Acidic numbers

Serial numbers on a stolen item may be removed in order to conceal its identity. A study focusing on the restoration of obliterated serial numbers is using an acid etching technique in a bid to restore identification details.

Steve Nunn, from the University of Derby’s department of biological and forensic sciences, is carrying out research that could help investigators identify stolen items which have had serial numbers concealed by criminals.

The study is already showing signs of success. “It has been shown that there is no statistical difference in the time it takes to restore an obliterated number when the amount of acid is varied,” said Mr Nunn.

“Our research has also shown that the letter that is being restored has an effect on the quality of the marks visible after restoration.”

Mr Nunn’s study focuses on restoration of obliterated serial numbers using an etching technique, a method known as ‘Fry’s Reagent’. Fry’s Reagent is the term used for the liquid solution that is applied to the area of interest on a carbon steel item. The solution is essentially a strong acid that is used to attack the metal and reveal areas of differing densities that are caused by the initial stamping process. In doing so, the acid will remove the uppermost surface – less compressed – allowing for any recoverable evidence to be visualised.

The technique has been used by forensic scientists and police forces for many years with great success. It has proved useful as it enables the investigator to reveal the true origin/identity of a recovered item.

However, Mr Nunn’s work goes further as it reviews the current method in a bid to determine the optimum conditions for its use. “The most common items that would be marked with serial numbers are vehicles and firearms,” he said. “These items would be encountered in the investigation of organised crime but the technique may also be applied to lesser crimes. These include thefts of items with a lower value such as computers, jewellery and any other items which have an identifying mark stamped into a metal component.”

The acid etching technique works as the area of metal surrounding the stamped letter is less compressed, and therefore more easily removed by the acid etching solution.

My Nunn carried out restorations of obliterated numbers using etching solutions containing varying amounts of acid.

Method

Once the numbers had been created, they were obliterated using a high-speed grinder. The surfaces were then polished using abrasive papers.

“The etching solutions were then applied to the obliterated numbers for two minutes using a cotton swab, followed by 15 per cent nitric acid application for one minute,” said Mr Nunn.

“If the number was not visible the process was repeated up to 15 times. If the letters could be seen, each one was given a score from one to three, with three representing the best restoration and one representing an unclear restoration. If the number was not visible after 15 applications then it was given a score of zero.”

Results

The experiment used the letters S, W and N. The acid etchant solution contained varying amount of acid ranging from 30ml to 50ml, in increments of 5ml.

“The letter S was more likely to be recovered than the other two letters and that the etchant solution containing 45ml of acid gave the fastest recovery times,” said Mr Nunn.

He then conducted the Mann-Whitney U-test – assessing whether two independent samples of observations come from the same distribution – to determine that there were significant differences in the quality of restoration between the letters.

“I then performed an ANOVA test – a statistical test of whether the means of several groups are all equal – where the amount of acid was compared to the number of applications required for each restoration. I found that none of the solutions restored the letters in a significantly lower time, meaning that no solution can