The disaster occurred at the Purity Distilling Company facility on January 15 1919, one day before the 18th Amendment (which prohibited alcohol production) was ratified. It was an unusually warm day. At the time, molasses was the standard sweetener in the United States. Molasses can also be fermented to produce ethanol, which is used in making liquor and was a key component in the manufacturing of munitions. The stored molasses was awaiting transfer to the Purity plant situated between Willow Street and what is now named Evereteze Way in Cambridge, Massachusetts.
Near Keany Square, at 529 Commercial Street, a huge molasses tank 50 ft (15 m) tall, 90 ft (27 m) in diameter and containing as much as 2,300,000 US gal (8,700,000 L) collapsed. Witnesses stated that as it collapsed there was a loud rumbling sound like a machine gun as the rivets shot out of the tank, and that the ground shook as if a train were passing by.
The collapse unleashed an immense wave of molasses between 8 and 15 ft (2.5 to 4.5 m) high, moving at 35 mph (56 km/h) and exerting a pressure of 2 ton/ft² (200 kPa). The molasses wave was of sufficient force to break the girders of the adjacent Boston Elevated Railway's Atlantic Avenue structure and lift a train off the tracks. Nearby, buildings were swept off their foundations and crushed. Several blocks were flooded to a depth of 2 to 3 feet. As described by Stephen Puleo,
Molasses, waist deep, covered the street and swirled and bubbled about the wreckage. Here and there struggled a form — whether it was animal or human being was impossible to tell. Only an upheaval, a thrashing about in the sticky mass, showed where any life was... Horses died like so many flies on sticky fly-paper. The more they struggled, the deeper in the mess they were ensnared. Human beings — men and women — suffered likewise.
The Boston Globe reported that people "were picked up by a rush of air and hurled many feet." Others had debris hurled at them from the rush of sweet-smelling air. A truck was picked up and hurled into Boston Harbor. Approximately 150 were injured; 21 people and several horses were killed — some were crushed and asphyxiated by the molasses. The wounded included people, horses, and dogs; coughing became one of the biggest problems after the initial blast.
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 sisters staring at him.
First to the scene were 116 sailors under the direction of Lieutenant Commander H. J. Copeland from the lightship USS Nantucket training ship that was docked nearby at the playground pier. They ran several blocks toward the accident. They worked to keep the curious from getting in the way of the rescuers while others entered into the knee-deep sticky mess to pull out the survivors. Soon the Boston police, Red Cross, Army and other Navy personnel arrived. Some nurses from the Red Cross dove into the molasses while others tended to the wounded, keeping them warm, and made hot coffee—ironically sweetened to taste with the surrounding molasses—as well as keeping the exhausted workers fed. Many of these people worked through the night. The injured were so numerous that doctors and surgeons set up a makeshift hospital in a nearby building. Rescuers found it difficult to make their way through the syrup to help the victims. It took four days before they stopped searching for victims; many dead were so glazed over in molasses, they were hard to recognize. Two who could not be identified were found on the fourth day.
United States Industrial Alcohol did not rebuild the tank. The property became a yard for the Boston Elevated Railway (predecessor to the Massachusetts Bay Transportation Authority) and is currently the site of a city-owned baseball field.
Several factors that occurred on that day and the previous days may have contributed to the disaster. The tank was poorly constructed and insufficiently tested. Due to fermentation occurring within the tank, carbon dioxide production may have raised the internal pressure. The rise in the local temperatures that occurred over the previous day also would have assisted in the building of this pressure. Records show that the air temperature rose from 2°F to 41°F (from −17°C to 4°C) over that period. The failure occurred from a manhole cover near the base of the tank, and it is possible that a fatigue crack grew here to criticality. The hoop stress is greatest near the base of a filled, cylindrical tank. The tank had only been filled to capacity eight times since it was built a few years previously, putting the walls under an intermittent cyclical load.
An inquiry after the disaster revealed that Arthur Jell, who oversaw the construction, neglected basic safety tests, such as filling the tank with water to check for leaks. When filled with molasses, the tank leaked so badly that it was painted brown to hide the leaks. Local residents collected leaked molasses for their homes.
Based on the date of the accident, some have claimed that the tank may have been overfilled so that the owners could produce as much ethanol for liquor as possible before Prohibition came into effect. But the 18th Amendment, enacting Prohibition, did not become law until more than a year later, and the Volstead Act did not ban the production of industrial alcohol, so these claims are groundless.
TodayThe sites of the molasses tank and the North End Paving Company have been turned into a recreational complex, officially named Langone Park, featuring a Little League ballfield, a playground, and bocce courts. Immediately to the east is the larger Puopolo Park, with additional recreational facilities.
A small plaque at the entrance to Puopolo Park, placed by the Bostonian Society, commemorates the disaster.