The bust is 28 inches tall and weighs 25 pounds, including its brass supporting structure, D'Angelo said. Made by Jean-Antoine Houdon in 1778, while Franklin was visiting Paris, the bust is one of four in existence, he said.
"It sat on the pedestal and just dominated the drawing room," D'Angelo said.
The three other busts are at the Philadelphia Museum of Art, the Metropolitan Museum of Art in New York, and the Louvre in Paris, according to D'Angelo. The Louvre version is made of terra cotta; the two others are marble.
When the marble bust was donated to the Philadelphia Museum of Art, it was valued at about $3 million, said D'Angelo, who believes the stolen work is worth at least as much.
It is unique, he said, because Franklin's expression seems to vary from different angles. The work is part of a collection of historical objects his family has collected, D'Angelo said.
Lawton and another woman were seen leaving the house Friday by a staff member, D'Angelo said. But because both had been recent employees there, their presence did not raise suspicion, he said.
The residence's housekeeper soon realized that the artwork was missing.
"It was a great shock," said D'Angelo, who reported that the intruder or intruders entered through a sun-porch window.
Township spokesman Thomas Walsh said Lower Merion police were working with Philadelphia authorities on the case. It was not known Saturday whether police were searching for a second suspect.
D'Angelo said he was considering offering a reward as an incentive for the safe recovery of the historic Franklin bust.
"I just hope it's returned," he said. "It's our history - American history."
Contact Jonathan Lai at email@example.com or 215-854-5151, or follow on Twitter @Elaijuh.