Forensic Importance of Tool marks. pptx.

Types of Toolmarks, Microscopic appearence, Evidence collection, serial number restoration, etching reagents, restoration procedure

1. FORENSIC IMPORTANCE OF TOOLMARKS PRESENTED BY PRIYANKA GADISETTY MSC FORENSIC SCIENCE

2. DEFINITION Tool marks are impressions produced by a tool on a receptive surface. In general, when any two objects come in contact, the softer of the two will be characteristically marked, i.e., damaged, by the harder. Any instrument which makes our work easy is called a tool. When a tool is used on an object it will leave a mark on the surface. The mark made is characteristic of the instrument used.

3. Importance of tool marks Tools are commonly used to break into a locked building or to open a safe door. Tool mark impressions are also microscopic in nature and are the result of the imperfections found on the cutting surface of a tool. These marks are the "signature" or identifying characteristics of a specific tool. A microscopic comparison of the tool marks left at a crime scene and test impressions produced by the suspect tool helps to identify the tool used to commit the crime

4. PRINCIPLE OF PROGRESSIVE CHANGE “EVERY OBJECT CHANGES WITH THE PASSAGE OF TIME TO DEVELOP INDIVIDUALITY”

5. PRINCIPLE OF PROGRESSIVE CHANGE- Individualizing characteristics can be obtained by the gradual wear and tear of an object. Marks left behind by such objects in crime scene can help to link the criminal with crime scene.

6. Types of Tool marks 1. Impressions marks Impressions are produced by perpendicular force acting against an object. Examples of tools that can make impressions include punches, hammers, and some gripping tools and also footwear impressions. The tip of a screwdriver or chisel that is used to indent a metal surface but does not penetrate the object will leave an impression by which the tool can be identified. Other tools like crowbars and even larger objects like car bumpers or bull bars may leave identifying marks and impressions .

7. Compression marks Compression marks are produced when a tool strikes the surface with a pressure.  These are produced by a single action of a tool in an area of contact e.g. impression produced by single action of a hammer. Such marks reproduce the outline of the edge of the instrument and reveal its approximate dimension. Individualization is possible by the presence of imperfections on the surface. Repetitive type marks are produced when a tool like a hammer strikes the surface a number of times

8. Striated marks Striated marks are produced when a tool scrapes the surface. Scrape marks are made by moving the tool laterally across the object. They include series of scratches consisting of grooves and ridges produced by defects in the edge of the instrument. Examples of such tools include flat bladed tools such as crowbars, pry bars and screwdrivers Striation marks of a screw driver

9. Microscopic appearance of striated marks

10. Shearing or pinching marks Shearing or pinching marks occur when the object is caught between opposing forces of two cutting actions. Scissors and tin snips cause shearing and wire cutting pliers cause pinching.

11. Information Determined from tool marks Generally two types of information can be obtained from tool marks. They are Class characteristics and Individual characteristics. The former are features produced by similar type tools. Individual characteristics give uniqueness. The manufacturing processes used to make the tools create unique imperfections on the tool surfaces and that uniqueness is reflected in the tool marks created when the tools are used. As the instrument ages, it receives additional damage which alters the contour of the edge and increases its uniqueness. Since the edge is unique the marks produced is also unique.

12. Extrusion marks Very often plastics bags are purchased as packets in retail outlets and collected from a consented plastic manufacturing factory produced by known machine. Sometimes, extrusion marks are found on most plastic bags.  The pattern of these extrusion lines are unique They are produced in transparent plastic bags during manufacturing process. These marks can link a bag back to a roll or box of bags or two torn pieces of plastic bag to each other.

13. Evidence collection If possible, collect the original article bearing questioned tool mark. If an object bearing a tool mark is too large to be easily collected, it must be photographed first and then cast. A silicon rubber cast is taken and the cast is submitted for comparison instead of the object that contained the tool mark. Detailed photographs, parallel to the tool mark with an “L” scale, should be taken prior to casting. The cast should be photographed before and after removing it from the tool mark surface. Dental casting material also gives fine details of tool marks.  Avoid any damage to the tool edge while sending. The surface area bearing the tool mark also be well protected

