Table 1

Substrate/Material	Typical Marking Method	Surface Groove Depth (µm)	Approx. Plastically Deformed Depth (µm)	Application
Lowcarbon steel	Mechanical stamping / roll	80-140	140-520	Firearm serial numbers, tools
Stainless steel	Mechanical stamping	60-120	180-400	Medical / industrial components
Cast / structural steel	Mechanical stamping	100-300	300-800	Frames, machinery parts
Aluminum alloys	Mechanical stamping	50-150	150-450	Vehicle VIN plates, engine parts
Copper / brass	Mechanical stamping	40-120	120-360	Nameplates, electrical components
Zinc / diecast alloys	Mechanical stamping	70-200	200-500	Engine blocks, housings
Steel (conventional laser)	Laser engraving (shallow)	May-25	Localized, near surface	Logos, lowstress ID marks
Steel (deep laser)	Deep laser engraving	50-500+	Similar order as groove depth	Durable industrial / firearm markings
Aluminum / Mg alloys	Chemical / electrolytic etch	10-100	Comparable to etched depth	Nameplates, aerospace parts
Polycarbonate (PC)	Thermal / laser marking	~100-150	Very limited true plastic zone	Polymer serials; swelling/heat used in recovery
Polyethylene (PE)	Thermal / laser marking	~200-250	Very limited true plastic zone	Polymer casings; depth per moulding conditions
Nylon / PA	Thermal / laser marking	~100-150	Very limited true plastic zone	Functional polymer parts
Other engineering polymers	Laser / hot stamp	20-200	Minimal subsurface structural change	Consumer goods, IDs, labels

Table 1: Typical depths of identification markings and associated deformation zones in common substrates

Table 2

Category	Technique	Principle	Key Notes	References
Destructive Methods	Chemical Etching	Differential corrosion: plastically deformed zones are more chemically reactive than undeformed regions	Requires mirror-polished surface; etchant selection depends on substrate (e.g., Fry's reagent for steel, other acids/alkalis for Al, Zn); restoration fails if deformation zone fully removed	[18-20]
	Electrochemical Etching	Anodic dissolution under applied current accelerates and controls etching in strained zones	Object serves as anode in cell; voltage ~3-6 V; electrolyte tailored to substrate; greater sensitivity & reproducibility vs. chemical etching, but still destructive	[21, 22]
Non-Destructive Methods	Magnetic Particle Inspection (MPI)	Flux leakage: plastically deformed zones reduce magnetic permeability, attracting magnetic particles	Works on ferromagnetic materials (e.g., steel); particles cluster along erased characters; widely used as first-line before destructive methods	[23, 24]
	Ultrasonic Methods	Cavitation bubbles collapse unevenly on distorted zones, eroding layers and enhancing contrast	Minimal surface preparation needed; applicable to metallic/non-metallic substrates; non-invasive and repeatable	[25, 26]
	X-ray Imaging / Micro-CT	Subsurface density differences detected via radiographic absorption and 3D volumetric reconstructions	Recovers structural deformation even if surface is obliterated; effective for metals, composites, bone, and delicate specimens	[29]

	Ultraviolet Illumination	Uses UV-induced fluorescence/ absorption differences in stressed areas	Reveals residual serials on polymers after abrasion via contrast in bright/dark	[27, 28]
	Infrared Imaging	Detects differences in IR emissivity and subsurface scattering	Visualizes hidden character patterns beneath smoothed polymer surfaces	[27, 28]
	Thermal Imprint detection	Monitors surface temperature patterns during controlled heating/cooling	Highlights shallow relief or density differences from original molded/embossed marks	[28]
	Digital Microscopy	High-resolution optical imaging of fine surface irregularities and restored features	Provides detailed images of faint or partially restored characters
	3D Surface Profilometry	Quantitative measurement of surface topography and relief variations	Produces objective, comparable datasets; complements optical microscopy for forensic interpretation

Table 2: Restoration Techniques and their Principles

Table 3

Parameter	Non-Destructive Methods (MPI, Radiography, Ultrasonics, Optical Profiling)	Destructive Methods (Chemical/Electrolytic Etching, Heat Tinting)	References
Principle	Detect variations in magnetic flux, acoustic impedance, X-ray absorption, or surface relief caused by subsurface deformation	Exploit differential corrosion/electrochemical reactivity between strained and unstrained regions	[16, 17]
Preservation of Evidence	High, sample remains intact; repeatable analysis possible	Low, irreversible alteration; material surface often consumed	[23, 20]
Sensitivity (Depth of Recovery)	Moderate, typically effective for shallow obliterations (≤150-300 μm depending on alloy and method)	High, capable of detecting deeply deformed zones (>300-500 μm in steels, depending on reagent)	[25,19]
Resolution (Clarity of Marks)	Dependent on imaging modality: micro-CT and confocal microscopy achieve high spatial resolution (micron scale)	Limited by etchant penetration and risk of over-etching; resolution may degrade with excessive corrosion	[24,18]
Speed of Analysis	Rapid, especially with digital instrumentation (real-time imaging)	Slower, requires reagent preparation, controlled exposure, repeated steps	[26,20]
Skill Requirement	High, specialized equipment, calibration, and trained operators	Moderate, relies on forensic technician skill in reagent handling	[21]
Reproducibility	High, digital imaging/software enhances consistency (low operator bias)	Moderate, variations occur due to reagent strength, surface prep, operator technique	[17, 22]
Courtroom Acceptance	Increasing, digital and 3D datasets seen as objective and less invasive	Traditionally accepted but methods may face defense challenges due to destructive nature	[23,20]
Cost/Accessibility	Higher, requires advanced instruments, computing resources, maintenance	Lower, acids, salts, and basic lab setup sufficient	[37]
Best Suited For	Evidence requiring preservation (firearms, VINs, luxury goods, delicate specimens like bone)	Cases of deep obliteration where non-destructive fails (e.g., heavily abraded steel surfaces)	[29]

Table 3: Comparative Evaluation of Destructive and Non-Destructive Methods for Restoration of Obliterated Identification Marks

Tables at a glance

Table 1

Table 2

Table 3

Figures at a glance

Figure 1

Figure 2

FIGURE 1

Graphical Abstract

FIGURE 2

Figure 1: Schematic illustration of Chemical and Electrochemical Etching for Restoration of Obliterated marks