A priming compound is a highly sensitive explosive chemical which, when struck by the firing pin or hammer of a weapon, will explode with great violence, causing a flame to ignite the propellant.
This explosive chemical is often mixed with other chemicals which provide oxygen to assist in the production of the flame, a fuel to increase the length and temperature of the flame and ground glass as an abrasive to assist in the initial ignition of the explosive.
In the realms of forensic science, the detection of primer discharge residue on the hands can provide crucial evidence as to whether a person has recently fired a weapon. To fully utilize the evidential value of gunshot residue (GSR) analysis, a basic understanding of the history, composition and manufacture of primers is essential.
Short history of priming compounds. The earliest priming compound was almost certainly mercury fulminate as used in the Forsythe scent bottle priming system which was introduced around 1806. This compound is highly sensitive and liable to spontaneously explode for no apparent reason. As a result, the Forsythe scent bottle, which required a considerable quantity of this compound to be carried in a container on the side of the pistol, did not achieve a great deal of popularity.
In 1807, Forsythe introduced a priming compound with a formula consisting of 70.6 parts potassium chlorate, 17.6 parts sulfur and 11.8 parts of charcoal. Whilst this was somewhat more stable than mercury fulminate, it was terribly corrosive.
The first real percussion cap (a small metal cup containing the priming composition which was placed on a nipple at the rear of the barrel) was introduced by Joshua Shaw in 1814 and contained mercury fulminate. As a result of the unpredictability of plain mercury fulminate, it was superseded in 1818 by a mixture of mercury fulminate, potassium chlorate, sulfur and charcoal. The residues produced by this mixture were, however, still terribly corrosive, requiring the weapon to be cleaned immediately after firing.
In 1828, Dreyse patented the 'needle gun ' , which had a paper cartridge case with the primer cup inside the case with the propellant. The firing pin on this weapon was a very long thin needle which penetrated the paper case striking the primer within. This primer cup contained a mixture of potassium chlorate and antimony sulfide.
It was found that purification of the mercury fulminate would lead to a more stable compound, and in 1873, a mixture of mercury fulminate, potassium chlorate, glass dust and gum arabic became the standard US military priming compound. This mixture suffered from two major drawbacks: (i) the mercury tended to make the brass cartridge cases brittle, which led to case failure on firing and dangerous leakage of high-pressure gas from the breech of the weapon; and (ii) the potassium chlorate left terribly corrosive residues in the bore of the weapon after firing.
As a result of these problems, the search began for a non-mercuric, non-corrosive primer composition. Early attempts revolved around the use of potassium chlorate as the main ingredient. Potassium chlorate is, however, a fairly unstable material and is very deliquescent, that is, it absorbs water from the atmosphere.
It also forms potassium chloride on decomposition, which is also deliquescent and is very corrosive to the weapon's bore.
Just prior to World War I, it was discovered that thiocyanate/chlorate mixtures were sensitive to impact. These, however, had the same drawbacks as straight chlorate primers, that is, they produced corrosive residues on firing.
The German company RWS was the first to substitute the potassium chlorate with barium nitrate. Lead styphnate was used as the main explosive component giving the first 'rust free ' primer. This was patented in 1928 under the name Sinoxid.
The first true non-corrosive, non-mercuric (NCNM) primers were commercially produced in America between 1935 and 1938. These, however, did not meet the stringent US government specifications as to storage, misfires, and so on, and military ammunition continued to use the old corrosive chlorate mixtures right through World War II.
In the United Kingdom, the change to non- corrosive military primers was even slower, and it was not until the early 1960s that all calibres in military and commercial primers used NCNM priming compounds.
Up to early 2000, the most common primer composition encountered was still the lead styphenate, barium nitrate, antimony sulfide and tetrazine type. In this priming compound, lead styphenate and tetrazine are the sensitive explosive ingredients; barium nitrate provides additional oxygen to increase the temperature of the flame, and antimony sulfide acts as a fuel to prolong the burning time. Aluminium, and occasionally magnesium, can also be encountered, but mainly in the higher-powered magnum pistol or rifle calibres.
Powdered glass was also often added to the mixture to increase the friction and to assist detonation when the mixture is crushed by the firing pin.
Modern 0.22" calibre rimfire ammunition is slightly different in that the composition almost invariably consists of lead styphenate, barium nitrate, tetra-zine and powdered glass.
Lead-free and non-toxic primers. It began to become apparent in the early 1970s that in heavily used training facilities, the range personnel were suffering from the symptoms of lead poisoning. Whilst a large proportion of this lead was being volatilized from the base of the bullets, a portion was obviously coming from the lead styphenate primer.
The US National Bureau of Standards claims that when lead-based primers are used, 80% of airborne lead on firing ranges comes from the projectile and 20% comes from the priming composition. These percentages obviously depend on whether the bullet is plain lead or jacketed. In the case of a non- jacketed bullet, the rifling will strip lead from the bullet' s surface, thus dramatically increasing the percentage of non-primer-based airborne lead.
The change to a bullet with a copper/zinc jacket extending over the base was a fairly simple matter of reducing the bullet sourced airborne lead, but finding a non-mercuric non-corrosive non-lead-based primer was another.
The problem was first solved in the early 1980s by Geco, who released a zinc- and titanium - based primer which they called " Sintox'. Since then, there have been a number of other lead-free primers produced by, for example, CCI Blazer, Speer, Federal and Winchester (Haag, 1995). The exact composition of the priming compounds used is not available, although SEM/EDX (scanning electron microscope/energy dispersive X-ray) analysis generally shows the presence of strontium in the Speer and Blazer cartridges, potassium in the Winchester cartridges and calcium and silicon in the Federal cartridges.
Most of the more recent primer formulations contain an initiator explosive compound called 'dinol', the chemical name of which is DDNP (diazodinitrophenol).
Other initiator explosives include:
• dinitrodihydroxydiazobenzene salt (diazinate);
• dinitrobenzofuroxan salts;
• potassium dinitrobenzofuroxan;
• various diazo, triazole, and tetrazaole compounds;
• perchlorate or nitrate salts of metal complexes of ammonium, amine or hydrazine an example of which is 2-(5-cyanotetrazolato)pentaaminecobalt III perchlorate (CP).
• potassium nitrate;
• strontium nitrate;
Fuel components include:
• amorphous boron;
• metal powders, such as aluminium, zirconium, titanium, nickel and zinc;
• metal sulfides such as:
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