Fingerprints Recoverable From Cleaned Metal

dstates points out a recent article from guardian.co.uk which discusses a new method by which to recover fingerprints from metal. The method relies on corrosion caused by sweat and other biological residues on the metal's surface. Quoting: "The patterns of corrosion remain even after the surface has been cleaned, heated to 600C or even painted over. This means that traces of fingerprints stay on the metal long after the residue from a person's finger has gone. The chemical basis of the change is not yet clear, but [Dr. John Bond] believes it is corrosion by chloride ions from the salt in sweat. These produce lines of corrosion along the ridges of the fingerprint residue. When the metal is heated, for example in a bomb blast or when a gun is fired, the chemical reaction actually speeds up and makes the corrosion more pronounced."

Comments

I wonder

[oodaloop]

how many peices of evidence for earlier crimes we can now find a print where we couldn't before? Maybe solve an unsolved crime or two, or free someone innocent? The ramifications for Iraq alone where we can match prints on IED remnants to current detainees is enough to keep me interested.

Re:I wonder

[txoof]

That's a really good question; it could be a huge boon for unsolved cases, vindicating wrongfully convicted individuals. I could also be a huge disaster for police departments. Thousands upon thousands of individuals appealing for reexamination of fingerprint evidence could swamp crime labs.

That being said, it is far worse to convict an innocent individual than to let a guilty man go free.

Re:I wonder

[Vellmont]

how many peices of evidence for earlier crimes we can now find a print where we couldn't before?

How many pieces of evidence are now ruined, because there wasn't a careful procedure followed in the chain of evidence where nobody touched it? A bullet casing or bomb fragment being criss-crossed with fingerprints isn't exactly going to make this technique any easier.

Re:I wonder

[cluckshot]

This really drives one to another issue. The longevity of the fingerprints will remove their value. All a finger print proves is proximity. As long as a finger print is there, it proves nothing but attendance. If it was a bio-subtance it had a short life span making it not only presence in definition but also proximity in time. That made finger prints useful. The problem here is that these now become "Undated" finger prints and as such unable to be related to events which was their only value in crime ID other than to have a list of suspects. This points out the most amazing reality about the crime "proof" we see in labs today. For the most part science is destroying evidence entirely. For example: Photos once were valuable. Then retouching started. Then it went to digital where retouching could be infinite. In the end, a photo is little more than fiction in court. Sound prints same. Now we see the ability of the police to fabricate evidence against someone to fullest extent unless we all are aware of what can be done.

Re:I wonder

[mikael]

Just imagine all the suspects involved with fingerprints on the brass cartridges:

1. The packing person who took the cartridges and placed them in a cardboard box.

2. The shop owner who took the cartridge out of the box to ensure it was a match with what the customer wanted.

3. The actual person who loaded the weapon.

If one fingerprint overwrites another, then it's not a problem. But what if the corrosion effect is additive and you get two patterns merged together. Would forensic experts be able to separate the two or would they get false positives with other fingerprints of innocent people?

Re:I wonder

[cos(0)]

This question was raised and discussed by Alexander Volokh in n Guilty Men.

Re:I wonder

[__aajfby9338]

Even though it's wrong target, you can still trace back to see to whom shop owner sold those cartridges.

Cartridges are not serialized. Even lot numbers are just marked on the carton, not on the cartridge, and any given production lot can end up being split between many, many sellers. With a shelf life measured in decades, a box of ammo might sit around on the shelf for a long, long time, and may change hands many, many times before being used. It's not even that unusual to use surplus ammo dating back to WW2 or before. A brass cartridge might have the year of manufacture stamped on the head (more likely for military ammo than for civilian ammo), and there are no markings at all applied to the projectile.

