Hazards Associated with Old Copper Based Blasting Caps
In this article we examine an explosives safety issue associated with older blasting caps known as the copper azide effect. Early designs (1918-1955) of blasting caps included copper shells filled with lead azide. When moisture comes into contact lead azide, a reaction occurs and hydrazoic acid is formed. Hydrazoic acid is gas at room temperature that is highly toxic and explosive. When in contact with copper, highly sensitive copper azides are formed. Copper azide is more impact sensitive, less friction sensitive, and orders of magnitude more static sensitive than lead azide. Further hydrolysis and oxidation creates what is known as explosive salts.
Explosive salts can form around various parts of the blasting cap including on the outside of the copper shell. Small amounts of static electricity such as that produced by the rubbing of clothes can be enough to initiate copper azide. UXO technicians dealing with old copper blasting caps should look out for discolorations in the copper such as a yellowish / green to blue / green color tarnish. If a cache of unopened blasting caps are found, the technician should be aware of the packaging used in the early blasting caps as an indicator that copper caps may be present. Early blasting caps were packed in cardboard boxes or tin cans, usually packaged in quantities of 100. The containers typically had the manufactures name, quantity of caps, and a general warning label printed on them. Copper azide can also be detected using a solution of 1 percent Ferric Chloride which forms a bright red dot in the presence of copper azide.
The threat posed by old copper based blasting caps can be significant. In 1997 two separate incidents occurred resulting in three civilian bomb technicians getting seriously injured and the death of another. When old copper blasting caps are found on a range, blow in place operations should be the first consideration. If material must be moved, precautions should be taken such as protective clothing and remote handling equipment. Anti-static clothing should be worn along with proper grounding.
