Topic 7 - Serial Number Restoration (Serial Number Restoration…
Topic 7 - Serial Number Restoration
Removing SNs removes traceability of firearm e.g. previous owners. Restoring the SN allows the firearm to be tracked.
Polycrystalline structure, composed of irregularly shaped crystals which form when molten metal cools to become a solid.
Cooling process affects the mechanical properties of a metal.
When SN is stamped, the crystalline structure is compressed altering the physical properties (hardness, strength and magnetic, electrical and chemical properties). These areas of altered crystalline structure are what is exploited to recover a SN that has been tampered with (as long as modified/stamped area still exists).
Serial Number Removal
: hand filing or high speed grinding wheel. Often polished afterwards and new SN stamped over.
: hammering over the SN with a round punch until hidden.
: stamping new characters over old ones. Curved numbers (2, 3, 5, 6, 9) over stamped with an 8. Straight number (1 and 7) over stamped with a 4. Larger number of 8s and 4s is suspicious.
: obliteration of whole SN, difficult to restore.
of SN and surrounding metal with drill. Often refilled with lead solder.
of SN by heating metal until the metal flows.
Serial Number Restoration
inspect for trace evidence and remove any material carefully.
note position of SN and the areas physical property (magnetic or non-magnetic.
determine location, size, font, hidden numbers and the number of characters found (if possible at this stage).
note the method of obliteration
Preparation of Surface
polishing, to clean, can remove scratches that obscure the SN. Polishing may be effective independently.
polish until surface is mirror-like, depending on extent of obliteration it may not be possible to remove all scratches.
known as ETCHING
same general technique for all metals, just with different chemical reagents (etchants).
first make sure etchant and metal reaction isn't too vigorous.
apply to restoration area with cotton swab. After 1 min rinse with acetone.
number should start to appear as a shadow. If not, repeat in intervals of 2, 5, 10 minutes up to 2 hours.
heating surface may speed up development and enhance the result.
Reagents used for MAGNETIC MATERIALS:
Reagents used for NON-MAGNETIC MATERIAL:
Acid ferric chloride
The aim is to erode the surface of the metal slowly, to reveal the SN stamping deeply penetrated into the metal
: anode and electrochemical cell used to apply current to restoration area. Current causes tiny irregularities on the surface to be dissolved, leaving a polished surface. Cotton swab or fabricated cloth containing electrolytic solution acts as the cathode.
Surface is slowly degraded.
acidic cupric sulphate with gelatin
acidic cupric ammonium chloride
17% sulphuric acid
2% fluoroboric acid
Magnetic Particle Method
used prior to chemical processing if appropriate.
material must be ferromagnetic (high susceptibility to magnetisation) and material must become temporarily magnetised.
Using plastic pipette, apply magnetic particle solution (Magnaflux bath) to restoration area while applying magnetic field through a magnet (electro-magnetic yoke or horseshoe magnet).
the solution will be attracted to the magnetised properties of the stamped area, with its altered crystalline structure.
gather in a way to represent the SN that has been tampered with.
Other methods of Restoration: x-rays (transmission or reflection), electrolytic etching, ultrasonic cavitation and relief polishing.