In general, it would be pretty hard to trace an arbitrary cartridge back to a particular seller or buyer without other evidence. About all that you can determine from a shell casing found at the scene of a crime would be the manufacturer, caliber, possibly the original year of manufacture (and that shell casing might have been reloaded numerous times after that), fingerprints of one or more persons who have handled it, and it may be possible to determine that it was fired in a particular firearm if (and only if) that firearm is recovered, and has not been modified, repaired, serviced, upgraded, or even fired a large number of times since that shell casing was fired in it.

You probably will not be able to trace a cartridge to a buyer or seller unless the box it came out of is also left at the scene with its credit card receipt taped to it, and even then it could be argued that a receipt indicating that particular brand and type of ammo (if the brand and type is even listed on the receipt) didn't correspond to that specific box of ammunition, and/or that the shell casing did not come from that specific box. It would be much like trying to match an individual paper napkin to a particular package, manufacturer, seller and buyer.

The FBI used to claim to be able to match a bullet to a specific manufacturing lot based on chemical analysis of the bullet's lead, but that technique has since been shown to be bogus.

Re:I wonder

[garett_spencley]

"We also tacitly accept a few wrongful convictions..."

Speak for yourself.

Re:I wonder

[Headw1nd]

I'll bite, because we don't need lofty quotes to prove it's worse to convict an innocent man than let a guilty man go free, I can do it with simple algebra.

Let's take the harm suffered by letting a guilty party go free. We can call it G. We will assume this is a positive value, since I think we can agree that letting guilty people go free is harmful to society.

Now, let's take the harm of imprisoning an innocent man, which we will call I. Also positive, since putting an otherwise useful member of society in jail for no reason is something I think we'll agree is harmful.

So let's look at the harm caused by each of our actions. Letting a guilty man free is of course G, as by our previous definition. Now to calculate the value of imprisoning an innocent man, we take our value I, and add G. Why? Well, in convicting the wrong man, we have inherently allowed the guilty party to go unpunished. So we can conclude that that G is less than I + G, i.e. it is better let a guilty man go free than to punish an innocent man.

Didn't think of that, did you?

So while convicting an innocent man might give you the opportunity to go tell that rape victim, "It's ok, we got him" it's a lie, and that lie not only destroys an innocent mans life, it lets the real rapist go free.

How Long Do They Have to be There

[theshowmecanuck]

How long do the fingerprints have to be on the metal to corrode it enough to get a good fingerprint from this method? For example, if the perpetrator uses a cloth to wipe the fingerprints off the metal immediately after the crime, will the metal have corroded enough to still give a fingerprint by this method? Or do the fingerprints need to be there for some time in order to corrode the metal enough to give a good print? And if they wipe the fingerprints off is there still enough residue to still corrode the metal, or will they need to wipe the fingerprints off using some sort of solvent or cleaner? etc. etc. etc. It would be interesting to here more.

Re:Plastic weapons

[shadow349]

This will open up the renaissance of plastic weapons.

Or, you know, gloves.

Seeing the wood for the trees

[Hognoxious]

If the fingerprints are that persistent, then lots of other marks are going to be there too - probably including lots of other fingerprints. The hard part's not going to be detecting the prints, but separating the relevant ones out from the rest of the item's history.

Archaeology Applications

[xdancergirlx]

Does this mean that we can see the fingerprints of people that handled old metal objects/chalices/swords/etc.? Maybe it would just be an item of curiousity to have a copy of Julius Ceasar's or Queen Elizabeth's fingerprints but I would put it on my wall! Maybe we could learn something about how fingerprints have changed (or not) over the course of history.

Wrong game

[spineboy]

He's not playing Rock-Paper-Scissor, he's playing Bat-AcidSoakedSponge-Saw. The hand motions are a little strange though.

Corrosion is a complicated subject

[Kupfernigk]

I have actually done research into chloride corrosion of brasses, and the answer is that it is enormously variable. Whether the brass is turned or stamped, the temperature, the number of steps in the stamping process, the sharpness of turning tools, the final treatment (grind to size, polish etc.) all affect the rate of attack. One would expect much the same for other metals, though considerable research would be needed. This will probably become a nice little earner for expert witnesses.