The critical moment in removing 17 tons of blasting powder from a ramp on the Springfield interchange came when emergency crews approached to undo the latch on the container of the overturned truck -- a step that fire and rescue officials were willing to take only after 12 hours of planning for it.

The worry: that the volatile powder had spilled out of some of the 680 plastic-lined boxes inside the cargo container, and that simply moving the latch would provide enough friction to set it off, or that a static spark from some other source would do the same.

"It would be a substantial explosion," said Brad Earman, a spokesman for the Washington field office of the Bureau of Alcohol, Tobacco and Firearms, which brought equipment for dealing with explosives to the scene to help emergency crews.

"Something as small as a spark or a static electricity charge could set it off," Earman said. "We're taking the utmost precaution."

That meant painstaking preparation. Crews who worked closest to the explosive were dressed in static-free clothing and used grounding materials and brass tools to prevent sparks. Water was sprayed on the container to keep it cool.

A crew finally opened the latch and was relieved to see that some of the packages were crushed but that neither black powder nor black powder dust was strewn throughout the 20-foot container.

The 34,000 pounds of black powder was imported from Brazil. It was shipped by sea to Newport News, Va., where it was strapped on the back of a flatbed truck and sent on its way to Vermont.

Oscar Rodes, president of Petro-Explo Inc., the company in Arlington, Tex., that imported the explosives, said the powder was purchased by Austin Powder Co., based in Cleveland, and was going to be used in the mining of slate.

Black powder, the first chemical explosive, was invented in China about 1,000 years ago by combining sodium nitrate, charcoal and sulfur. It was used mainly in warfare until the 17th century, when miners began using it industrially.

Pound for pound, the explosion caused by black powder is less intense than that caused by dynamite, TNT or other so-called high explosives. But black powder is far more volatile because it doesn't require a shock from a blasting cap or similar source to set it off.

When black powder is loose and catches fire, it burns fast and hot. But when it is held in a container or packaged in large quantities as in the truck, a spark can cause a violent explosion, officials say.

For safety, Rodes said, the explosive was packed in plastic bags and stored in 50-pound quantities inside plastic-lined cardboard boxes. The boxes were then packed, according to Department of Transportation regulations, inside a 20-foot, wood-lined container, he said.

Each year, industry experts say, 5.5 billion pounds of explosives are used in the United States. Much of that product is shipped on the nation's roads by truck.

There are no state or federal regulations prohibiting the transport of such explosives on major highways such as Interstate 95, nor are there restricted hours for transport.

The limitations on hazardous material transport in Virginia include some restrictions on the amount and type of material passing through tunnels, such as those in the Hampton Roads area, and on some neighborhood streets.

There are, however, strict controls on how explosives are to be packed for transport. In the case of black powder, the boxes it is shipped in cannot be constructed with metal staples, but are glued together. Also, drivers of hazardous material rigs are required to receive special training within 90 days of being hired and receive additional training at least once every three years.

"Normally, explosives are highly regulated by the Department of Transportation," said Chris Ronay, president of the Institute of Makers of Explosives, the safety association of the commercial explosives industry. "It's one of the most highly regulated industries in the world."

Staff writer Alan Sipress contributed to this report.

CAPTION: A truck carrying 17 tons of blasting powder lies overturned on a ramp as a car passes on the outer loop of the Beltway.

CAPTION: Workers deal with a cargo of blasting powder on a truck that overturned at the Springfield interchange. The cargo container was opened only after 12 hours of preparation.