Solving the mystery of the ‘tsunami of molasses’
Harvard University students tackle bizarre disaster that killed 21 people a century ago
“A dull muffled roar gave but an instant’s warning before the top of the tank was blown into the air,” the New York Times wrote in 1919. “Two million gallons of molasses rushed over the streets and converted into a sticky mass the wreckage of several small buildings which had been smashed by the force of the explosion.”
“Wagons, carts, and motor trucks were overturned. A number of horses were killed. The street was strewn with debris intermixed with molasses and all traffic was stopped.”
It was January. The place was Boston. And when 8.7 million litres of molasses burst from a gigantic holding tank in the city’s North End, 21 people were killed and about 150 more were left injured. The wave of syrup — some reports said it was more than 10 metres tall — rushed through the waterfront, destroying buildings, overturning vehicles and pushing a firehouse off its foundation.
For nearly 100 years, no one really knew why the spill was so deadly.
But at a meeting of the American Physical Society last month, a team of scientists and students presented what may be an important piece of the century-old puzzle. They concluded that when a shipment of molasses newly arrived from the Caribbean met the cold winter air of Massachusetts, the conditions were ripe for a calamity to descend upon the city.
By studying the effects of cold weather on molasses, the researchers determined that the disaster was more fatal in the winter than it would have been during a warmer season. The syrup moved quickly enough to cover several blocks within seconds and thickened into a harder goo as it cooled, slowing down the wave but also hindering rescue efforts.
“It’s a ridiculous thing to imagine, a tsunami of molasses drowning the North End of Boston, but then you look at the pictures,” said Shmuel Rubinstein, a Harvard professor whose students investigated the disaster.
When the molasses arrived in Boston’s harbour, it was heated by just a few degrees. The warmer temperature made it less viscous and therefore easier to transport to a storage tank near the waterfront.
When the tank burst two days later, the molasses was still probably about 4 or 5 C warmer than the surrounding air, said Nicole Sharp, an aerospace engineer and science communications expert who advised the Harvard students.
The students performed experiments in a walk-in refrigerator to model how corn syrup, standing in for the molasses, would behave in cold temperatures. With that data in hand, they applied the results to a full-scale flood, projecting it over a map of the North End. Their results, Sharp said, generally matched the accounts from the time.
“The historical record says that the initial wave of molasses moved at 35 miles per hour (55 km/h),” Sharp said, “which sounds outrageously fast.”
“At the time, people thought there must have been an explosion in the tank, initially, to cause the molasses to move that fast,” she added.
But after the team ran the experiments, she said, it discovered that the molasses could, indeed, move at that speed.
“It’s an interesting result,” Sharp said, “and it’s something that wasn’t possible back then. Nobody had worked out those actual equations until decades after the accident.”
If the tank had burst in warmer weather, it would have “flowed far- ther, but also thinner,” Rubinstein said. In the winter, however, after the initial burst — which lasted between 30 seconds and a few minutes, Sharp said — the cooler temperature of the outside air raised the viscosity of the molasses, essentially trapping people who had not been able to escape the wave.
About half the people who were killed “died basically because they were stuck,” Rubinstein said.
A firefighter who survived the initial wave managed to stay alive for nearly two hours while he waited to be rescued, they said, but he drowned. “Men and women, their feet trapped by the sticky mass, slipped and fell and were suffocated,” the Boston Globe wrote in 1968. “The stronger tried to save others, and many of them died for their heroism.”
The exact cause of the tank’s failure has never been known. Last year, a team of engineers using modern methods to analyze the century-old disaster blamed poorly designed steel tanks.
The project at Harvard grew out of Rubinstein’s Introduction to Fluid Dynamics class, which asks students to create a final project.
“Choose an interesting project and make an appealing video,” he said.
Rubinstein and Sharp said they would like to eventually build an entire course around the disaster.
Students could apply what they learn in other classes to understanding not just why the molasses behaved the way it did, but also what other forces shaped the events of that day in 1919.
The Boston molasses disaster, Rubinstein said, is “a beautiful story for teaching.”