The most discussed tool mark is the serial number that in stamped on any metal or on any object which is produced when the serial number was stamped by the machine into the metal, which is the areas that is below the stamped letter but the bonds of metal are weakened.
During the criminal activity, the attempt of removal of serial numbers was done by fining technique wing gun or grinding off the serial numbers. Due to these techniques the number get disappeared but the thief has no idea that strained metal that is still present below the serial numbers is like the retention of numbers. If any solution that is discussed in the later section dissolves the metal and then swabbed on the surface that is under investigation, the number will reappear for the temporarily basis from the area where the serial number was present at that time. The camera should be there at the time of swabbing process through which the recording of serial numbers should not be lost.
Some valuable items have registered serial numbers, so the manufacturers of these items like firearms, watches, art objects, automobiles etc. and their serial number on the products were known for the means of identification or a relationship may be established. There are some inscriptions on the expensive silver or stainless items that is made by the individual who possessed that particular item. While doing any criminal activity, criminal might try to alter or destroy the identification marks without leaving any evidence behind. The restoration of obliterated marks mainly performed at laboratories.
When any item recovered from the crime scene was located so it becomes necessary for an investigator to determine its origin. The physical characteristics of weapon give an ideal of the make and model and about its history like when and whom the weapon was transferred, the ownership which can only be obtained by knowing the serial number.
The markings which are obliterated were identifiable and are restorable for example the markings that got obliterated by the melting process of metal in, welding, puddling. Other than metal they can also be restored on other materials such as wood, fiberglass and plastic. There are some specific methods for different metals and alloys for the purpose of restoration of obliterated marking.
Restoration Principle
A body of metal or frame or on a plane is used on which the serial numbers are usually stamped with dies of hand steel that forcibly strikes the metal surface which usually results in microcrystalline structure of the metal produces compressed and strained surfaces. A short distance was extended by a strained surface below the intended number. After the application of chemical reagent, at different rates the strained and unstrained areas will dissolve the metal.
Metals that are used having polycrystalline structure which are originally comprises of crystal grains of irregular shape and forms when the metal in the molten state cools to the solidification point. At the time of manufacture, the metal cooling controls the grain or crystal size which affects the mechanical properties of the metal. The crystalline structure that surrounds the stamped number was distorted and the structure of grain or crystal is compressed which sometimes diminish the grain or crystal size of any metal through which its physical and mechanical characteristics were altered.
This altered crystalline structure resulted altered features such as hardness, strength, electrical, magnetic and chemical properties. In case of crystalline structure that is compressed, the depth of the metal depends upon the metal and the force that is applied while punching. So by applying these methods the original serial numbers may be developed again and made visible.
The area of crystalline structure that is altered con discovered by the application of an etching reagent that is suitable through which a change in structure of crystal from a compressed area to non-compressed area can be determined. The crystalline structure behaves differently from the areas which are not compressed after the application of magnetic field and these variations can become visible using finely grounded iron dust.
Methods of Obliteration
The serial numbers were obliterated in case of stolen weapons for in an attempt to hide the origin of weapon. The following categories were discussed about the methods of obliteration.
• Filing or grinding This process is the number was removed by the technique filing or grinding using carborundum grinding wheel of high speed. Before performing this technique, the surface was polished and after that over-stamping with a new number.
• Over stamping- In this method a new number was stamped over old one. The numbers having curved surface such as 2, 3, 5, 6, 9 and 0, the most often chosen stamp is 8 whereas 4 stamp is chosen for the numbers having straight surfaces like 1 and 7. Serail numbers having dominance of numbers 8 and 4 should most often treated with suspicion.
• Drilling – The number and the surrounding metal was completely removed and space ie hole left is mainly fed up with weld material or lead solder
• Centre punching – The entire numbered surface was obliterated with a pointed punch.
• Welding – In this process the surface was heated until the flow of metal occurs with either a welder of oxy-acetylene or an are welder.
Peening-The surface was basically hammered with a round punch for the purpose the hiding the number
• Non recoverable methods for removal of numbers-For the removal of punched serial numbers there are some methods which are used like drilling and welding will help in removal of all traces of original numbers that can be recovered permanently. The crystalline structure that is altered and the traces that is recoverable can be removed with the help of drilling. There are many factors on which the Centre punching depends like the area of the serial number covered and while punching the applied force and these factors makes interpretation difficult to restore any number.
The basic drawback behind the restoration of Centre punching is that this type of punching alter the crystalline structure and restoration that is done will restore the Centre punching as well the serial numbers. In case when an oxy-acetylene torch or are welder was applied which was performed until the flow of metal will remove all the traces of the crystalline structure. After this the metal is heated and re melted which allows the crystalline structure to regain into a new form or constant pattern on cooling Thus, at last the crystalline pattern that is altered have been completely lost with the original crystalline structure.
