Magnetic-Particle Method
Definition
Non-destructive restoration for ferromagnetic metals. The polished surface is magnetised and dusted with fine iron particles; the particles cluster at the strained sub-surface zone where magnetic permeability differs, revealing the original digits without removing material.
- Applicable materials
- Ferromagnetic metals only
- Mechanism
- Iron particles cluster at the strained sub-surface zone
- Destructiveness
- Non-destructive, no material removal
- Use
- Restoring erased or stamped serial numbers
Common questions
Why does the magnetic-particle method only work on ferromagnetic metals?+
The technique relies on magnetic permeability changing in the cold-worked zone left by stamping, a property only ferromagnetic materials like steel and iron exhibit; it will not reveal marks on aluminium, brass, or other non-magnetic metals.
How does the magnetic-particle method compare to chemical etching for restoring erased numbers?+
Chemical etching dissolves the surface to reveal strain patterns and is destructive to the metal, while the magnetic-particle method only magnetises the surface and applies particles, preserving it for further examination.
Related terms
- Davis Reagent
- Variant etchant for steel using cupric ammonium chloride (Cu(NH4)2Cl4) in dilute HCl. Slower acting than Fry's, sometimes preferred when contrast on Fry's...
- Fry's Reagent
- Classical etchant for plain carbon and low-alloy steel: 40 g cupric chloride (CuCl2) + 90 mL concentrated HCl + 100 mL water....
- Heat-Tinting
- Restoration by controlled heating of the polished surface. Strained metal oxidises at a different rate from unstrained metal, producing colour contrast (straw...
- Peak Contrast
- The narrow window during a chemical etch when the original digits are most legible. Continuing past peak contrast overetches the unstrained metal...
- Stamped Marking
- A serial, chassis, engine or part number impressed into metal by a hardened steel die under high force. The visible impression is...
- Strained Zone
- The region of cold-worked, plastically deformed metal lying beneath an original stamp. The crystal lattice is distorted, dislocation density is elevated, and...
- Villela's Reagent
- Etchant for stainless and high-chromium steels: 1 g picric acid + 5 mL concentrated HCl + 100 mL ethanol. Targets prior austenite...