TYPE OF TOOL MARKS –
1) COMPRESSION MARKS
2) STRIATED MARKS
3) COMBINATION OF COMPRESSION AND STRIATED MARKS
4) REPEATED MARKS
Compression type tool marks are produced when a tool strikes the surface with a pressure. Striated type tool marks are produced when a tool scrapes the surface. Repetitive type marks are produced when a tool strikes the surface a number of times.
The features of the tool marks can be divided into three parts as class characteristics, sub class characteristics and individual characteristics.
1. Class Characteristics: These are the features produced by similar type tools.
2. Sub class characteristics: These are the features produced by tools of one type manufactured on the same machine.
3. Individual characteristics: Some individual characteristics are developed by a tool while in use due to wear and tear. No two tools can produce identical random individual marks.
Examination of tool mark
Whenever a tool mark is found at the scene of crime the following procedure should be adopted for its collections and examination.
1 Photography: Tool marks should be photographed. At least two photographs should be taken, one showing the background with the impression and another a close up of the impression. Scale should always be kept at the same level of the tool marks while taking the photograph. Camera should be kept perpendicular to the surface of the tool marks. More details are obtained by using oblique illumination.
2 Tracing: Tool marks can be traced on a tracing paper. This helps in comparing the class characteristics.
3 Lifting of tool marks: A cast or mould is simply a reverse or negative three-dimensional image of an impression. The following materials are generally used for making a mould of a tool mark.
Plastic/rubber: This is a material for lifting fine details of a tool mark. A lump of material is softened by pressing in fingers and then applied on the surface. It is pressed carefully as it is likely to be disturbed by out side pressure.
Dental casting material: This also gives fine details of tool marks.
4 Plaster of Paris: This material is used when the impression is of large size. Plaster of Paris is partly hydrated calcium sulphate (CaSO4)2 1/2 H2O. First on the surface of tool marks talcum powder is sprayed. This helps in removing the cast without disturbance. Then solution of plaster of Paris is put on the surface. This is allowed to dry and the cast removed carefully. For preparing plaster of Paris solution, water is taken in a container and plaster of Paris added until water does not absorb any more plaster of Paris. 7 parts of plaster of Paris are generally put in 4 parts of water. To increase rigidity of surface details a thin layer of shellac dissolved in alcohol is sprayed on the surface by means of sprayer of the type used for spraying insecticides. The shellac is carefully sprayed from a distance of several feet so that air pressure does not disturb the details. The plaster is then poured in. When the caste has hardened the shellac is peeled off. The talcum powder permits the peeling off the shellac without affecting the cast.
5 Metal casting: The mould of tool mark can also be made by metal casting. The suitable metal for this purpose is known as woods metal. This is a metal of low melting point (71oC), which is suitable for casting tool marks. Its composition by weight is Bismuth 50%, Lead 25%, Tin 12.5% and Cadmium 12.5%.
6 Cellulose acetate: Tool marks from stone, concrete, wood, etc. can be lifted by cellulose acetate dissolved in acetone. A layer of 1/16 inch is made. However the cellulose acetate cast should be photographed, with scale, immediately otherwise the layer is likely to shrink.
Examination of foreign material/materials on Tool
The suspected tool is examined for presence of all traces of metal or wood and foreign particles sticking to the tool/blade using the stereo microscope. The physical and chemical characteristics of these particles should be compared with the material of the surface on which the tool mark was made.
Taking test impression
The material on which the test impression is to be made must be chosen carefully. It must be soft enough so that the tool edge will not be altered. When the original mark is in wood, paint or soft metal, the standard should be made with a similar material. Several test impressions should be made at various angles with the tool.
When the original mark is on hard metal such as steel or brass, it is desirable to use the same material for the standard. Sheet lead, tin or aluminium may be employed successfully.
Comparison of tool marks
The tool mark is negative replica of the tool and mould is negative replica of the tool mark. A tool mark must be compared with tool mark, mould with mould and photograph with photograph having same magnification. The comparison is best done by using comparison microscope and with oblique illumination. Magnification of test and crime sample should be identical.
1- To determine whether a lock is forced open or not
The key manufactured for a specific lock always plays through a prescribed path inside the lever mechanism of the lock with out causing any scratch on the surface of the levers. On the other hand, if a duplicate key, foreign tool, etc, are used to open a lock, then several fresh scratches can be observed on the levers and internal surfaces of the questioned lock. If the lock was opened by applying heavy mechanical thrust, tool marks can be found on the body of the lock.