Methods of Restoration
This altered crystalline structure resulted altered features such as hardness, strength, electrical, magnetic and chemical properties. In case of crystalline structure that is compressed, the depth of the metal depends upon the metal and the force that is applied while punching. So by applying these methods the original serial numbers may be developed again and made visible.
The area of crystalline structure that is altered con discovered by the application of an etching reagent that is suitable through which a change in structure of crystal from a compressed area to non-compressed area can be determined. The crystalline structure behaves differently from the areas which are not compressed after the application of magnetic field and these variations can become visible using finely grounded iron dust.
Methods of Obliteration
The serial numbers were obliterated in case of stolen weapons for in an attempt to hide the origin of weapon. The following categories were discussed about the methods of obliteration.
• Filing or grinding This process is the number was removed by the technique filing or grinding using carborundum grinding wheel of high speed. Before performing this technique, the surface was polished and after that over-stamping with a new number.
• Over stamping- In this method a new number was stamped over old one. The numbers having curved surface such as 2, 3, 5, 6, 9 and 0, the most often chosen stamp is 8 whereas 4 stamp is chosen for the numbers having straight surfaces like 1 and 7. Serail numbers having dominance of numbers 8 and 4 should most often treated with suspicion.
• Drilling – The number and the surrounding metal was completely removed and space ie hole left is mainly fed up with weld material or lead solder
• Centre punching – The entire numbered surface was obliterated with a pointed punch.
• Welding – In this process the surface was heated until the flow of metal occurs with either a welder of oxy-acetylene or an are welder.
Peening-The surface was basically hammered with a round punch for the purpose the hiding the number
• Non recoverable methods for removal of numbers-For the removal of punched serial numbers there are some methods which are used like drilling and welding will help in removal of all traces of original numbers that can be recovered permanently. The crystalline structure that is altered and the traces that is recoverable can be removed with the help of drilling. There are many factors on which the Centre punching depends like the area of the serial number covered and while punching the applied force and these factors makes interpretation difficult to restore any number.
The basic drawback behind the restoration of Centre punching is that this type of punching alter the crystalline structure and restoration that is done will restore the Centre punching as well the serial numbers. In case when an oxy-acetylene torch or are welder was applied which was performed until the flow of metal will remove all the traces of the crystalline structure. After this the metal is heated and re melted which allows the crystalline structure to regain into a new form or constant pattern on cooling Thus, at last the crystalline pattern that is altered have been completely lost with the original crystalline structure.
Methods of Restoration
The surface is first polished which is the preliminary process which is accomplished by initially removing all surface markings, scratches, grow marks. The surface was degreased and cleaned using chloroform or acetone after the polishing of the surface. The application of all these chemical techniques is same for all metals but the thing that differs mostly is chemical formula that varies from metal to metal. Heat was applied to the surface till it got so hot t touch and then cool down
Chemical Techniques
With the help of cotton ball that is attached to glass rod or some type of stick, reagent was applied to the surface. Then after some time approximately 1 minute, rinsing of reagent was performed using acetone and then viewed. The number that is read will started appearing as a shadow when viewing through an oblique light. If in case after 1 minute nothing will happen then this process was repeated for 2. 5. then 10 minutes consecutively for 2 hours.
The selection of etching reagent depends upon the type of metal surface used,
1. Tests for Iron and Steel
• Fry’s reagent – 90g Cupric Chloride, 120 mil Hydrochloric acid and 100ml water. The solution was applied as described above that particular area where solution was applied is then washed with acetone and then adding 15% solution of Nitric acid. Till the development of number the process was repeated.
• Cast Iron and Cast steel-On the surface, solution of potassium dichromate 10% solution of Sulphuric acid was applied
• Wrought, drawn and forged iron and steel-For the examination of these items two reagents were used,
Reagent 1-Alcohol -50ml, Conc. Hydrochloric acid-80ml, Cupric Chloride 129 g. Water-60ml
Reagent 2-15% Nitric acid The reagent was poured in the cotton swab and cotton soaks the reagent (1), after applying on the surface of metal wait for some time. Then the reagent (2) was applied on the metal surface but prior applying (2) clean the previous one from the surface. The continuation of cycle goes on until the development of number.
• Solution of Sg Ferric Chloride, 5oml Hydrochloric acid and 100ml water is prepared
• Solution of ethyl alcohol is prepared with saturated solution of picric acid
2. Tests for Aluminum and alloys of Aluminum
- Villela’s reagent- This reagent was considered as the best reagent for the restoration techniques of Aluminum surface. Glycerine-30ml, Hydrofluoric acid-20ml, Nitric acid-10ml
- The Hume-Rothery reagent – This is considered safer but are not much efficient as Villela’s reagent. Cupric Chloride-200g, Hydrochloric acid-5ml, Water-1000ml. Because of the faster reaction cure has to be taken while using Fry’s reagent in aluminum.
