Tool Mark Forensics: Information for Writers

. (Photo credit: Wikipedia)

Your investigator is on the scene, looking for any clues that could get the villain off the streets. 

Maybe the villain was smart enough to wear gloves, but was he smart enough to buy a new tool and make sure that it was destroyed afterward? 

No? You might just have written a plot twist. 

Imagine if the perpetrator was trying to set someone else up with the crime. The villain borrows the hero's tools, and then carefully puts them back in the hero's garage and phones in a 1-800 tip to the police, sending the authorities to your hero's home with a search warrant in hand. 

This would be especially damning if the only prints left on the tools were the hero's. Can he talk his way out of this? Did he have an alibi? Or is he  now in jail leaving your intrepid heroine to carry on without his help?

Crime Scene 101 article
Warrants Blog Article
Finger Prints article

Types of Tool Marks Left Behind

* Shape of the tool
* Random microscopic irregularities and imperfections produced in
   manufacturing. These are frequently invisible to the naked eye.
* Wear-use - as a tool is used over time it develops new and unique
   features such as abrasions and erosion of the surface which is
   more likely to be visible upon inspection.

Wear Use on a Tool

* Wear-use takes place prior to the crime but can also be altered as
   the criminal uses the tool during the crime. 
* The wear use can help a forensic scientist make a match and
   show a probability that the investigators had found the right
   tool/weapon. 
* Unfortunately, this can also work against investigators should the
   tool not be found immediately and the tool is then used elsewhere
   and changed.

Information Left by a Tool

 (Photo credit: Wikipedia)

The forensic scientist will determine the class of weapon. Class of weapon simply identifies the general tool - was it a screw driver that gauged the hole in the window frame? Was it a saw that took out the balustrade?

When the tool leaves an impression then the scientists can
hypothesize about the tool - for example a round mark on the
wood indicating someone was swinging a hammer.

The forensic scientist must take three different surfaces into consideration when making determinations:
1. The tool
2. The surface that the tool interacted with. Obviously a machete
    attack on a mattress will have a different imprint than the 
    machete against a marble floor - though it will leave marks in
    both. So as you choose and implement your tool in your plotting
    consider the interaction with the contact material and what kinds
    of marks/information that you want to be discovered in forensics.
3. Use Edge - when a tool has an edge such as a shovel or pliers
    scissors etc. So for example, were scissors used to cut? Or were
    scissors used to bang? Or were scissors used to gauge?

 (Photo credit: Wikipedia)

* No tools are identical
   because of microscopic 
   anomalies from
   manufacturing and use.

Another idea that could inform your plot is the Locard's Principle. 

Locard said that whenever two surfaces come into contact one with the other there will be an exchange -

*If the tool is harder than the
  contact material then it
  might pick up materials like
  paint chips on the crowbar.
* If the surface is harder than the tool then the tool
   might leave trace evidence behind such as a car
   hitting a wall would leave the paint chips on the
   wall.
* Investigators should never try to fit a tool
   back into the indentation because it can
   alter the mark - so if you need a 
   wrench (*giggle*) thrown into your plot
   line you could have a newbie do just that.

 (Photo credit: Wikipedia)

How is analysis done?

On the scene -
* Tools found at the scene are packaged and transported back to 
   the lab for research.
* Transportable surfaces are collected
   `A door can be removed
   `A portion of the surface can be cut and a portion collected
* Package each item separately so there is no cross contamination

In the lab -
* Test marks are make using the tool in question
* In the crime lab they will test the edge of a tool with a soft
   material - ex. lead
* After impressions are made into a soft material - then the
    investigators might try to replicate the strike into the original
    surface to see if the can match the strike/interaction
    (evidentiary marks).
* Duplication is difficult because of the pressure and angle of the
   strike
* The test marks are microscopically compared with the marks on
   the crime scene materials. This includes microscopic
   photography.
* Non-transportable surfaces are:
   ` Photographed with a ruler in place
   ` A cast is made - using liquid silicone or dental plaster - doing
     cast work is not ideal. The impressions do not allow for
     microscopic comparisons.

Video Quick Study (3:00) A forensic scientist mixing the Microsil, application and removal

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