The lock is examined under a stereo microscope to locate the point of disturbance and tool marks on the body of the lock Then the questioned lock is cut open in such a way so that the lever mechanism remain intact. The lever mechanism is to be observed under a stereomicroscope in oblique illumination for any tool marks or other disturbances. Usually, the recent marks and scratches have bright metallic lustre. If the surface of the lock bears any tool marks or deformation caused due to hammering with tools, then the entire defect and abnormality are to be matched with respect to its original position of locking
2- Tool marks in the case of cutting of electric wires/cables
Crimes involving the use of wires are not infrequent. The clipping of cables and wires is found in wire and transformer theft case sabotage cases etc. The cut wire is used as evidence material. If the wire cutters are also seized, the tests impression/exemplars are produced and compared with those on the evidence cut wires.
3- Tip of the broken tool
If a broken piece of tool tip is found embedded in the surface, the physical matching of the broken part with the suspected tool gives a definite proof of the tool used in the crime. The broken part may also be found fallen on the ground at the scene of crime.
4- Staple marks
Use of staple is increasing day-by-day due to its low cost and convenience. Two types of problems are encountered in the staple cases. One is source identification and second is to link the stapler with the staple. For source identification the marks of manufacturing of the staple can be compared under microscope. Material of the staple can be compared with elemental analysis techniques. To link the stapler with the marks on the pin, the test marks are made with the questioned stapler and compared with the marks present on the crime pin, by conventional tool marks comparison techniques.
Methods Used to Obliterate Identification Marks
1 Filing or grinding- The original number would have been filed away or ground with a power grinder followed by polishing and then over stamping with a new number.
2 Peening- This involves hammering the surface with a round punch to hide the number.
3 Over stamping- Here a new number is simply stamped over the old. For numbers with curved surfaces i.e. 2,3,5,6,9 and 0, the stamp 8 is the one most often chosen. For numbers with straight surfaces i.e. 1 and 7, the stamp 4 is the obvious choice. Serial numbers with a preponderance of ‘8’ or ‘4’ numbers should be treated with suspicion.
4 Centre punching- The surface bearing number is obliterated with a pointed punch.
5 Substitution- Substitution of an iron plate with a new number over the original surface by pasting or welding.
6 Drilling- It removes the number and the surrounding metal with a drill. The cavity is usually filled up with either lead solder or welding material.
7 Welding- Heating the surface with either an oxy-acetylene welder or an arc -welder until the metal flows.
8 Occasionally an original finish would be given to a previous obliterated number surface
Chemical etching method for restoration
The chemical etching method is the simplest and most effective method for restoring obliterated numbers. It is simple to apply and it requires no expensive equipment. It works well on any size or type of object.
The techniques involved require considerable skill and great patience. The materials used are potentially dangerous and should be used with full awareness of health and safety requirements. They should only be used in areas specially set aside for this purpose. Rubber gloves will protect the hands from corrosive acid and when using volatile liquids work in the open air away from any seat of naked flame, if laboratory conditions are not available.
In case of motor vehicles remove the engine from the vehicle if necessary for ease of access to the engine number surface, and for better quality of photographs of the restoration process.
1 Preliminary Examination
Examine the metal surface after cleaning oil and dirt away, using acetone. Examine the surface with a hand magnifier and see whether any erasure has taken place at all. Look for any disturbance in the pattern at the background. This pattern will be present either in the form of milled marks caused by grinding the surface before stamping the serial numbers, or cast marks produced during the manufacturing process. If it is disturbed, suspect erasure. Even 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 number4 . Remove the pasted plate if any, from the original surface by using trifluoroethanol, after recording the original appearance. Remove the welded plate too. Examine the lower surface for any obliterated marks. Note how the mark has been erased and whether it has been repaired after erasure. See if any digits or parts of digits are visible. Note these down. Examine the surface by carefully adjusted illumination preferably oblique lighting to see the erased number.
For photography, use a single light for striking the surface bearing the serial number at a low angle. Take several exposures, the light being moved in each instant to strike the surface from a different position. Use process film. This method assists to bringing out the faint serial numbers.
Identify the type of metal from which the object is made. This is necessary to choose the appropriate chemical etchants.
2 Preparation of surface
(i) Clean the surface using preferably benzene or acetone to free it from grease or paint. Solvents such as gasoline, commercial paint remover or 50/50 mixture of acetone and chloroform may also be used. To assist the solvent a soft toothbrush should be used to dislodge deposits from the stamped surface.
(ii) Hand polish the area to a smooth, mirror like finish or a reasonably smooth surface with emery cloth, or other fine abrasive. Emery paper with coarse grade is used by first removing all scratches, and other gross marks with fine emery. Do not remove more metal than is necessary. Leave deep scratches. Examine the surface during polishing, as digits are sometimes revealed during the process.