- Zinc and alloys of Zinc/aluminum- Solution of 50% Nitric acid and 50% of Hydrochloric acid
- Copper, Brass and other alloys of Copper- Solution of Cupric Chloride-1.40g, hydrochloric acid-150ml and water. Solution of 19g of Ferric Chloride, 6ml of Hydrochloric acid and water (100ml) .
Electrolyte Methods
In an electrochemical cell, the specimen was made anode is a process called Electro polishing. Minute quantities of irregularities was dissolved when current is applied which leaves a surface that is highly polished. Due to the compressed structure of crystal, the electrical properties in the stamped area are altered carefully and the compressed areas will be different from non compressed areas on the basis of dissolution.
This has been proved to be an effective method for the process of serial number restoration that was first developed 25 years ago. This method includes both anode and cathode, the specimen was made at the anode and electrolytic solution in a swab of cotton becomes cathode. DC voltage from the external source at variable ranges was applied that was somewhat greater than the minimum voltage that is required for the necessary current to flow which is called as critical voltage. The critical voltage of steel is approximately 6V, TV for brass and 7.5V for aluminum. The reagents which were used are
For steel-Ig Cupric Chloride, 15ml Sulphuric acid, 1g gelatin, 500ml water, 2,5g Cupric ammonium chloride, 50ml hydrochloric acid, 50ml water.
For aluminum 17% Sulphuric acid and 2% fluoro boric acid
Ultrasonic Restoration method of Restoration
In a liquid, when there is a creation of air bubbles which may be due to the reduction in pressure which is very much localized, is the process called cavitation. High speed motor boat propellers can be mainly observed where a stream of bubbled is produced and this phenomenon is considered as very much same to the boiling process in which the bubbles made up of vapors are formed because of the localized rise in temperature. In case if any metal objects undergoes cavitation process the effect is the etching of surface. Sometimes this process of etching on the surface can further used for the restoration of erased serial numbers.
A method that is used in ultrasonic restoration is discussed below. In this piezoelectric transducer is used by which an electric current of high frequency approximately 20 kHz is delivered and then that will be converted into vibrations that are mechanical that have the same frequency. In water, the mechanical vibrations were introduced directly using a probe or by the help of some second method, then the vibrational cavitation (ultrasonic cavitation) of the water will start.
The sample was simply placed in the stream of cavitation bubbles and as a result of cavitation in the sample erased numbers easily got visible through which the metal in less amount were removed from area where stamping occur than the nom-stamped areas.
The basic reason behind this is stamped area is the work hardened area which is due to the compression of any crystalline structure and then this area that is work hardened that is being more strong than the neighboring area which is considered as more resistant to the process of cavitation. This method is applicable to all metals which is considered as the main advantage and treatment is not required with any chemical reagents.
Additional Restoration methods
Heat etching is defined as the heating is done with oxyacetylene torch to heat tint the object or surface or the object was heated in an inert atmosphere. The cooling of the object was also reported till the formation of frost of water vapor on the surface. The success was not reported of all these methods.
The other method also been suggested which is totally opposite to the method of heat etching but it was also not very much successful. Other methods that were review includes
- Electrolyte etching
- Profile measurements of hardness
- Relief polishing
- X-rays (reflection and transmission)
- Scanning acoustic microscopy
- Electron channeling contrast
The simplest methods that were discussed above is chemical etching but it does not require any expensive equipment and the results are then photographed. Fry’s reagent is the best considered method for steel.
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The surface is first polished which is the preliminary process which is accomplished by initially removing all surface markings, scratches, grow marks. The surface was degreased and cleaned using chloroform or acetone after the polishing of the surface. The application of all these chemical techniques is same for all metals but the thing that differs mostly is chemical formula that varies from metal to metal. Heat was applied to the surface till it got so hot t touch and then cool down
Chemical Techniques
With the help of cotton ball that is attached to glass rod or some type of stick, reagent was applied to the surface. Then after some time approximately 1 minute, rinsing of reagent was performed using acetone and then viewed. The number that is read will started appearing as a shadow when viewing through an oblique light. If in case after 1 minute nothing will happen then this process was repeated for 2. 5. then 10 minutes consecutively for 2 hours.
The selection of etching reagent depends upon the type of metal surface used,
1. Tests for Iron and Steel
• Fry’s reagent – 90g Cupric Chloride, 120 mil Hydrochloric acid and 100ml water. The solution was applied as described above that particular area where solution was applied is then washed with acetone and then adding 15% solution of Nitric acid. Till the development of number the process was repeated.
