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Home / Law / Introduction to Forensic Science / Toolmarks

Toolmarks

Toolmarks
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This video will cover toolmarks– how they are compared, and what class and individual characteristics are. There are four classifications of toolmarks. The first classifies impressed marks, such as when a screwdriver is used as a lever to force a door or a window. The second is striated toolmarks, such as a screwdriver scraped along a car door. The third is crushed toolmarks, such as pliers cutting wires. And the final category is for multi-stroke toolmarks, such as a saw being used to cut wood. The types of crimes where toolmarks can be found include burglaries and car theft. Marks are also created when a firearm is fired due to the action of the firing mechanism.
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These can be found on the cartridge or projectile involved. At the crime scene, we must record the toolmark. A toolmark can be cast using a silicone casting material called Copyrite. This is capable of recording very fine details, is easy and safe to use, sets quickly, and forms a stable and robust object.
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Apart from shotguns, which have a smooth barrel, firearms have rifled barrels. This is a set of spiral groves that run from the chamber to the muzzle in order to grip the projectile, and cause it to spin. This increases the range and accuracy of the projectile. These rifling marks are classified as impressed toolmarks. These rifling marks can indicate a particular type or brand of firearm, but not that a specific firearm was used to fire the projectile. Within these grooves, scratches are formed during the firing action. These scratches are referred to as striations, so are a form of striated toolmarks.
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These striations can also be present on the spent cartridge cases from the interactions between the firing pin and the primer cup, and from the breechface. If the firearm is an automatic, marks can also be caused on the base of the cartridge by the ejector, and on the rim by the extractor. Other marks on the cartridge case wall can also result from chambering, discharge, or extraction. If a tool is recovered during an examination of a crime scene, control toolmarks must be made so that they can be compared to the crime scene toolmark cast in Copyrite. Control toolmarks are made by the forensic scientist using the tool and a sheet of lead.
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Lead is used as it is a soft metal, so the tool will leave an impression in it. The aim is to make representative marks from the tool to establish if it can replicate the appearance of the crime scene toolmark. If so, the control mark will be cast in Copyrite. This is so that when making comparisons the scientist is comparing like with like. That is, the crime scene Copyrite toolmark cast is compared with the control Copyrite toolmark cast. In a firearm case, projectiles and cartridge cases can be recovered from the crime scene. In order to make a comparison the scientist would compare like with like. This is achieved by test firing the recovered weapon using similar cartridges.
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The video provided shows how this is achieved by firing the projectile into a tank of water. The water slows down the speed of the projectile without damaging it, whereas a hard surface could potentially damage the projectile. The test-fired projectile would then be compared with the crime scene projectile and the cartridge cases would also be compared. Regardless of whether you are examining Copyrite casts, projectiles, or cartridge cases, the examinations would be undertaken using a comparison macroscope. The image provided shows a comparison macroscope. You can see in the image that the macroscope has two stages. This is so the crime scene toolmark can be placed on one stage, and the control toolmark can be placed on the other stage and examine simultaneously.
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Each toolmark can be independently manipulated in order to see if there are any points of agreement or a complete disagreement between the toolmarks. In the examination process, the scientist is comparing class and individual characteristics. Class characteristics derive from the manufacture of the tool, firearm, or cartridge. These relate to the design and quality control levels of the manufacturer. The size of the projectile in a cartridge would also be consistent in ammunition production. And a gun manufacturer would want to ensure that the diameter of the barrel was also consistent in every gun of that type that they produce. Individual characteristics, on the other hand, are specific to a particular tool.
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These are produced by damage in the use of the tool, such as wear on the jaws of pliers, when the ejector removes a spent cartridge case, or when the projectile is passing through the barrel of the firearm. As such, these toolmarks are created accidentally though use. Therefore, they can be used to show that a particular tool produced a particular toolmark, or a specific firearm fired a projectile. It is important to note that toolmark examination is subjective. It is dependent on the opinion of the examiner, and it is possible that two examiners could come to different conclusions about the same toolmarks they are examining.
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You should now be able to describe the different types of toolmarks, describe how to undertake a toolmark examination, understand the difference between class and individual characteristics.

Now that we know how firearms work, the next thing to look at are toolmarks and how these are created and examined.

The approach taken in the examination of toolmarks is essentially the same approach taken when examining the marks on bullets or cartridge cases made when they are fired, and which are used when attempting to link them to each other or to a weapon that may have fired them.

The video also discusses two very important concepts which are class characteristics and individual characteristics, and how these are used in the comparison of toolmarks to provide information in a forensic context.

You will also get some information about casting toolmarks using different techniques.

NB: The text and narration refer to something called a macroscope. This is not an error! These are low power mono or stereo magnifiers – typically in the 15 to 40X range – and are very useful in some toolmark examinations. For example, the detail in a tyre tread or footwear impression is often too fine to be clearly seen with the naked eye but too large for microscopy.

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This content is taken from University of Strathclyde online course
Introduction to Forensic Science
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