(iii) Clean again the area with a solvent, such as acetone or other grease solvent. Do not touch the area with bare fingers because fingerprints can interfere with the reaction.
(iv) Heating- Treat the area with a blow lamp or Bunsen burner. Care should be taken not to over heat. If the metal is heated to red hot, the temperature is sufficiently high to soften the metal and, on cooling, the metal becomes homogeneous and can no longer be differentiated. A useful guide is to heat the metal surface until it is just too hot to touch. The metal should be allowed to cool before etching begins 1, 2.
(v) Photograph the entire item, and record details of the obliterated area by close-up photography. Use 35 mm camera with a macro lens and extension and a fast film, 400, ISO.
Etching procedures on different surfaces
1 Steel surfaces (chassis and engine of cars, guns, gas cylinders etc.)
(i) The etching reagents are in two solutions.
Solution 1 (Fry’s reagent)
Crystalline cupric chloride 90gms.
Concentrated hydrochloric acid 120ml.
15 per cent Nitric acid.
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 (not water). Examine again.
Apply the Solution 2 by swabbing the surface for approximately 1 minute with cotton wool dipped in this solution. Look for any digits revealed and record these.
Continue etching alternatively with Fry’s and 15% nitric acid etching reagents until the complete number is visible. This may take up to 2 or 3 hours. A consistent check should be made on the appearance of the mark since faint marks appear and then disappear again before the mark is completely etched.
(ii) Alternate Solution
Hydrochloric acid 80ml.
Water 60 ml.
Copper Chloride 12.9 gms.
Alcohol 50 ml.
15 per cent Nitric acid.
The method is similar to the previous one. Apply with a swab a little of the acid copper chloride solution and continue to apply for 60 seconds. Dry with cotton wool and then apply the nitric acid for a similar period.
This alternate swabbing with solutions (1) and (2) is continued until the number is developed. Time, 5 minutes to as long as 1 hour.
(iii) Ferric chloride etching reagent can also be successfully used on steel surface.
1. If etching results in areas of dark contrast surrounded by metal with normally etched appearance, this indicates that the mark has been “torched” with a welding torch, and further efforts to restore the mark are a waste of time.
2. When nitric acid reagent is applied after the Fry’s reagent, copper is sometimes deposited on the surface. This does not matter. It can usually be removed by rubbing vigorously with the nitric acid swab, or alternatively, it will disappear when Fry’s reagent is applied again.
2 Copper, Brass, German Silver and other Copper Alloys
These metals react to:
Ferric Chloride 19 gms.
Hydrochloric acid 6 ml.
Water 100 ml.
Apply the reagent as a swab until the number appears. When the reagent is slow to develop the erased mark, form a wall of plasticine around the erased mark. Use the solution as a bath. Let the reagent remain on the metal for 24 hours. Some workers have found that 20 gms. Ammonium persulphate made up to 100cc., with water gives the best result for brass and copper.
3 Stainless Steel
Swab either with dilute sulphuric acid or 10 per cent solution of hydrochloric acid in alcohol.
4 Lead (Motor car batteries etc.)
Glacial acetic acid 3 parts
This solution has yielded excellent results. Time 10-30 minutes. When the erased number ppears, clean the metal in concentrated nitric acid.
5 Zinc Alloys
(i) Sodium hydroxide 10 per cent solution in water.
Development is slow. So, use the plasticine bath method. Time 10-16 hours.
(ii) Alternate solution
Chromic acid 20gms.
Na2 SO4 1.5 gms
Water (or) 100 ml.
Nitric acid 25%
6 Cast Iron and Cast Steel
Apply constantly a 10 per cent solution of sulphuric acid plus potassium dichromate. Action will be slow. Apply constantly the reagent. It may even be necessary to build a wall of plasticine around the number and fill the hollow with the reagent. Remove the solution at intervals, examine the surface, replace fresh solution. Over development results in the obliteration of the restoration.
7 Aluminium Alloys (Engine surface of the motor bikes, auto-rickshaw engine, vehicle identification plates etc.)
(i) The best reagent is Villela’s solution.
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.
(ii) An alternative and safer formula is Hume Rothery solution and is made up as follows:
Copper chloride 200 gms.
Hydrochloric acid 5 ml.
Water 1000 ml.
As soon as this solution is applied a copper deposit is formed. Remove this copper deposit by alternative swabbing with Hume Rothery reagent and distilled water, until the number appears. Time 5 minutes to 1 Hour.