• Cast Iron and Cast steel-On the surface, solution of potassium dichromate 10% solution of Sulphuric acid was applied
• Wrought, drawn and forged iron and steel-For the examination of these items two reagents were used,
Reagent 1-Alcohol -50ml, Conc. Hydrochloric acid-80ml, Cupric Chloride 129 g. Water-60ml
Reagent 2-15% Nitric acid The reagent was poured in the cotton swab and cotton soaks the reagent (1), after applying on the surface of metal wait for some time. Then the reagent (2) was applied on the metal surface but prior applying (2) clean the previous one from the surface. The continuation of cycle goes on until the development of number.
• Solution of Sg Ferric Chloride, 5oml Hydrochloric acid and 100ml water is prepared
• Solution of ethyl alcohol is prepared with saturated solution of picric acid
2. Tests for Aluminum and alloys of Aluminum
- Villela’s reagent- This reagent was considered as the best reagent for the restoration techniques of Aluminum surface. Glycerine-30ml, Hydrofluoric acid-20ml, Nitric acid-10ml
- The Hume-Rothery reagent – This is considered safer but are not much efficient as Villela’s reagent. Cupric Chloride-200g, Hydrochloric acid-5ml, Water-1000ml. Because of the faster reaction cure has to be taken while using Fry’s reagent in aluminum.
- Zinc and alloys of Zinc/aluminum- Solution of 50% Nitric acid and 50% of Hydrochloric acid
- Copper, Brass and other alloys of Copper- Solution of Cupric Chloride-1.40g, hydrochloric acid-150ml and water. Solution of 19g of Ferric Chloride, 6ml of Hydrochloric acid and water (100ml) .
Electrolyte Methods
In an electrochemical cell, the specimen was made anode is a process called Electro polishing. Minute quantities of irregularities was dissolved when current is applied which leaves a surface that is highly polished. Due to the compressed structure of crystal, the electrical properties in the stamped area are altered carefully and the compressed areas will be different from non compressed areas on the basis of dissolution.
This has been proved to be an effective method for the process of serial number restoration that was first developed 25 years ago. This method includes both anode and cathode, the specimen was made at the anode and electrolytic solution in a swab of cotton becomes cathode. DC voltage from the external source at variable ranges was applied that was somewhat greater than the minimum voltage that is required for the necessary current to flow which is called as critical voltage. The critical voltage of steel is approximately 6V, TV for brass and 7.5V for aluminum. The reagents which were used are
For steel-Ig Cupric Chloride, 15ml Sulphuric acid, 1g gelatin, 500ml water, 2,5g Cupric ammonium chloride, 50ml hydrochloric acid, 50ml water.
For aluminum 17% Sulphuric acid and 2% fluoro boric acid
Ultrasonic Restoration method of Restoration
In a liquid, when there is a creation of air bubbles which may be due to the reduction in pressure which is very much localized, is the process called cavitation. High speed motor boat propellers can be mainly observed where a stream of bubbled is produced and this phenomenon is considered as very much same to the boiling process in which the bubbles made up of vapors are formed because of the localized rise in temperature. In case if any metal objects undergoes cavitation process the effect is the etching of surface. Sometimes this process of etching on the surface can further used for the restoration of erased serial numbers.
A method that is used in ultrasonic restoration is discussed below. In this piezoelectric transducer is used by which an electric current of high frequency approximately 20 kHz is delivered and then that will be converted into vibrations that are mechanical that have the same frequency. In water, the mechanical vibrations were introduced directly using a probe or by the help of some second method, then the vibrational cavitation (ultrasonic cavitation) of the water will start.
The sample was simply placed in the stream of cavitation bubbles and as a result of cavitation in the sample erased numbers easily got visible through which the metal in less amount were removed from area where stamping occur than the nom-stamped areas.
The basic reason behind this is stamped area is the work hardened area which is due to the compression of any crystalline structure and then this area that is work hardened that is being more strong than the neighboring area which is considered as more resistant to the process of cavitation. This method is applicable to all metals which is considered as the main advantage and treatment is not required with any chemical reagents.
Additional Restoration methods
Heat etching is defined as the heating is done with oxyacetylene torch to heat tint the object or surface or the object was heated in an inert atmosphere. The cooling of the object was also reported till the formation of frost of water vapor on the surface. The success was not reported of all these methods.
The other method also been suggested which is totally opposite to the method of heat etching but it was also not very much successful. Other methods that were review includes
- Electrolyte etching
- Profile measurements of hardness
- Relief polishing
- X-rays (reflection and transmission)
- Scanning acoustic microscopy
- Electron channeling contrast
The simplest methods that were discussed above is chemical etching but it does not require any expensive equipment and the results are then photographed. Fry’s reagent is the best considered method for steel.
For more updates, go through this link.
