Surface Preparation
Before applying chemical methods, the metal surface needed to be polished. It is the second step after photography. Following are procedures:
- Remove all scratches and other gross marks with a fine emery paper.
- Polished the surface to a mirror like finish.
- Final polished surface finish should be done by using jweller’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 surface.
Procedure:
- Apply the respective chemical reagent using cotton wool swab.
- Rinsed the surface with acetone, after application of one minute.
- Numbers should begins to appear which can be make more visible with oblique lightining.
- If NO impression is visible, reapply the reagent and after 1 minute rinsed the surface with acetone.
- 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
| 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 Cheimcal 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 storing in glass containers. |
| Hume-Rothery Reagent | 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 | reaction is very fast and lower obervering 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 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
| 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 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