Peak Contrast
Definition
The narrow window during a chemical etch when the original digits are most legible. Continuing past peak contrast overetches the unstrained metal and erases the recovered pattern, which is why interval photography is mandatory.
- Definition
- The narrow window when etched-out digits are most legible
- Risk of overrun
- Continuing past it overetches unstrained metal and erases the pattern
- Mitigation
- Interval photography during etching
Common questions
Why can't an examiner simply etch longer to be sure the pattern is fully developed?+
Etching works because strained metal beneath a stamped or struck mark corrodes faster than the surrounding unstrained metal. Once that difference is fully revealed, further etching starts eroding both areas at similar rates, which washes out the contrast that made the pattern readable in the first place.
What does interval photography protect against if peak contrast is missed?+
Because the legible window can be brief and the process is not reversible, photographing the surface at short intervals throughout etching ensures the examiner captures the pattern at its clearest point even if that point is not obvious until reviewed afterward, rather than relying on a single end-point observation.
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...
- Magnetic-Particle Method
- Non-destructive restoration for ferromagnetic metals. The polished surface is magnetised and dusted with fine iron particles; the particles cluster at the strained...
- 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...