Chicago ? For some 67 million years, the Tyrannosaurus rex now known as “Sue” was allowed to sit in the South Dakota ground, layer upon layer of rock attaching to its bones.

Now, picked clean, patched and perched in the Field Museum’s vast entry hall, those same bones can’t go six months without being brushed free of accumulating sediment.

Visitors, on the semiannual cleaning days, get a surprise glimpse behind the curtain at a leading public attraction. And Sue, the 42-foot long, most complete T. rex skeleton ever found, gets to glisten a bit, the brown of its approximately 321 bones a little less gray.

You don’t do a job like this with Windex and paper towels. You don’t do it with Chuck from maintenance, either — no offense to Chuck.

Cleaning crew at work

So one day recently, Bill Simpson, a high-ranking Field paleontologist, was standing on an UpRight MX 19 micro-scissor lift, 6 1/2 feet in the air, doing housework.

“It’s the only specimen I clean,” said Simpson. “One could make a case for this being the most famous fossil in the world. It’s such a high-value object that we don’t want (the) exhibits (department) to have the liability.”

How do you clean a dinosaur?

First, you call security and get the alarms turned off.

“Could you repeat that?” said the guard’s voice on the walkie-talkie speaker.

“You’re going into the Sue corral? Stand by. I’m going to disarm it.”

After that, though, the answer is, perhaps, not as high-tech as you’d expect for an exhibit bought at auction in 1997 for $8.4 million, the Rosetta Stone for research on the species.

The only one to clean Sue in its Field tenure, Simpson, manager of the geology department’s 100,000-specimen fossil vertebrates collection, has settled on doing it with a portable vacuum cleaner, set to blow rather than suck, and a synthetic feather duster.

For almost two hours, Simpson first huffed and puffed with his DeWalt portavac, better than the cans of compressed air he used to use. The resultant gentle dust shower dropping to the “land form” on which Sue is mounted brought to mind dirty snow or, if you stretched a bit, volcanic ash covering Pompeii.

At each of eight or nine stops around the display’s perimeter, the highly trained paleontologist would then stroke the great beast with the duster, and not just any old one.

First, Jasna Srndic from the exhibits department, Simpson’s aide and scissorlift driver on the project, came out with a bright orange one. After trying it on the plastic skull — the actual fossil head, too heavy to mount gracefully, is on display upstairs — Simpson asked her to go back and get “the duster that’s my favorite.”

The right tool for the job

As he stood on the battery powered lift, a sort of platform on wheels, he explained his temporary inactivity.

“We’re waiting for a feather duster,” he said. “I have my standards.”

When he at last had the preferred lavender and pink model in hand — one that he chose for its flexibility from among the exhibits department’s proffered tools at his first Sue cleaning, back in 2000 — he resumed the work in motions that resembled nothing so much as petting.

“Nice dinosaur,” you could imagine his last words being. Indeed, while Simpson was working the teeth, Anthony Turner, a school bus driver and teacher’s aide helping escort a school group from Hillside, Ill., said, “That’s cute. I like that.”

Then he seemed to reconsider: “I’d hate to be in those jaws back in the day.”

At the dinosaur’s delicate “fingers,” Simpson used his own, rubbing the bone clean by hand. “There’s no elasticity,” he said. “They’re just thin pieces of rock at this point.”

Walking with the dinosaur on the land form, he stood up inside the rib cage. He poked the brush into the nooks and crannies like a hungry man buttering a fork-split English muffin.

Out of the ankle joint he coaxed another species, commonly referred to as the “dust bunny.”

The giant, saucer-shaped hip bones, were nothing you’d want to eat out of, either. “The hips always get the dustiest,” Simpson said.

Despite the name, Sue is not necessarily female. Its age — about 28 — is known; its sex is not. It’s called that after its discoverer, Susan Hendrickson, an amateur fossil hunter who decided to look around when she got a flat tire one day in 1990 in the Hell Creek Formation in western South Dakota.

The Field, with help from McDonald’s, bought the late Cretaceous period skeleton at a Sotheby’s auction, dug out the fossil and had it ready for display by May 2000.

It has been a huge hit, seen by more than 7 million people. Two replicas travel the globe, earning revenue for the museum and renown for Sue.

The recent cleaning came about because the exhibits department, anticipating spring break crowds and dust, asked. There’s no set schedule, Simpson explained, just a sort of compromise between exhibits, which would like to clean it more, and geology, which would like to touch it less.

Risky business

“We are putting it at some risk when we clean it,” he said.

Seeing him and Srndic up on the lift, the platform’s extension slid toward the skeleton and Simpson extended further outward to reach the tip of the tail in an almost balletic pose, you couldn’t help but think about the dinosaur disaster scene in the great screwball comedy “Bringing Up Baby.” In Howard Hawks’ 1938 film, socialite Katharine Hepburn is on a swaying ladder, talking to paleontologist Cary Grant on a raised platform. She slips and brings down the brontosaurus skeleton Grant has been painstakingly assembling.

Neither Simpson nor Srndic — and perhaps this is a good thing — has seen the film. But, Simpson explained, it would take more than a skinny movie star falling to take Sue down.

“Oh, it’s stable,” he said. “I don’t think we could knock it over if we wanted to. Sometimes I wish I could push a button and have the bones disappear temporarily so people could see the artistry of the bent metal that supports it.”

While Simpson worked, visitors didn’t ask as many questions as they usually do, he said, but they were impressed.

“We picked a great day,” said Brian Konen, in from Sugar Grove, Ill., with his wife, Ann, and three young kids.

“I don’t know if I’ve ever seen it clean before.”

“Do you clean your room as good as he’s cleaning Sue?” volunteer docent Donna Stone asked a group of schoolchildren, most of whom claimed they do.

“I’ve been here three years, and I always come to watch him clean,” said Kenny Addison, a museum store employee. “That’s the guy you want in there. He knows what spots to hit and how to hit ’em.”

After it was over, as Srndic walked around the land form vacuuming up the fallen dust, Simpson said he enjoys the cleanings, partly because he’s out in public, partly because it challenges a stereotype: “I think a certain segment of the public thinks paleontology is about dusty old bones,” he said. “And, you see, we do our best to get away from that.”

Cleaning Sue, he added, is not even close to the dirtiest job he’s had. During a break from college, he was hired to clean up an old tire plant.

“Now, that,” he said, “was dust.”