Surface Preparation
Before applying chemical methods, the metal surface needed to be polished. It is the second step after photography. The following are the procedures:
- Remove all scratches and other gross marks with a fine emery paper.
- Polished the surface to a mirror-like finish.
- The final polished surface finish should be done by using jeweller’s rouge or flour paper.
- 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 the surface.
Procedure:
- Apply the respective chemical reagent using a cotton wool swab.
- Rinsed the surface with acetone, after application of one minute.
- Numbers should begin to appear which can be made more visible with oblique lightning.
- If NO impression is visible, reapply the reagent, and after 1 minute rinsed the surface with acetone.
- Repeat in 2min, 5mins, and 10 mins for up to 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
| Reagent | Chemical Components |
|---|---|
| Nitric Acid | 1% to 15% Nitric Acid in Water |
| Fry’s Reagent | 90g cupric chloride, 120 mL HCI, 100 mL H2O |
| Turner’s Reagent | 2.5g to 5g cupric chloride, 40 mL HCI, 25 ml ethyl alcohol, 30 mL H2O |
| Davis Reagent | 5g CuCl2 + 50 mL HCI + 50 mL H2O |
| Reagent 1 | g ferric chloride, 50 ml hydrochloric acid, 100 mL H2O |
| Chromic acid | 40g chromic acid, 50 mL H2O |
| Picric Acid | saturated solution of picric acid in ethyl alcohol |
Table B: Etching Chemical for Aluminium and Aluminium Alloys
| Reagent | Chemical Components | Remarks |
|---|---|---|
| Villela’s Reagent (Nickolls, 1965) | 30 ml glycerine, 20 ml hydrofluoric acid, 10 ml nitric acid | best reagent, but very corrosive and should avoid stored in glass containers. |
| Hume-Rothery Reagent (cast having high silicon alloy) | 200 g cupric chloride, 5 ml hydrochloric acid, 1000 ml water | safer alternative but not efficient as Villela’s reagent, |
| Fry’s Reagent | 90g cupric chloride, 120 mL HCI, 100 mL H2O | the reaction is very fast and lower observing time. |
Table C: Etching Reagents For Zinc And Brass
| Metal | Chemical Components |
|---|---|
| Zinc | 50% hydrochloric acid + 50% nitric acid |
| Brass | 40 g cupric chloride, 150 ml hydrochloric acid, 50 ml water |
| Brass | 19 g ferric chloride, 6 ml hydrochloric acid, 100 ml water |
Electrolytic Method of Restoration
In this technique, the number plate/specimen is attached to an anode in an electrochemical cell arrangement. It is used for polishing the surface before the application of chemicals (also called electropolishing).
When a minimum DC voltage (critical voltage) is applied, minute irregularities on the surface get dissolved, leaving a highly polished surface.
- Anode: restoration plate specimen
- Cathode: cotton swab containing an electrolytic solution
- Critical DC Voltage: defined as the minimum voltage necessary for the current to flow.
- Steel: 6V
- Brass: 7V
- Aluminum: 7.5V
Reagent Used After Electropolishing
| Metal | Chemical Reagent |
|---|---|
| Steel | 1 g cupric sulphate, 15 ml sulphuric acid, 1 g gelatin, 500 ml water |
| Steel | 5 g cupric ammonium chloride, 50 ml hydrochloric acid, 50 ml water |
| Aluminium | 17% sulphuric acid |
| Aluminium | 2% fluoroboric acid |
References:
- Forensic Science by Nordby
- Forensic Examination 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