The U.S. Army has destroyed about 90 percent of its aging chemical weapons after it wraps up work this week in Utah, where it has kept its largest stockpile -- a witches' brew of toxins, blister and blood agents that accumulated through the Cold War.

The Army's Deseret Chemical Depot in Utah's west desert burned its last hard weapons in a 1,500-degree furnace on Wednesday -- projectiles that contained mustard agent, which can produce painful skin blisters. The last tray of 23 projectiles came out of a furnace at 2:11 p.m. after baking for two hours, a process that rendered the mustard agent harmless.

The depot -- which at its peak held some 13,600 tons of chemical agents, making it the world's largest -- expects to complete the job by the weekend when it incinerates bulk supplies of Lewisite, a powerful skin, eye and lung irritant.

"It gives me great joy and satisfaction to be done," said Ted Ryba, the Army's project manager at the depot, after the last of the mustard agent projectiles were seen emerging from the furnace on a conveyor belt.

The U.S. is part of an international treaty to rid the world of chemical weapons, a campaign taking place with spotty success around the globe. The goal was supposed to be accomplished by April 29 but will take years longer.

"Clearly, it's still a tremendous example of what the world can do," said Craig Williams, director of the Chemical Weapons Working Group in Berea, Ky., an advocate for safe disposal. "You've got 188 of 194 countries on the planet signing the treaty. It's an impressive effort, a great step forward for the safety of the world."

The U.S. has acknowledged it will take as long as 2021 to finish destroying the final 10 percent of its chemical weapons at depots in Pueblo, Colo., and Richmond, Ky. Russia is farther behind in its effort, having destroyed only about 48 percent of a large cache of chemical weapons, according to the Organisation of for the Prohibition of Chemical Weapons in The Hague, Netherlands.

An international tribunal voted last month to waive trade or other sanctions and instead subject the U.S. and Russia to increasing pressure and inspections. Each country must submit plans by April 29 detailing how they will finish the job "in the shortest time possible."

A third country, Libya, also is expected to miss the deadline. The recent uprising in Libya interrupted that country's work and exposed more chemical weapons depots than were thought to exist, Williams said.

In the U.S., the Army has finished destroying chemical weapons at depots in Anniston, Ala.; Pine Bluff, Ark.; Newport, Ind.; Aberdeen. Md.; Umatilla, Ore.; and a Pacific atoll where the work started in 1986, according to the Army's Chemical Materials Agency.

That leaves a stockpile of mustard agent in Pueblo., Colo., and a mixed inventory of mustard and nerve agents at Kentucky's Blue Grass Army Depot.

The Deseret Chemical Depot in Utah once contained 44 percent of the nation's supply of chemical agents. The depot didn't just hold obsolete U.S. weapons. A supply of nerve agent seized from Nazi Germany at the end of World War II was destroyed only months ago.

The heavily guarded Utah incinerator sits in the middle of a desolate base of nearly 3 square miles, surrounded by barbed wire and chain-link fences in remote Rush Valley. Underground bunkers were used to store the explosive shells, mortars, land mines, projectiles, rockets, spray tanks for use by war planes and bulk storage containers.

The rusting weapons had seals that often leaked, sounding alarms. It began almost routine for workers in gowns and breathing masks to have to enter bunkers to package the "leakers." In 2002, a pipefitter was exposed to nerve agent but recovered and was back on the job the next day. Two years earlier, the incinerator was forced to shut down for a summer after a drop of GB nerve agent escaped the emissions stack, which got a new safety valve.

Chemical weapons were introduced into warfare during World War I, killing 90,000 troops on battlefields, according to the Organisation of for the Prohibition of Chemical Weapons. More recently, the regime of former Iraqi dictator Saddam Hussein lobbed a chemical cocktail on a Kurdish town in 1988; that became the inspiration for the 1997 treaty to rid the world of chemical weapons.

As far as is known, the U.S. has never fired a chemical weapon in anger, although some consider the use of the defoliant Agent Orange during the Vietnam War a chemical attack, Williams said.