Shortly after noon on January 15, 1919, a giant steel tank on the Boston waterfront split open and let out 2.3 million gallons of molasses. A brown wave as tall as a two-story house rolled through the North End at about 35 miles an hour. It knocked buildings off their foundations, bent the steel of an elevated railway, and killed 21 people. It sounds like a joke, and people still laugh when they first hear about it. But the physics of thick, heavy, cooling syrup made it one of the strangest and cruelest disasters in American history, and the court fight that followed helped change how buildings get approved.
A tank full of future alcohol
Molasses is the thick, dark syrup left over when sugar is boiled out of sugarcane juice. Today most of us know it from gingerbread and baked beans. In 1919, a lot of it was headed somewhere else. Molasses can be fermented into ethanol, and ethanol was a key ingredient in munitions as well as in liquor.
The Purity Distilling Company kept a huge storage tank at 529 Commercial Street, right on Boston Harbor. Ships from the Caribbean pumped molasses into it, and a pipeline later carried it to the company’s alcohol plant across the river in Cambridge. Since 1917, Purity had been owned by a larger firm, the United States Industrial Alcohol Company, usually shortened to USIA.
The tank stood 50 feet (15 meters) tall and 90 feet (27 meters) across. Full, it held about 2.3 million gallons, which weighs roughly 13,000 tons. That is a lot of weight pressing outward on a wall of riveted steel plates.
Around the tank was one of the most crowded neighborhoods in the city. The North End was home to thousands of Italian immigrant families. Next to the tank ran Commercial Street, with warehouses, a paving company’s yard, a firehouse, homes, and the steel trestle of the Atlantic Avenue elevated train.
A warm day in January
The weather that week did something odd. Just days earlier the temperature in Boston had been about 2°F (−17°C). By January 15 it had climbed above 40°F (about 4°C). A ship from Puerto Rico had just topped up the tank, and that fresh molasses had been warmed so it would pump more easily. The tank was close to full.
At about 12:30 p.m., people nearby heard a deep rumble, like a train passing overhead. Others described a crash, a growl, or a sharp bang. Some said it sounded like a machine gun, which was probably the rivets holding the steel plates together shooting loose one after another.
The tank came apart. Its wall split and its steel sheets flew outward, and the molasses came out all at once.
A wave you could not outrun
Molasses is about one and a half times as dense as water. A full tank of it held a huge amount of stored energy, and when the walls gave way, that energy turned into a wave. Accounts put it at about 25 feet (8 meters) high at its peak, moving at around 35 miles an hour (56 kilometers an hour).
Pieces of the tank smashed into the girders of the elevated railway and bent them. A streetcar was briefly lifted off its tracks. Wooden houses were swept off their foundations and crushed. A firehouse was pushed partly off its foundation. A truck was reportedly thrown into the harbor. Several blocks were left under 2 to 3 feet of molasses.
One newspaper, The Boston Post, described people and horses struggling in waist-deep syrup, so coated that it was hard to tell which was which. The more they struggled, the paper wrote, the deeper they sank.
Anthony di Stasio, a boy walking home from school with his sisters, was picked up by the wave and carried along on its crest, almost like a surfer. When it dropped him, the molasses rolled him over and over. He heard his mother calling his name but could not answer because his throat was full of syrup. He passed out and woke to find three of his four sisters standing over him. That account comes from a 1983 Smithsonian magazine story by Edwards Park.
Why thick syrup moved so fast
This is the part that sounds wrong. Molasses is the classic example of something slow. There is even a saying about it. How could it outrun people?
The answer has to do with viscosity, which is how thick and slow a liquid is. Viscosity tells you how a liquid resists flowing, but it is not the only thing that matters. When a huge, heavy mass of liquid is let go all at once, gravity drives it outward and downward. Scientists call this a gravity current. Avalanches, mudslides, and lava flows behave the same way. A big enough, heavy enough flood of molasses can move fast at first, even though a spoonful of it oozes.
In 2016, aerospace engineer and science writer Nicole Sharp, working with Harvard physicists Jordan Kennedy and Shmuel Rubinstein, took a closer look. The idea grew out of a Harvard fluid dynamics class, in which students built a small model of the neighborhood and released a tank of cold corn syrup into it while high-speed cameras filmed it. Sharp then read through old newspaper accounts, studied century-old maps, and even asked the National Weather Service for the weather records. Kennedy measured how blackstrap molasses behaves at different temperatures.
Their findings, presented at an American Physical Society meeting that November, backed up the old reports. A wave moving about 35 miles an hour was physically believable. They also found that temperature mattered far more than anything else. Molasses is roughly 4,000 times more viscous than water to begin with. Cooling it from 10°C to 0°C (50°F to 32°F) made it about three times thicker again.
That explains the cruelest part of the story. The molasses came out of the tank warmer than the winter air. As it spread through the streets, it cooled quickly and thickened. What had been a fast, crushing wave turned into something closer to wet cement. People who survived the first hit were stuck. Rescuers could barely move through it. Sharp has pointed out that at least one victim seems to have died hours later, still trapped. On a hot July day, the molasses would probably have spread thinner and stayed runnier, and rescues might have been easier.
