By Carolanne Sacry Feiste, NAVAIR Weapons Divison

A common household product used to remove the hard-water ring from lavatory basins has a new use. Pumice, a natural occurring foamed volcanic glass with thin membranes enclosing air-filled cells, is strong yet lightweight as a sponge.

Fifteen years ago, a dedicated ordnance research team, headed up by Carl Halsey, began conducting tests on pumice at the NAVAIR Weapons Division China Lake, Calif. The tests were to determine the feasibility of using pumice as a buffer to suppress sympathetic detonation (the phenomenon in which the detonation of one explosive item causes the detonation of adjacent items). If one bomb explodes near others, there will often be a ripple effect, hence the term sympathetic detonation. It not only creates a larger explosion, it destroys valuable assets and dramatically increases the threat to life. The stored missiles and bombs on board a Navy ship, for example. If one piece of ordnance detonates, the risk of sympathetic explosions is significantly increased from fragments, the blast wave and heat.

The research thus far has led to profound changes in the technology and application of pumice for the protection of government assets and personnel.

In one of the test using live ordnance with pumice-filled boxes between the bombs, Halsey’s group determined that pumice provided heightened blast and fragment protection. Small (about 3/8-inch-diameter) pieces of pumice can be coated and bonded together with epoxy and formed around containers or poured into hollow cavities - providing a surprisingly strong shield around the explosive items.

In another test, a container with a donor explosive was placed only three inches from an acceptor explosive - both packed within the bonded pumice. The donor explosive was detonated but the acceptor had no reaction.

The Navy, Air Force and Army currently use pumice in portable explosives storage magazines and hand-carried explosives boxes.

The term “hazard footprint,” describes the size and shape of the area determined as “unsafe” around an explosion. No vehicles or personnel allowed within the hazard footprint.

The ordnance evaluation branch at the Weapons Division China Lake conducted further tests in which a large explosive charge was detonated next to a concrete wall. Potentially lethal concrete fragments were thrown 800 feet, creating a very large hazard footprint.

The test was repeated, using the same size explosive charge and at the same distance from the wall - except this time the wall was made of pumice. The explosion produced a small indentation in the earth where the wall had stood, and all that was left was a dusting of gray powdered pumice. There were no fragments at all: the hazard footprint was virtually eliminated.

Other tests showed that incredibly high temperatures are needed to noticeably affect pumice. With an extremely low thermal conductivity, pumice is very difficult to burn. It would be better described as “melting” rather than burning, and then only at temperatures in the neighborhood of 2,500 F.

Halsey continues to have great visions for pumice and its use, including structure and personnel protection as well as anti-terrorist applications. He has designed protective barriers for buildings, and boxes that can be placed over potentially explosive devices, allowing security personnel to clear the area and assess the risk.

The bottom line…pumice can provide protection from fire, lower the risk of sympathetic detonation and property damage plus prevent the loss of human life.