(iii) FBI Laboratory recommends the following for etching aluminium surfaces
Apply the ferric chloride reagent to the surface to be etched. Several hours of application may be necessary to reveal the erased mark. Various methods of application can be used to avoid constant swabbing for hours, such as:
Soak a large piece of cotton wool in the reagent and it will stick to the metal surface by surface tension. The cotton wool must be removed and resoaked in the reagent at 20 minutes intervals, and the effect closely noted. If the surface to be etched can be made to be horizontal, a “puddle” of reagent can be left over the area, and replaced when it has lost its colour. Do not, however, build up a “plasticine” bath around the area as the etching reagent attacks plasticine.
Inspect the surface periodically. When the number is visible, clean the surface with acetone and dry it.
Hydrochloric acid 10 per cent solution.
Alternate swabbing and washing. Time 10-20 minutes.
(i) German Silver
Ferric chloride 25%
Hydrochloric acid(conc.) 25%
Swab the area. Time 10-20 minutes.
Concentrated nitric acid
Immerse the specimen for few seconds. After each dipping wash with running water.
9 Gold and Platinum
Nitric acid 1pt.
Hydrochloric acid 5pts.
Distilled water 6 pts.
Carry out etching at 15oC. Time 1 hour or longer.
10 Wood (punched marks)
Many wooden articles are stamped with identifying markings which may be erased by cutting, filing or the use of abrasive. Provided the area beneath the exposure has been disturbed by the original punching, legible results may be obtained. Play a jet of steam onto the erased surface. The steam softens the wood and causes the fibres which were bent during punching the number to spring back and the broken fibres to swell. Where the mark has been, the wood now projects above the surface and the mark can be deciphered. Pretreat with a solution of caustic soda or potash for hard woods such as are employed for tool handle. This softens the wood and accelerates the treatment.
(i) Try all the methods of development given below, as the treatment of leather is empirical. Clean the surface with cotton-wool swab soaked in chloroform, and examine under ultraviolet light. This works admirably when the embossed letters have been filled with an oil paint. The oils are brought to the surface by the chloroform and outline the letters as a bright fluorescence.
(ii) Swab the surface with cotton wool soaked in 2N caustic soda solutions. The loose disturbed surface absorbs the solution, softens and darkens. The compressed leather, which has been embossed, does not absorb the solution so quickly. This differential absorption enables the erased mark to be deciphered.
(iii) When the carbon ink has been used on the embossed number and the same has been soaked into the surface.
Clean the surface as above and take an infrared photograph. When the ink has been a carbon ink and has soaked into the surface, cleaning in this way gives a surface which photographs as white under infra-red while the traces of carbon ink are photographed intensely black.
Some rubber articles are stamped, and when stamping is erased the suspect area can be treated by lightly swabbing with petrol or even on occasions carbon-disulphide. This latter solution acts upon rubber very rapidly and should be used with caution.
(i) It is not possible to make use of alteration of structure in the case of polymers. These materials have no organized macro structure and the die does not, therefore, produce alterations of structure capable of being treated with reagents. Numbers or other markings in a polymeric material are presently produced using a heated stamp. Laser drilling is also used to produce an identification mark.
Polish the surface as before and then flood the surface with a deeply colored solution such as a dye solution. When excess solution is removed, the dye soaked into the crevices of the mark is plainly visible and the mark can be deciphered.
(ii) Conduct experiments using swelling techniques with various solvents, relief polishing and heat-treatment. Follow the table below.
Polyethylene (high density) (PE-HD)
Heat (100oC, 5 min.), swelling with ethanol, light petroleum (40-60)
Styrene acrylonitrile (SAN)
Polishing heat (140oC, 10 min.), swelling with light petroleum (40-60)
Acrylonitrile butadiene styrene (ABS)
Polishing heat (100oC, 10 min.), swelling with light petroleum (40-60)
Polyamide (PA 6)
Polishing heat (150oC, 5 min.), swelling with light petroleum (40-60)
Polishing heat (150oC, 30 min.), swelling with light petroleum (80 oC)
Polybutylene terephthalate (PBTP)
Polishing heat (200 oC, 10 minutes)
Swelling with ethanol, ethyl ether, heat (120 oC, 5 min.)
Epoxy resin (EP)
Swelling with acetone, formic acid, ethyl acetate, light petroleum (40-60)
Visualization and photography of the restored marks
Restored marks are to be distinct enough to be visualised and photographed, as far as possible. Better visualisation and photography can often be obtained if the etched surface is moistened slightly with etchants. There should be sufficient lighting to view the restored number because of the difficulty to view properly the number especially on the engine and the chassis of the motor vehicle. Portable beam lighting can be positioned to produce low angle illumination across the surface area for viewing.
