A Wild Idea Jonathan Franklin. Pavnil Patel. Views Total views. Actions Shares. No notes for slide. Serial number restoration 1. These number are linked to the buyer of the firearm.
Whether it is a handgun or motor vehicle, it takes little effort on the part of the perpetrator to grind off these identifying marks. Obliteration The serial number is no longer visible due to the removal of the top layer of the metal. The area below the obliterated layer is affected by the stamping process and is the reason serial number restoration is possible.
Serial Number Restoration An obliterated serial number typically has a surface that has been scratched so that the number is no longer recognizable 7. Polishing the surface will help in visualizing the serial number. The polished surface gives us specular reflection even, consistent reflection of light where as the area that is attacked by the acid will yield diffuse reflection. The dark appearance of the numbers is the result of diffuse reflection.
This can be done by writing down the numbers or photographing the results. Additionally, the chemicals used to etch the metal should be documented in the notes.
Total views 2, On Slideshare 0. From embeds 0. Number of embeds 1. Downloads Shares 0. An indented character is left behind. This method is commonly used on metal substrates, particularly in the motor vehicle manufacturing industry. In this case, the characters to be stamped are applied to the substrate with a slower, more steady pressure. Its application is in cases in which the substrate could be damaged by the sudden impact used in die stamping. However, by heating a die prior to stamping, it can be used to produce a suitable impressed character.
By arranging these dots into the appropriate pattern, the requisite characters are formed. The process is analogous to dot matrix computer printing. The die is pushed on to the plate from behind.
The methods include ink processes such as offset printing, hot foil stamping, screen printing, laser printing, etc. Traditionally, methods used to remove serial numbers involve a physical abrasion of the substrate until the number is no longer visible. Methods to do this include filing, grinding usually with an electric or air-powered angle grinder , sanding with sandpaper or emery paper, or scraping with a sharp implement, especially on plastic substrates.
Numbers can also be obliterated so that they are no longer decipherable. A relatively sharp object, such as a centre punch or cold chisel, is hammered on to the area of the number until it is damaged beyond legibility.
In the case of printed numbers, chemicals can be used to wash away or blur the numbers. Solvents such as alcohol, acetone or similar can be used. Forensic practitioners are most often confronted with die-stamped serial numbers which have been ground off metal substrates.
This is particularly common in cases involving theft of motor vehicles or the use of firearms. If a number is ground off a surface until no visible sign of the characters remains , it does not necessarily follow that all traces of the number have been obliterated. The process of cold working involves transmitting a shock into the substrate, and compressing the metal structure such that the material which originally occupied the volume taken by the characters is displaced downwards into the substrate Fig.
If the top layer of the substrate is subsequently removed, a damaged area of material underneath the original characters may remain Fig. Note that a number of factors may influence how far into the substrate the damaged area extends. These include the composition of the material, as well as the force used to stamp the number.
If the surface is ground off deeply enough, it is possible that all traces of the damaged area will be obliterated. Figure 1 Damaged area underneath a stamped character. Figure 2 Residual deformation still present after obliteration of character.
There are two major methods employed for recovering obliterated serial numbers in metal substrates. The purpose of both processes is to create a visible contrast between the damaged and undamaged regions of the substrate. This contrast is the result of differential reflection or scattering of light from the damaged area compared with that from the undamaged area Fig.
Before either method can be applied, the surface must be prepared. This involves removing as much as possible in the way of residual irregularities, in order to obtain as smooth a surface as possible. A compromise must occasionally be reached when deep grooves are present, as removing the surrounding metal may achieve total obliteration of any remaining traces of the original characters.
Some practitioners use small hand-grinding or sanding tools to smooth the surface, but this can run the risk of uncontrolled heating of the area, as well as having less control over the depth of the abrasion. For these reasons, hand sanding is generally preferred. This is perhaps the most commonly used technique for recovering obliterated numbers.
It originates from metallurgical examinations, in which etching solutions are applied to cross-sections of metal in order to observe the crystal structure under reflected light.
Figure 3 Reflection of light from smooth surface compared to rough surface. For recovering obliterated numbers, the method relies upon the phenomenon that the rate of reaction between the applied chemicals and the substrate can differ in the damaged and undamaged areas.
The damaged area usually has a different electrochemical potential to the undamaged surroundings, so can therefore be attacked selectively, or at least at a different rate, in comparison with the surroundings. The chemicals used depend upon the composition of the substrate. The solutions are generally applied by wiping them across the surface of the substrate with a cotton-wool or cloth swab.
This process is repeated regularly, and the surface is observed closely between applications. Any visible characters should be noted immediately, as it is quite common for different characters to appear at different times during the restoration. It is also common for some characters to disappear shortly after becoming visible. Another method of application, particularly useful for aluminum alloy engine block numbers on horizontal surfaces, is to form a dam of plasticene around the area, then pour the etching solution into this to a depth of mm.
For example, different chemicals are used to etch aluminum, zinc and steel alloys. Although the BCA routinely restores serial numbers on the metal surfaces of firearms, examiners also have procedures for obliterations on wood and plastic surfaces.
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