When a toilet becomes a time capsule

Boston’s city archaeologist Joe Bagley, standing in a recently uncovered privy hole.

Most people would not be happy to uncover a massive deposit of bathroom waste on the job.

But Joe Bagley isn’t most people, and he doesn’t have most jobs.

Bagley is Boston’s city archaeologist. For him, the contents of a privy (what people called toilets before the age of indoor plumbing) might as well be a gold mine. And right now, he is on the site of what could be one of the biggest privies ever found in this city: Behind what used to be a small home for impoverished girls in Dorchester.

“As an archeologist, 99 percent of what we look at is garbage,’’ Bagley says. “And so we want to go where we find the most trash, because trash tells the secrets.’’

They often find the best trash in privies, which served as dumping sites for all kinds of waste — household trash, illicit materials where no one would ever want to look, and, of course, the stuff privies were built to contain. Privy holes are excellent preservers of organic material — fabric and wood, for instance — that would otherwise decompose.

What was once a toilet is now a time capsule.

“They’re the greatest thing ever in historic archaeology,’’ Bagley says.

In Boston and beyond, privies have produced significant historical finds. The privy of Katherine Naylor, who lived in Boston’s North End in the 17th century, contained many items that Puritans would not have been expected to own: expensive fabrics, fancy shoes, ornate glassware, and a wooden bowling ball.

The last item was especially exciting: It is the oldest bowling ball ever found on this continent. Adding an extra layer of interest was the fact that it was illegal for Naylor to own it at the time (the Puritans frowned on fun).

There were also plum and cherry pits and parasite eggs, signs of what city dwellers of the time ate, and health issues they faced.

This summer, Bagley and a small team of assistants have been digging in the backyard of 232 Centre St. in Dorchester, behind a large brick building that now belongs to Epiphany School but used to be the Industrial School for Girls. Founded in the 1850s, it housed around 30 young ladies aged between 10 to 15 — though some were as young as 6.

It was probably not a happy place. Girls were sent to live there by their families when they couldn’t take care of them. They were very poor, mostly immigrants, and relied on the charitable donations of the rich Beacon Hill residents who supported the school.

But little else is known about the students. They lived in poverty, and therefore, often, obscurity. There are no tax records, Bagley says. Census records give some sense of the population, but they were only collected every 10 years, skipping over any students who attended in between decades.

Bagley wasn’t sure where to find the privy. The lot is too big to dig up entirely, so he had to make his best guess. Old documents showed a carriage house used to be on the property, and its general location. Bagley suspected the privy would be located either within or adjacent to it.

Screening privy contents for more finds.

He was right twice over: The carriage house foundation was roughly where he expected it to be, and a double brick wall jutted out from it. It was the privy.

Bagley’s team has spent the last week excavating two pits. There is space for a third, though it’s not yet known if there is a pit there or if it just served as a waiting area. If it is a third pit, Bagley says it will be the largest privy yet discovered in the city.

The pits are nearly two meters deep, and were mostly backfilled with sand when the privy fell out of use. But there, at the bottom, Bagley’s team found what it sought: Mixed within what is probably 150-year-old fecal matter was the evidence that this was once a girls’ school. (Bagley said the “night soil’’ didn’t smell, although privy remnants often do. Even Naylor’s privy was pungent, 400 years after the fact.)

So far, finds from the privy include beads, buttons, tiny teacups, sewing pins, a slate pencil, hair bands, and parts of dolls. Bagley suspects the nearly identical buttons and hair bands were part of the uniform the girls wore. The dolls are all different, he says, suggesting that they were among the girls’ few personal belongings, and may not have been issued by the school.

One thing the dolls have in common, Bagley says: “They are always creepy.’’

Some of the privy’s treasures: a broken mug, a small intact plate, a marble, and a creepy doll’s head.

Other privy contents also provide valuable information. Animal bones will show which animals they ate and which cuts of meat. Seed casings and bug carcasses will show other parts of their diet. Parasites will give a sense of their health.

Bagley, 30, became Boston’s city archaeologist in 2011 but participated in his first dig in 2002, when he was just 17. He then studied archaeology at Boston University and UMass Boston. During the recession, when finding a job in archaeology was almost impossible, he became a professional papercut artist.

He loves his job.

“It’s outdoors, it’s fun, it’s random so you never know what you’re going to find or what you’re going to do,’’ he says. “I really like digging, it’s fun. I like getting messy. I don’t like wearing a suit.’’

This might be Bagley’s best dig yet.

“I’ve been on a lot of digs where we’ve found a lot of cool stuff and there’s a lot of really cool sites that I’ve been on,’’ Bagley says. “The difference between this one and all of those is that this one started off with me looking at the history of the site and going, ‘God, this would be a really interesting site if we actually found anything’ … And then we actually found something.’’

Once the dig is complete, Bagley estimates he’ll have a year’s worth of research to do. He hopes that by this time next summer, he’ll have finished a report on the lives of the Industrial School for Girls’ students, the most comprehensive record of such to date.

“We’re trying to give them more of a voice,’’ Bagley says.

With that, he climbs back down into the privy, and looks for more.

Photos: When Boston was a Big Dig of a mess

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