Surface Preparation

Before applying chemical methods, the metal surface needed to be polished. It is the second step after photography. Following are procedures:

  1. Remove all scratches and other gross marks with a fine emery paper.
  2. Polished the surface to a mirror like finish.
  3. Final polished surface finish should be done by using jweller’s rouge or flour paper.
  4. Cleaned and degreased the surface with chloroform or acetone.

Procedure For Chemical Etching for Restoration of Numbers

The procedure for chemical etching is the same but they differ in chemicals based on surface.

Procedure:

  1. Apply the respective chemical reagent using cotton wool swab.
  2. Rinsed the surface with acetone, after application of one minute.
  3. Numbers should begins to appear which can be make more visible with oblique lightining.
  4. If NO impression is visible, reapply the reagent and after 1 minute rinsed the surface with acetone.
  5. Repeat in 2min, 5mins, 10 mins for upto 2 hours.

Note: The application of heat could increase the speed of development. Avoid heating too much, just enough that you can bare it in your hands.

Reagent Used Based on Various Metal Surface

Table A: Chemical Etching Reagents For Iron and Steel Metals

ReagentChemical Components
Nitric Acid1% to 15% Nitric Acid in Water
Fry’s Reagent90g cupric chloride, 120 mL HCI, 100 mL H2O
Turner’s Reagent2.5g to 5g cupric chloride, 40 mL HCI, 25 ml ethyl alcohol, 30 mL H2O
Davis Reagent5g CuCl2 + 50 mL HCI + 50 mL H2O
Reagent 1 g ferric chloride, 50 ml hydrochloric acid, 100 mL H2O
Chromic acid40g chromic acid, 50 mL H2O
Picric Acidsaturated solution of picric acid in ethyl alcohol

Table B: Etching Cheimcal for Aluminium and Aluminium Alloys

ReagentChemical ComponentsRemarks
Villela’s Reagent (Nickolls, 1965) 30 ml glycerine, 20 ml hydrofluoric acid, 10 ml nitric acidbest reagent, but very corrosive and should avoid storing in glass containers.
Hume-Rothery Reagent200 g cupric chloride, 5 ml hydrochloric acid, 1000 ml watersafer alternative but not efficient as Villela’s reagent,
Fry’s Reagent90g cupric chloride, 120 mL HCI, 100 mL H2Oreaction is very fast and lower obervering time.

Table C: Etching Reagents For Zinc And Brass

MetalChemical Components
Zinc50% hydrochloric acid + 50% nitric acid
Brass40 g cupric chloride, 150 ml hydrochloric acid, 50 ml water
Brass19 g ferric chloride, 6 ml hydrochloric acid, 100 ml water

Electrolytic Method of Restoration

In this technqiue, number plate/specimen is attached to anode in an electrochemical cell arrangement. It is used for polishing the surface before application of chemical (also called electropolishing).

When a minimum DC volatage (critcal voltage) is applied, minute irregularites on the surface gets dissolved, leaving highly polished surface.

  • Anode: restoration plate specimen
  • Cathode: cotton swab containing electrolytic solution
  • Critical DC Voltage: define as the minimum voltage necessary for the current to flow.
    • Steel: 6V
    • Brass: 7V
    • Alumiuium: 7.5V

Reagent Used After Electopolishing

MetalChemical Reagent
Steel1 g cupric sulphate, 15 ml sulphuric acid, 1 g gelatin, 500 ml water
Steel5 g cupric ammonium chloride, 50 ml hydrochloric acid, 50 ml water
Aluminium17% sulphuric acid
Aluminium2% fluoroboric acid

References:

  • Forensic Science by Nordby
  • Forensic Exmaination and Interpretation of Toolmarks by David Baldwin et.al.
  • Handbook of Firearm and Ballistics by Brian J Heard
  • Forensic Examination of Unusually Firearms by J.K. Sinha

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