Fry's Reagent
Definition
Classical etchant for plain carbon and low-alloy steel: 40 g cupric chloride (CuCl2) + 90 mL concentrated HCl + 100 mL water. The workhorse reagent for restoring serial numbers on firearm frames, engine blocks and machinery.
- Primary metals
- Plain carbon and low-alloy steel; modified versions for stainless and high-carbon steels
- Main components
- Cupric chloride, concentrated hydrochloric acid, water
- Common applications
- Firearm frames, engine blocks, machinery serial numbers, and erased markings
Common questions
What is Fry's reagent used for?+
Fry's reagent is a chemical etchant that restores obliterated serial numbers and markings on steel surfaces, commonly used on firearms, engine blocks, and machinery. When applied to steel, it causes differential corrosion between cold-worked (damaged) and unstressed zones, revealing hidden digit geometry that was erased or filed away.
What's in Fry's reagent?+
The classical formula mixes cupric chloride, concentrated hydrochloric acid, and water. The exact proportions vary slightly (commonly 40 g CuCl2 + 90 mL HCl + 100 mL water, or 90 g CuCl2 + 120 mL HCl + 100 mL water depending on the source). Modified versions adjust the ratio for stainless and high-carbon steels.
Why does Fry's reagent reveal hidden serial numbers?+
Erasing a serial number damages the metal through filing, grinding, or stamping, leaving deformed (cold-worked) zones under the surface. Fry's reagent etches these stressed areas faster than the surrounding unstressed metal, creating visible contrast that outlines the original digit shape.
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...
- Electrolytic Etching
- Electrochemical alternative to chemical etchants. The part is the anode in an electrolyte bath; controlled current selectively dissolves the strained metal at...
- Magnetic Particle Inspection (MPI)
- A non-destructive testing method that reveals magnetic-domain contrast between cold-worked and unstressed zones in ferromagnetic metals using a ferrofluid under UV illumination;...
- ASTM E2885-13
- ASTM International standard practice for restoration of obliterated vehicle identification numbers; the reference protocol for VIN casework in US and internationally aligned...
- Backscattered Electron (BSE) Imaging
- An SEM imaging mode sensitive to atomic number and crystallographic texture; can reveal grain-boundary elongation patterns in the sub-surface cold-worked zone after...
- Cold-Working Stress Field
- Residual plastic deformation and elevated dislocation density in a metal lattice created by stamping; persists below the visible surface even after surface...
- Heat-Tinting
- Restoration by controlled heating of the polished surface. Strained metal oxidises at a different rate from unstrained metal, producing colour contrast (straw...
- IMEI (International Mobile Equipment Identity)
- A unique 15-digit number permanently assigned to a mobile device's hardware. Used by networks to identify and block stolen devices, and by...
- INTERPOL iARMS
- INTERPOL's Illicit Arms Records and tracing Management System. Multilateral database enabling law enforcement in INTERPOL member countries to trace recovered or seized...
- 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...
- Neutron-Induced X-Ray Fluorescence (NIXRF)
- A synchrotron-based method using neutrons to probe deep obliterations; can detect digit geometry buried several millimetres beneath a machined surface, entirely non-destructively.
- Obliteration
- Covering of existing writing with an opaque layer (ink, correction fluid, or correction tape) so that the underlying text is invisible in...
Explained in these topics
- Restoration of Erased Markings on Metal SurfacesClassical etchant for plain carbon and low-alloy steel: 40 g cupric chloride (CuCl2) + 90 mL concentrated HCl + 100 mL water. The workhorse reagent for restori...
- Restoration of Erased Serial NumbersAn aqueous solution of hydrochloric acid and copper(II) chloride used to chemically etch steel substrates; differential corrosion between cold-worked and unstr...
- Serial Number and Erased Mark RestorationThe classical chemical etchant for ferrous metals: 90 g cupric chloride, 120 mL concentrated HCl, 100 mL water. Modified Fry's adjusts the ratio for stainless...