Great Molasses Flood



A tank holding two million gallons of molasses exploded in Boston, MA



Evidence showed the tank itself contributed to the explosion



Shmuel M. Rubinstein, a Harvard University professor, conducted experiments with his class to learn more about the disaster 

As Slow as Molasses in January


A dull muffled roar gave but an instant’s warning before the top of the tank was blown into the air. 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. The New York Times, 1919


A Molasses Flood?

Did you ever wonder where the expression, “As slow as molasses in January,” came from? On January 15, 1919, at around 12:30 in the afternoon, in Boston MA there was an explosion of a container holding over two million gallons of molasses. At a speed of 35 miles per hour, and causing a wave estimated at 40 feet tall, the brown sticky substance flowed through the waterfront area of Boston. Twenty-one people were killed, 150 people injured, cars overturned, and buildings destroyed. Until recently, no one has known the reason for the explosion.


What Happened?

The molasses was being held in a large tank at the Purity Distilling Company, having arrived recently from the Caribbean. The outside temperature rose quickly that day to above 40° F causing a suspected partial fermentation of the molasses*. The storage tank had not been built to specification** and the carbon dioxide produced by the fermentation raised the internal pressure of the molasses. Finally, the pressure caused rivets to pop, stress fractures to occur, and the explosion ensued. It was discovered, after the fact, that the construction engineer, Arthur Jell, had only tested the tank with water to check for leaks; however, once filled with molasses, cracks developed and leaks occurred. The fractures were subsequently painted brown to hide the fact.


Why Would Molasses be Dangerous?

The consistency of the molasses and the speed at which it flowed, caused people to slip, become stuck, and drown in the knee-deep substance.

Anthony di Stasio, walking homeward with his sisters from the Michelangelo School, was picked up by the wave and carried, tumbling on its crest, almost as though he were surfing. Then he grounded and the molasses rolled him like a pebble as the wave diminished. He heard his mother call his name and couldn’t answer, his throat was so clogged with the smothering goo. He passed out, then opened his eyes to find three of his four sisters staring at him. Smithsonian, 1983

Cleanup took weeks as crews used salt water from a fireboat to wash the molasses away and used sand for absorption. The harbor was molasses colored until summer.


An Experiment in Fluid Dynamics

In 2014, an investigation discovered the issues with the tank itself that contributed to the explosion. In 2016, a Harvard professor and his fluid dynamics class conducted experiments on all aspects of the explosion. They found that the Caribbean molasses was warmer than the cold January air of Boston, and that the temperature difference caused the flow to thicken and hamper rescue efforts – if it had been summer, the molasses would have flowed farther, but would have been thinner, perhaps not trapping people.


* Molasses can be fermented to produce rum, ethanol, and was a component in munitions.

** There was some speculation that the Purity Distilling Company was trying to beat prohibition; the 18th Amendment of the Constitution was ratified the day after the flood.


Did You Know?

  • During the early leakage of molasses from the tank, nearby residents collected it for home use.
  • On hot days in the summer, Boston locals swear that molasses still “bleeds” from the streets.



Watch Along with Harvard Students as they Perform the Molasses Flood Experiment



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