ASHKELON, Israel — To the naked eye, the white, powdery substance appeared to be plaster. That’s what the professional and volunteer archaeologists at a dig in Israel concluded.

To be certain, though, they subjected the chalky dust to spectroscopy and a petrographic microscope, only to discover that it was not a manufactured substance, but decayed plant life and fecal matter.

What that meant to the archaeologists from the Leon Levy Expedition to Ashkelon — a former seaport south of Tel Aviv that was home to successive civilizations over thousands of years — was that structures thought to have been inhabited by people were more likely occupied by animals. That revelation upended their view of what they were excavating.

“For archaeologists,” said the expedition’s co-director, Daniel M. Master, a professor at Wheaton College in Illinois, “it was the difference between a palace and a stable.”