It had been awhile since I'd caught up with Philadelphia developer/contractor John Fries.

In the early years of the last decade, I wrote about his efforts, in partnership with fellow developer Blake Ingram, to convert a former Methodist church in the city's Wissahickon neighborhood to condominiums.

Fries also was the contractor on several renovations I wrote about even earlier, including a Society Hill project converting a Federal-era single-family home turned to multifamily use in the 1960s back into a large single home.

Fries acknowledged that he was still working as a small-scale developer, renovating a rowhouse on Hancock Street in Fishtown, and that he had done another recently on Frankford Avenue for his son, a Temple student "who is paying me rent."

But it was his four-year-old "hobby" that intrigued me. Fries and Doug Ganci, whom he called "my mentor," excavate backyard privies, looking for hand-blown glass bottles that were produced before about 1860.

Ganci, a.k.a. Philo Gideon, is a scientist by training and the author of Cullen's Egg Bottle, which uses archaeological evidence to trace the origins of the commercial soda-water industry. He started digging through privies at age 19, "and I'm 65 now," the Ohio native said.

The hand-blown glass bottles Fries and Ganci seek have marks at the bottom made by iron rods called pontils, which held the glass in place as the bottles' tops were tapered.

A scar or mark on the base of a bottle, usually with a small deposit of iron from the tool, remained after the pontil was removed, Fries said.

That was actually the third of three phases in the evolution of bottle-making, Ganci said: the first, a sand pontil "using little grains of sand;" the second, "reversing the blow pipe, leaving a circular mark at the bottom."

"All pontils were eliminated by 1857, and a snap case with a spring that held the bottle in place was used," Ganci said.

Bottles were machine-made by 1900.

Fries emphasized that he and Ganci don't sell the bottles, they collect them for historical research.

"This is not a profit-making enterprise," said Fries, who has dug 100 privies since he came upon a crew from Baltimore excavating one outside the post office in Francisville in winter 2009.

"The real value of what I write about is the flowing of the story of our country," Ganci said, "using artifacts whose importance is more valuable to that story than the greatest paintings hanging on our walls."

Ganci and Fries sought and received permission to dig from the developers of a project on Spruce Street that was converting adjacent buildings with five apartments back into two single-family houses.

Both houses dated from the 1830s, in the right ballpark for bottle-searching, Fries said. (In fact, they discovered several Philadelphia-made mineral-water bottles there.)

Yet as old as these houses are, they replaced buildings built a century or more before, occupied by artisans or less affluent Philadelphians. One of the Spruce Street developers, Murat Aslan, is keen on historic restoration. He took bits of 18th-century redware pottery Fries and Ganci found and reassembled them as decoration.

After two centuries of dealing with the smell of privies, especially in the summer, the city switched to sewers - first made of terra cotta, then cast iron - in the 1880s, Fries said.

"Homeowners were required to clean out their privies, but a lot of them were just covered, leaving a couple of feet of waste that also contained a lot of other stuff they threw away," he said.

There's a lot of history in those layers, Fries said.