Scientists make use of a curious fact involved in firing to test it. Each cartridge used in firing contains a propellant charge or powder which burns during he firing action. The commonly used powders are either black powder or smokeless powder. Black powder, also known as gun powder, consists of charcoal (15%), sulphur (10%) and potassium nitrate (75%). When ignited, it produces a lot of smoke. Smokeless powder contains either nitrocellulose alone (in which case it is known as a single base powder) or nitrocellulose mixed with nitroglycerine (in which case it is known as double base powder).

Besides the propellant mixture, the cartridge also contains some special chemicals called the primer. These chemicals have the special property of getting ignited when subjected to great pressure. Several types of primers may be used, but the most commonly used are lead azide, lead styphnate, mercury fulminate, barium nitrate, potassium chlorate and antimony sulphide. In fact, when the trigger of a gun is pressed, the firing pin strikes the cartridge at a point where the primer is kept. This cause the primer to ignite. The flames thus produced ignited the propellant charge. The burning of the propellant charge produces large amounts of gases, which finally propel the bullet.

So much so about the primer and black powder. Now how does the scientist make use of these facts? Actually when a gun is fired, the products of combustion (of propellant and primer) come out of the barrel of the gun. Some may leak through the back of the gun and may get deposited on the hands of the person who has fired. Depending of whether the gun was handled by the right or the left hand, the products of combustion get deposited on certain specific areas of the hand which are exposed (fig 1).

Now if somehow, one can detect the products of combustion from the hands, one can be in a fairly good position to say whether the accused did handle the gun. The first such idea occurred to Teodoro Gonzalez of the Criminal Identification Laboratory, Mexico City police headquarters, as far back as in 1933. He employed, what later became famous as the "paraffin test", also known as the "Dermal nitrate" or "diphenylamine test". In this test, the hands of the suspect are coated with a layer of paraffin. After cooling, the casts are removed and treated with an acid solution of diphenylamine. This reagent detects nitrites and nitrates that originate from gun powder and are deposited on the hands of the criminal. A positive test is indicated by the presence of blue flecks in the paraffin.

In 1959, two scientists Harrison and Gilroy introduced another test. This test was designed to detect chemicals such as barium, antimony and lead. Those chemicals are produced by the burning of the primer and may get deposited on the hands of the criminal. This test, too, is used frequently to determine whether a criminal indeed fired a shot. In this test, a square of white cotton cloth is moistened with hydrochloric acid. This is a common chemical used in most laboratories. Most people know it by the name tezaab, which is sometimes used to clean toilets. However, the acid that is used in the test is quite diluted; so it does not burn the hands of the suspect. The cotton cloth, moistened with this diluted hydrochloric acid, is used to swab the hands of the suspect. If the suspect has fired the gun, the particles of the chemicals mentioned above will get transferred to the cotton. The cotton swab is then treated with a special chemical triphenyl methyl arsonium iodide also known as "antimony detector" (because it detects antimony). To be more accurate, the cotton swab is split into two. One half is treated with the above chemical, which detects antimony and the other half with another chemical called sodium rhodizonate. This chemical detects the presence of barium and lead, by changing the color of the cotton, treated with either chemical changes, then it is confirmed decisively that the person is the culprit.

Several times, I have been asked by the police to opine if a particular criminal fired the fun or not and I have used these two tests to my benefit with great accuracy. Most of the time, criminals have been prosecuted on this evidence. SO next time when you read in newspapers that the scientists determined the actual gun man by conducting certain tests, don't be in awe. You know exactly what chemicals and tests have been used. Who knows, you might even give a hint or two to the police!

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