1. Serial Number Restoration Laboratory Exercise

2. Serial Number Restoration is a procedure commonly performed in forensic crime laboratories to determine the original owner of an item.
Criminals commonly obliterate a serial number in an attempt to make tracing the number impossible.
When serial numbers are obliterated, criminals typically only scratch the surface enough so the numbers are no longer visible to them.

3. Step 1. Examine the metal to see if any partial characters are visible.
Note the partial arc in the area of obliteration. This is indicative of a number with a curved upper area such as an 8, 9, or 0. The number 6 can be excluded due to its extending along the right side.

4. Step 2. Smooth the surface
Starting with course sandpaper, begin smoothing the surface of the area of obliteration

5. Step 3. Polish the surface
Using progressively finer sandpaper, polish the surface until it’s smooth
Remind the students that one of the principals of Serial Number Restoration is the ability to see the number by Specular and Diffuse Reflection (lights reflection off smooth and rough surfaces) Therefore, for the best results, the surface should be as smooth as possible to give the maximum contrast when viewing the rough surface caused by the etching of the acid.

6. Step 4. Application of Acid
Acid is swabbed with a cotton swab or dropped onto the area of obliteration to begin the etching process
Take notes on the effect of the acid on the metal
While the acid is on the surface, the cotton swab can be used to wipe the surface to facilitate viewing the number
It may take attempts with different acids to achieve results
It is important to try the different acids on the metals to note their reaction

7. Step 4. Application of Acid
It is important to try the different acids on the metals to note their reaction. There is a difference between an aggressive and an effective acid. The reaction with the metal should be somewhat slow and consistent. If an acid is too aggressive, the metal will be etched too quickly and there is a risk the serial number will be “burned” out.
Because of the variety of compositions of metals and alloys, it is necessary to try different acids to see which one will work best.
The ideal acid for your metal will allow you to view the serial number while the restoration process is occurring and is effective but not too aggressive.

8. Step 4. Application of Acid
The laboratory exercise contains two types of metals, stainless steel and carbon steel.
Stainless steel reacts best with Fry’s Reagent and Acidic Ferric Chloride
Carbon steel reacts best with 25% Nitric Acid and Acidic Ferric Chloride
Carbon Steel can also be called “hot rolled” or “cold finished”. This refers to the forming technique.

9. Step 4. Application of Acid
Some acids will take longer than others to react with the metal. If an acid doesn’t react within a few minutes, a new acid should be used.
Acids can be blotted up with a paper towel for removal, however caution should be used as these acids are strong and will burn your skin.
As with all laboratory exercises using acids, safety glasses and protective clothing should be worn. The use of acid should be regulated to a fume hood or other well ventilated area.

10. Step 5. Record Serial Number
Once the serial number is visible, record your results
There are times when the numbers are easier to read when they have solution on top of them. Students should be try reading the numbers both with and without solution so they can see the difference. 1 2 3 4 5

11. Review Original Serial Number Obliterated
1. Look for visible number fragments
2. Sand with course sandpaper
3. Polish with fine sandpaper
5. Record restored serial number
4. Apply acid

12. Documenting the Restoration
As numbers appear during the restoration process, write them down. At times, the complete number may be unclear. If that is the case, write down the possible numbers could be. 5 4 ? 7 ?
3 or 8 5 3 4 ? 7
0 or 9 5 3 4 2 7
In the second position we see “3 or 8”. This is an example of where we were unsure if the number was a 3 or an 8 because the bottom and left side of the number isn’t yet visible. We want to document it just in case the number disappears when more acid is added.
The question marks are where we know a number should be present, but we are unable to determine the number at that time.

13. Documenting the Restoration
On the bottom of the worksheet the reaction should be documented. Different reagents will react differently with the various metals.