The same rule shows up in a gentler form in the world’s slowest laboratory experiments, where a funnel of pitch in Australia drips more slowly after its building got air conditioning. Colder liquids get thicker, sometimes by a lot.
Digging out a sticky city
The first rescuers on the scene were 116 cadets from the USS Nantucket, a training ship of the Massachusetts Nautical School that happened to be docked nearby. They ran several blocks to the flood and waded into knee-deep molasses to pull people out. Boston police, Red Cross nurses, and Army and Navy personnel soon followed. Doctors set up a makeshift hospital in a nearby building.
The search went on for four days. Many of the dead were so coated in molasses that they were hard to recognize. Some victims had been swept into the harbor and were not found for months. In the end, 21 people died, including children, and about 150 were hurt. Several horses were killed too.
Cleaning up took weeks. Crews sprayed salt water from a fireboat to wash the molasses away and spread sand to soak it up. The harbor stayed brown until summer. Rescuers, workers, and sightseers tracked the syrup all over the city, onto subway platforms, streetcar seats, pay phones, and into homes. One report said that everything a Bostonian touched was sticky. For decades afterward, longtime residents said the North End still smelled of molasses on hot days.
What went wrong with the tank
USIA said the tank had not failed at all. It claimed that anarchists had blown it up. That was not a crazy thing to say in 1919, a year when anarchist bombs really were going off in American cities, and the company’s alcohol went into munitions. But the evidence pointed somewhere else.
The tank had been built in a hurry in 1915. The man in charge of the job was Arthur Jell, USIA’s treasurer, who had no engineering or architecture training. He skipped a basic safety step: filling the new tank with water to check for leaks. When it was filled with molasses, it leaked so much that the company painted it brown so the streaks would not show. Neighbors collected the dripping molasses to take home. People also reported that the tank groaned when it was filled.
Nearly a century later, structural engineer Ronald Mayville took a modern look at the tank’s design. His analysis, published in 2014, found that the steel walls were only about half as thick as they should have been, even by the looser standards of the time. The riveted seams were overstressed too. Using computer modeling, he found that the highest stress was around the rivet holes near a 20-inch manhole at the base of the tank, the same spot where experts at the trial had said the crack began.
Mayville also found a problem no one in 1919 could have known about. The steel had very little manganese in it, which made it more brittle in the cold. Steel like that can switch from bending to cracking at a certain temperature, and for this tank that switch may have been as high as 59°F (15°C). On the day it failed, the air was around 40°F. A crack that started at a rivet hole could race across brittle steel instead of stopping. The Boston Globe noted that it was the same type of steel used on the Titanic.
Other ideas have been raised too. Fermentation inside the tank may have made carbon dioxide gas and added pressure, and the fresh, warm molasses may have expanded as it mixed with the cold molasses already inside. But the basic story most engineers accept is simpler. The tank was too weak for what it held, and nobody had checked.
The court case that followed
Families and businesses sued. In all, 119 claimants brought a combined lawsuit against USIA, one of the first class-action cases in Massachusetts. The court appointed an auditor, Hugh W. Ogden, to hear the evidence. The hearings ran for about three years and heard from a long line of engineers, scientists, and eyewitnesses.
Ogden rejected the bomb theory and found USIA responsible. The company paid about $628,000 in damages, roughly $11.7 million in today’s money. The families of those who died received around $7,000 each.
The case had a lasting effect. Building rules changed in its wake, including requirements that plans for structures like this be checked and signed off by a licensed architect and engineer. Building codes that we take for granted today owe something to a tank of molasses that nobody tested.
Where the tank stood
USIA never rebuilt the tank. The site later became a yard for the elevated railway, and today it is part of Langone Park, a city park with a Little League baseball field, a playground, and bocce courts.
A small green plaque near the entrance to Puopolo Park next door marks the disaster. Oddly, it says the wave was 40 feet high, taller than most estimates. On the 100th anniversary in January 2019, researchers used ground-penetrating radar to find the outline of the tank’s old concrete base, still buried about 20 inches under the baseball field. People at the memorial stood in a circle along its edge while the names of the 21 victims were read aloud.
Molasses is still dangerous
Molasses is not usually treated as a hazardous material, and that has caught people out more recently too. In September 2013, a leaking pipe spilled about 233,000 gallons of molasses into Honolulu Harbor while it was being loaded onto a ship. Unlike oil, which floats, the molasses sank to the bottom and used up the oxygen in the water as bacteria fed on the sugar. More than 26,000 fish and other sea creatures suffocated. No one had a plan for a molasses spill, because no one had thought they needed one.
Boston’s flood is still the one people remember. It is part tragedy, part physics lesson, and part warning about what happens when a big structure goes up fast with no one checking the math. The next time a recipe tells you to wait for molasses to pour, it is worth remembering that a whole tank of it once moved faster than a person could run.
Further reading and a taste of molasses
Dark Tide: The Great Boston Molasses Flood of 1919 (100th Anniversary Edition) — Stephen Puleo’s history of the disaster, from the hurried building of the tank to the long court fight, told through the North End families who lived beside it.
Golden Barrel Unsulfured Blackstrap Molasses, 32 oz — A quart jar of dark, strong-flavored blackstrap molasses for gingerbread, baked beans, and barbecue sauce, made in Lancaster County, Pennsylvania.