However, for photography a powerful light source adjusted suitably for catching their images on the camera screen should be available. Flexible powerful fibre optics illuminators are excellently suited for this purpose. A 35 mm camera screwed on a tripod for time exposure will be very useful. A macro lens with extension rings would enable the camera to keep a close distance from the number area and ensure a magnified image on the film. Use a fast film 400 ISO. A photograph with a contrast film (100 ISO) should also be made. Process the negatives in the usual manner. Print on a normal grade paper for normal contrast.
Methods for restoration of obliterated engraved marks
Identification of articles of plate and jewellery, and common articles like stainless steel and tool steels by restoring the obliterated engraving identification marks.
2 Engraved Marks
Engraved marks are made on plate and jewellery using an engraving tool and engraved marks on hard metals like stainless steel and tool steels are produced by an electric engraver. All engraved marks are made by removing the metal with the tool. The underlying metal is not seriously disturbed, and no routine guaranteed method of restoration is possible.
Methods of Restoration
(a) Use (HCL) etching methods for restoring engravings on plated articles. Use acidic ferric chloride on chromium plated on brass plate. Use Fry’s reagent for chromium-plated on mild steel plate. Choose an etching solution which preferentially etches the two metals.
(b) In case of electrical engraving, the instrument heats the metal at the point of engraving, and partially fuses it. If the number has been filed out, polish the surface carefully. The remains of the fused areas reflect light slightly different from the rest of the metal. Therefore, throw a beam of light onto the polished surface and view through the correct angle. The erased number will be seen as a dark shadow on the bright background
1 Preservation of the restored punched and engraved marks
1. If a mark is successfully restored, it is important that the number is recorded and, as far as possible, preserved. In an attempt to preserve the mark, the area should be washed with dilute ammonia solution to neutralize the acid (if the surface is acid-etched), cleaned thoroughly with acetone and dried. Clear lacquer should then be thickly sprayed over the mark.
2. It is also important to clean up around the mark. Etching reagents are often splashed or dropped over surrounding parts of the article being examined, and if not cleaned off, corrosion can result.
3. Remember that different parts of marks will appear and disappear as restoration proceeds. A series of records may be required.
Methods for restoration of obliterated painted numbers and other marks
Identification of motor vehicle, registration number plate etc., by restoring the obliterated original painted number, and sticker numbers.
Methods of obliteration
Some number plates consisted of letters and figures painted onto the metal plate. Obliteration normally consisted in painting over the surface with black or other paint and inserting the false numbers. In a similar manner some floral and ornamental designs, registration numbers and some other identifying features all originally made on the painted motor vehicle surface are obliterated with new designs and numbers.
Methods of restoration
1- Take photographs the suspected obliterated surfaces before treating it in any manner. Examine the number plate and the vehicle surface at different illuminations, especially the oblique one. The obliterated marks may sometimes be visible.
2- Wash away the fresh paint and the fresh number with chloroform or dioxan. Remember that this action has its dangers because the underlying paint and the marks tend to wash away along with the top layer. Try suitable solvents mixtures of solvents at some other place and thus choose the suitable solvent or mixture of solvents for application in the obliterated surface
3- Apply the chosen solvents in cotton swabs over the paint surface suspected of being obliterated. Wait till the top layer just swells. Remove this layer carefully by gentle rubbing with cotton. Look for the obliterated marks and record them by photography and detailed notes. Take care that the underlayer is not washed away and removed along with top layer. Use the above procedure to discover more than one obliterated marks underneath
4- X-ray shadowgraph- If the article is portable try this method. Best results are possible when the underlying layers are painted with a paint containing heavy elements, such as lead, while the top paint is free from such elements. With the correct exposure the outlines of obliterated marks can be deciphered.
5- In the case of number plates where original sticker number is removed or replaced, try specular reflection. Throw an even light on the plate from an angle and photograph the plate from above. The brightly polished areas, which have been protected by the lettering, reflect the light specularly away from the camera. The weathered areas diffuse the light to some extent and some of this diffused light enters into the camera. As a result, the lettering appears black on a light background
6- Expose the plate to ultraviolet radiations. If the area fluoresces in ultraviolet light record the appearance by photography. Photograph following the techniques of UV fluorescence photography.
7- A photographic records of the restored marks should be made.
Laboratory Procedure Manual- Forensic Physics: Directorate of Forensic Science, MHA, Govt. of India
Dr. Sally Lukose: Class Notes.