1. Serial Number Restoration

2. Serial Number – A unique number typically applied to an object by a manufacturer
Serial Number Restoration – The practice of restoring an obliterated serial number by using scientific methods

3. Commonly Marked Items
Many items have serial numbers placed on them by the manufacturer. Stereos, computers, vehicles, televisions, and almost all electronic equipment have serial numbers.
Firearms are the most commonly encountered item in crime laboratories with obliterated serial numbers.
The firearms are made from a variety of materials including aluminum alloy, stainless steel, and steel.

4. Types of Serial Numbering Application Methods
Serial numbers can be applied in a variety of methods. Each method has unique characteristics which enable them to be differentiated from each other.
Dot Matrix Engraving

5. Dot Matrix
Dot matrix engraved serial numbers are fairly shallow and are made by a succession of dots

6. Dot Matrix - Examples

7. Laser Engraving
Serial numbers engraved with a laser are shallow and have a “burned” appearance.

8. Stamped Serial Numbers
Stamped serial numbers are made by using a stamp with a positive impression of a number and applying sufficient pressure to impart a negative impression of the stamp on the object
Stamping of serial numbers on objects encountered in crime laboratories (firearms, vehicles, etc) is the most common method. Because of this, the lecture and laboratory exercise will concentrate on this type of serial number application.

9. Stamped Serial Numbers - Theory
When a serial number is stamped in metal, plastic deformation of that metal occurs
After the force causing the deformation is removed, the impression remains
“Plastic Deformation” is an engineering term used to indicate permanent deformation.
The samples of aluminum with the kit are included to demonstrate the effects of plastic deformation and should be passed around the class at this time so students can examine this phenomenon.

10. Stamped Serial Numbers

11. Stamped Serial Numbers - Theory
The compressed area at the base of the stamping is strained below the surface of the stamp mark
The depth of the strained area is dependant on the type of metal. For example, the strained area of steel may be affected for approximately 6 times the depth of the stamp
The diagram on the right is meant to show a side view of the distortion the metal is encountering from the stamping process.
As the force of the stamp (depicted by the green arrow) encounters the metal, we see the resulting strain throughout the thickness of the metal. The amount of distortion is shown by the bending in the lines.
The raised areas around the sides are caused by the metal swelling from the force of the stamp. This was evident on the previous slide where the number “7” had swelling around the sides.
The bottom of the valley of the top line is the area which came in contact with the tip of the stamp. This is the lowest area of the stamping and it the what is visible.

12. Stamped Serial Numbers - Theory
Stamping a serial number while the metal is below it’s recrystallization temperature is known as “Cold Working”.
When metal is cold worked, the areas that are worked will change properties. These changes include: increased hardness and tensile strength. However, there are decreases in it’s malleability and ductility
Additionally, the position of the metal in the electromotive series is raised which lowers it’s resistance to oxidation
The lowered resistance to chemical oxidation allows for the use of acid to etch more efficiently in the area where the serial number was stamped.
The temperature below it’s recrystallization temperature is that below it’s molten to stage. Therefore, any manipulation of the metal at room temperature will cause this phenomenon.
Malleability – A metals likelihood to be deformed
Ductility – A materials ability to yield under shear stress
Electromotive Series – A metals relative tendency to be oxidized
The fact that the lowered resistance to oxidation allows us to use acid must be stressed. It is the key factor in our ability to restore serial numbers in metal.

13. Obliteration
When serial numbers are obliterated, they are generally ground down beyond the lowest point of the original stamping.
This results in the visible portion of the serial number being removed.
By removing the serial number, criminals hope that the object can not be identified as having been stolen.
The red line represents the area ground in an attempt to obliterate a serial number. As you can see, it has been ground beyond the area of the lowest point of the stamp. This would result in the serial number no longer being visible.
Law enforcement uses the serial number of firearms to determine if a firearm has been reported stolen. The absence of a serial number makes it impossible to find the original owner of the firearm.

14. Obliteration
The area below the obliterated layer is affected by the stamping process and is the reason serial number restoration is possible.
The red line represents the new surface of the metal with the valley of the original stamping removed.

15. Serial Number Restoration - Chemistry
Steel is comprised of iron and carbon. Additional elements such as nickel, manganese, chromium, cobalt, vanadium, and molybdenum are added to enhance it’s properties. Sulfur and phosphorus are typically present as impurities.
The elements are bound at the atomic level
Copper salts (Copper Chloride) are common in many metal etching reagents for the purpose of assisting in the oxidation of iron
Now that we understand the mechanics of the stamping and obliteration process, we will examine the process of restoring the serial number.
We first need to look at the chemical process of acid etching. An example of this is given above.
Anodic Reaction: Fe Fe e-
Cathodic Reaction: Cu Cu0 + 2e-

16. Visualizing Restorations
The reason serial numbers appear when they are acid etched is due to specular reflection.
Specular reflection – light reflecting off a smooth surface
Diffuse reflection – light reflection off rough surfaces
Specular Reflection
Diffuse reflection
Rough Surface
Smooth Surface

17. Common reagents in restoration
Turner’s Reagent
2.5 grams Cupric Chloride (CuCl2)
40 mL Hydrochloric Acid (HCl)
25 mL Ethyl Alcohol (CH3 CH2OH)
30 mL Water (H2O)
Acidic Ferric Chloride
25 grams Ferric Chloride (FeCl3)
25 mL Hydrochloric Acid (HCl)
100 mL Water (H2O)
Ferric Chloride
25 grams Ferric Chloride (FeCl3)
100 mL Water (H2O)
25% Nitric Acid
75 mL Water (H2O)
25 mL conc. Nitric Acid (HNO3)
Fry’s Reagent
90 grams Cupric Chloride (CuCl2)
120 ml Hydrochloric Acid (HCl)
100 mL Water (H2O)
Some used alone or in combinations

18. Serial Number Restoration
An obliterated serial number typically has a surface that has been scratched so that the number is no longer recognizable

19. Serial Number Restoration
The first step in the process of restoring the serial number is to polish the surface by using sand paper in progressively finer grits.
Polishing the surface will help in visualizing the serial number Specular reflection
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 (scattered reflection of light)

20. Serial Number Restoration
Acid Etching – The acid reacts with the metal and produces hydrogen gas bubbles

21. Serial Number Restoration
The result of the acid etching is a restored serial number. The dark appearance of the numbers is the result of diffuse reflection.

22. Documentation
The final step in performing a serial number restoration is documenting the recovered serial number. This can be done by writing down the numbers or photographing the results. Photographing serial number restoration may be difficult due to the lighting requirements.
Additionally, the chemicals used to etch the metal should be documented in the notes.

23. Case Studies
An ATV was suspected of having been stolen from a local dealership.
When police attempted to read the VIN (Vehicle Identification Number) it was observed that it had been obliterated.
The number in question was located on the engine

24. The surface with the obliterated number was located on the top of the engine block

25. Step one: sand it smooth
This is the area on top of the engine where the VIN number was stamped.
The first step in the restoration process will be to sand it smooth.

26. Step 2: apply reagent
After the surface was sanded, Acidic Ferric Chloride was applied to the engine block.

27. The Vehicle Identification Number was restored to H310E-06674
The Vehicle Identification Number was restored to H310E Using this information, police were able to verify the ATV was stolen and press charges against the suspect. H 3 1 E - 6 6 7 4