Author(s): Angelo Verzoni. Published on May 1, 2017.

Cold Molasses

New research helps explain the deadly Boston molasses flood of 1919


The Great Boston Molasses Flood, which occurred on January 15, 1919, was triggered when a large molasses storage tank, housed just a few feet from one of Boston’s busiest streets, burst open and sent a wave of the sticky substance through the neighborhood. An estimated 2.3 million gallons of molasses—enough to fill three Olympic-sized swimming pools—gushed from the tank, forming a wall that was initially 25 feet high and 160 feet wide. Twenty-one people, including a firefighter, were killed, and 150 people were injured.


Nearly 100 years after the disaster, a team of researchers from Harvard University believes it knows why the peculiar incident was so deadly: frigid winter temperatures rapidly cooled the molasses as it exited the tank, making it thicker. The phenomenon not only prevented people from escaping it, but also hindered efforts to rescue those who were trapped in it, according to the researchers. Shmuel M. Rubinstein, a Harvard engineering professor whose students assisted in the research, told The New York Times that he believes fewer people would have died if the tank had burst in warmer weather.

Researchers studied hundreds of historical documents and evaluated the flow of molasses at different temperatures. They presented their findings in November at a meeting of the American Physical Society’s Division of Fluid Dynamics. Newspaper accounts of the molasses flood describe the bizarre scene. “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,” The New York Times wrote on the day of the incident. “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 tank was owned by Purity Distilling Co., a subsidiary of U.S. Industrial Alcohol, which used the alcohol distilled from the molasses for weapons manufacturing. According to historian and author Stephen Puleo, the tank burst because of shoddy construction, coupled with the fact that fermentation triggered by mixing hot and cold molasses was occurring inside the tank, forming gas and increasing pressure.

A three-year court case led to the adoption of engineering certification laws and stricter rules about engineers and architects filing building plans with cities nationwide. In his book about the incident, Dark Tide, Puleo noted that “the Boston molasses flood did for building construction regulations nationwide what a subsequent Boston disaster, the great Cocoanut Grove nightclub fire, did for fire code laws.”

ANGELO VERZONI is staff writer for NFPA Journal. Top Photograph: Getty Images