a basic footprint analysis skill, but not footprint collection capability.
After finishing the work, Ren Qiang arranges for his car to take Lu Chuan back to the Criminal Investigation Team.
The man has a dinner appointment tonight and naturally shouldn't be late.
As for Action Team No. 2: "Get everyone to compare fingerprints; whether we can catch the suspect depends on these four sets of fingerprints!"
Returning to the team, Lu Chuan puts back the crime scene kit, changes out of police uniform, and puts on casual clothes.
"How's the arson case handling going over there?"