14. Packing and forwarding Do not place the tool and tool mark together as it may compromise the integrity of the tool and/or tool mark; additionally trace materials could be transferred by this process. Never change the position of a tool's working edges such as on a slip jawed pliers, while packing. Do not package a suspect tool and an object possessing a tool mark together, they must be in individual containers. Items that have blood or other biological fluid must be air dried prior to packaging and labeled appropriately with a BIOHAZARD label

15. Comparison of tool marks The tool mark is negative replica of the tool and mould is the negative replica of the tool mark. A tool mark must be compared with the tool mark and mould with mould, and photograph with photograph having same magnification. Test impression is made with the suspect tool and characteristics of the test impression are compared with those of the impression found at the scene of crime, which helps to identify the tool. The comparison is best done by using comparison microscope and with oblique illumination

16. Extractor and ejector marks Used cartridge casings are first removed from the chamber by extractor mechanisms. The casings are then dragged across the edge of the ejection port as they are ejected from the weapon. These extractor and ejector marks can be matched.

17. Bullet Comparisons

18. SERIAL NUMBER RESTORATION Serial numbers or monograms are inscribed on manufactured goods including firearms , vehicles and other machines to uniquely identify them. when these goods are stolen, in order to defeat this identification, criminals erase the incriminating mark by filing, grinding, or peening.  Restoration of obliterated serial numbers is possible through chemical etching HCl-CuCl 2

19. TYPES OF MARKS 1. Cast marks These are marks raised above the surface. They form the part of the mould used in casting. If the cast marks are removed it is impossible to restore them. 2. Engraved marks These marks are made by removing the metal with engraving tool either by making dot like impressions or by continuous impressions. Here also restoration is not possible. 3 . Punched Marks Majority of serial numbers are punched by stamping the numbers in to the metal by striking a die bearing the number forcibly into the metal with a hammer. Erased punch marks can be restored provided the filing has not been done too deep.

20. Principle of Restoration When the numbers are stamped the crystalline structure of the metal is deformed and compacted below the indentation and puts them under strain. Restoration is possible by releasing the tension in the metallic surface. The strained and unstrained metal reacts differently with etching solution.  This unequal rate of reaction allows the number to be recovered even if it has been obliterated and is no longer visible to the naked eye. 

21. Etching The most common method to restore serial numbers is chemical etching.  This method uses the oxidation properties of chemicals to restore the obliterated number. Different chemicals are utilized depending on the metallic properties of the obliterated metal, such as an aluminium alloy, stainless steel etc.

22. Etching reagents 1 . Fry’s reagent for Steel surfaces (e.g. chassis and engine of cars, guns, gas cylinders etc.) The etching reagent consists of two solutions. Solution 1 (Fry’s reagent)        Crystalline cupric chloride                   90gms.    Concentrated hydrochloric acid          120ml.        Distilled Water                                    100ml Solution 2 15 % Nitric acid Procedure Apply the Solution 1 by swabbing the surface for approximately 2-3 minutes with cotton wool dipped in the reagent.  Look for any digits revealed and record these.  Clean the surface with acetone   Examine again.

23. Restoration procedure (1) Cleaning Clean the surface using preferably benzene or acetone to free it from grease or paint. (2)    Polishing   Hand polish the area to a smooth, mirror like finish with emery cloth, or other fine abrasive. Examine the surface during polishing, as digits are sometimes revealed during the process.  (3)     Clean again the area with a solvent, such as acetone or other grease solvent. (4) Apply etching reagent

24. DECIPHERING OF ERASED NUMBERS

25. 2. Aluminium Alloys E.g.- Engine surface of the motor bikes, auto-rickshaw engine, vehicle identification plates etc.   The best reagent is Villela’s solution consisting the following:                    Glycerin                30 ml.                    Hydrofluoric acid   20ml.                    Nitric acid             10 ml.  Apply the reagent as a swab until the number appears.  Use the reagent with caution as it attacks the fingers vigorously.  Time 5-15 minutes.

26. Welding If no erasure is noticed, remove the paint over a wide area around the surface to check whether the portion carrying the chassis number was removed by cutting and substitute for it by welding or pasting a metal plate with a new number   .  Remove the pasted plate if any, after recording the original appearance.  Remove the welded plate too.  Examine the lower surface for any obliterated marks.  Examine the surface by oblique illumination to see the erased number .