In the past, fingerprint collection generally divided into two categories: physical methods and chemical methods.

In recent years, optical fingerprinting has become more common as the use of lasers has become more widespread.

The criminal police of the second group of operations generally use physical methods to collect fingerprints, not because they don't want to use more methods, but they are not very good at others.

Among the various hair methods of physical fingerprint collection, it is the most convenient and fast to use transparent glue to paste it.

Of course, this fast fingerprint collection method has great disadvantages, the integrity of fingerprint collection is not high, and the damage to fingerprints is great.

In addition, the application scenarios are also limited, and transparent adhesive can only collect fingerprints on hard non-absorbent materials such as glass, floor tiles, stainless steel, etc.

As for the fingerprints on leather, paper and other materials, it is necessary to use chemical methods.

Of course, the fingerprint collection method that will be used by the second action group will be used in addition to transparent glue, and the general powder method will also be used.

For example, the common silver powder is widely used because of its strong adsorption.

Even a professional criminal police officer like Lu Chuan will use silver powder.

But at the scene of a fire, it is clear that fingerprints cannot be collected by ordinary methods.

There are two reasons why fingers can leave fingerprints on surfaces.

One is that there are irregular lines on the surface of the fingers, and the presence of these lines can reduce the friction between the fingers and the object.

This is different from the general understanding, it is common sense that rough surfaces can increase friction, but for fingers, the skin itself is rough and uneven.

The coefficient of friction is already very large, and the uneven fingerprint reduces the contact area between the skin and the object, so it reduces the friction.

This is to protect the skin from friction.

Second, there are many sweat glands and sebaceous glands on the fingerprint, and the mixture of the two substances is left on the surface of the object, and the fingerprint can be obtained by allowing these substances to develop.

What Lu Chuan is doing now is collecting fingerprints through a combination of physical and chemical methods.

Fingerprint collection at the scene of the fire has always been a difficult point in the field survey.

One is that after the fire, the fingerprint carrier itself will be damaged, and in this case, the gods will not be able to collect fingerprints when they come.

Second, after the fire, even if the fingerprint carrier itself is not completely damaged, the sweat stains that can develop the fingerprint evaporate, and the proteins and amino acids in the oil secreted by the sebaceous glands are also damaged by high temperature.

The physical method is to collect fingerprints by using sticky attachment to the fingerprints containing sweat and oil.

However, at the scene of the fire, the sweat evaporated and the grease was destroyed, so it was naturally impossible to collect fingerprints in this way.

Most chemical methods use chemicals to react with proteins and amino acids in oils and fats to develop.

At the scene of the fire, it is obvious that it cannot be done either.

Therefore, theoretically speaking, among the fingerprint substances, the substances that cannot be damaged by high temperature have become the key to collecting fingerprints.

And the premise of all this is that fingerprints are found.

This is what Lu Chuan is doing now.

Along the part of the fire point, Lu Chuan carefully swept it over with an LV brand brush.

Not every intact surface has been spared.

Ren Qiang, who was watching, nodded frequently.

The most important thing about being a criminal police officer is not how much super logical thinking or reasoning ability you have.

What is needed most is patience.

Just like Lu Chuan is doing now, following the possible direction, all the details have not been let go, bit by bit, and he will do it steadily.

Only in this way can we ensure full coverage of the murderer's path of action at that time.

This sweep is three hours.

The hard work paid off, and Lu Chuan found five sets of points where fingerprints could be extracted.

After that, Lu Chuan took out a bottle of solution and began to extract fingerprints.

"It's ...... Silver nitrate? ”

Ren Qiang has been a criminal police officer for so many years, and he also knows some of the belongings under investigation.

Lu Chuan nodded: "Overheated fingerprints, only the sodium chloride in sweat stains can be preserved at high temperatures, and this substance can be extracted with silver nitrate solution." ”

Lu Chuan explained while operating: "But silver nitrate and sodium chloride have reacted, and black fingerprints appear. ”

At the scene of the fire, there were black and black fingerprints everywhere, almost the same as nothing.

"So the next step is to use copper powder to develop the fingerprints, so that we can extract the fingerprints we need."

"Why don't you use silver powder?"

Silver powder is cheaper and has stronger adsorption.

"The extraction of this fingerprint is not the stronger the adsorption, the better, the fingerprint after the development of silver nitrate is very fragile, and too strong adsorption will cause the fingerprint to crack, causing unnecessary breakpoints."

Ren Qiang nodded understandingly, but his eyes looking at Lu Chuan gradually changed.

In the morning in the field survey office, Ren Qiang heard Lu Chuan say that he had studied fingerprint collection, but in fact, he did not have any special expectations.

It's just that Lu Chuan has a precedent for meritorious service, plus at least he is from a professional class, so he is always better than himself and others.

Have a try-and-see attitude.

As a result, after Lu Chuan collected fingerprints on the spot, Ren Qiang knew.

Lu Chuan's research on fingerprint collection is not ordinary, but extremely powerful.

He and Liu Guodong have worked together all their lives, and naturally they have seen each other collect fingerprints.

Although it is two streets away from him, he is ...... than Lu Chuan.

This Lu Chuan...... It's amazing.

After everything was done, it was already half past five in the afternoon.

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

It's a pity that the collection of the fifth group of fingerprints failed, and when Lu Chuan threw copper powder, his hand shook a few more grams.

But these few grams of copper powder caused the fingerprints to break and were completely unusable.

"Very good, very good, these four groups are enough!"

Looking at the golden and clear fingerprints on the photo, Ren Qiang felt that there was no need to find Liu Guodong and them to compare.

With such a clear fingerprint, even the new college students in his group can be compared with the naked eye.

Lu Chuan nodded and began to pack up his current survey box.

"In fact, at this kind of arson scene, the extraction of footprints will be more convenient, but unfortunately the ground traces are too badly damaged."

Ren Qiang agreed, since the murderer set fire to the warehouse, he would definitely leave footprints.

And in the process of ignition, it is likely to be covered by some collapse, which will be preserved.

However, due to the rescue at the scene and the trampling of the second group of the operation, the ground was already in a mess, and there was no possibility of collecting footprints.

At least, with Lu Chuan's current ability, he can't collect it.

The system rewarded him with basic footprint analysis skills, but he did not have the ability to collect footprints.

After finishing his work, Ren Qiang arranged his own car to send Lu Chuan back to the criminal investigation detachment.

People have dinner in the evening, so naturally they can't be late.

As for the second action group: "All the personnel came to compare the fingerprints, whether they can catch the suspect or not, it depends on these four sets of fingerprints!" ”

Returning to the detachment, Lu Chuan put the current survey box back, went to the office to change into a police uniform, and put on civilian clothes.

"How's the arson case going?"