The Great Molasses Flood

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On January 15, 1919, the city of Boston suffered its greatest disaster when a storage tank filled with over 2 million gallons of molasses burst and killed 21 people and injured 150 more. 

Researchers have been studying the unique circumstances surrounding this industrial accident ever since.

Learn more about the Great Boston Molasses Flood on this episode of Everything Everywhere Daily.

If you are familiar with molasses, you recognize it as an ingredient usually used in baking. It is a thick, sweet syrup which if you pour it out of its containers will pour very slowly. Because it is so thick and has such a high viscosity, it seems like it would just easy to avoid a molasses flood by just walking away. 

If actual events weren’t so tragic, the idea of a molasses flood would actually sound comical. 

To understand how the flood happened and why it was so deadly, we need to understand what the molasses was doing there and what it was used for.

Molasses is a byproduct of the refining of sugar. Since the very start of European colonization of the Americas, molasses was used as a principal source of sugar for the production of alcohol, in particular, rum. 

Alcohol, in particular ethanol, is also used in the creation of explosives. In 1919, the United States was coming off of World War I and the demand for industrial alcohol was at its highpoint. 

Also, the day after the flood, January 16, 1919, was the day that the 18th Amendment passed, which would ban the sale of alcohol 12 months after passage. The company had to make a whole lot of booze in the next year before they couldn’t make it anymore. 

The Purity Distilling Company was located in Boston and had a facility on the harbor where they would process incoming molasses shipments, usually from Cuba. It was a subsidiary of the larger United States Industrial Alcohol Company. 

As part of their facility, in 1915 they build a large, 50-foot tall storage tank for all the molasses which came in from ships. The tank dominated the local neighborhood and it looked very similar to the large fuel storage tanks you might have seen in ports or near oil refineries. 

Not surprisingly, the construction of the storage tank was where the problem started. 

The tank was poorly designed and tested. For starters, the tank was designed and approved by a man named Arthur Jell, who was the treasurer for US Industrial Alcohol. He wasn’t an engineer, and he didn’t even know how to read a blueprint. His primary concern was cost, not design or safety.

The steel used in the tank was not adequate for the task. A modern analysis of the tank determined that the steel was only half as thick as it should have been. The steel was also brittle due to a lack of manganese, and the rivets weren’t installed properly.

The tank was also never adequately tested. Before it was put into use, it was only filled a quarter of the way with water. Not only didn’t they fill the entire tank, but they used water, which is much less dense than molasses. 

Almost immediately after being put into use, the tank began leaking. 

Molasses would seep through the rivet holes. Locals would go to the tank to collect free molasses. 

The company’s solution to this was to paint the tank brown, so no one would notice the molasses leaks. 

In the aftermath of the disaster, it was found that almost everyone who lived in the area expected the tank to fail at some point. 

Here I need to take a detour to explain a bit behind the physics of fluids, more specifically, molasses.

The first is to understand the concept of viscosity. Without getting too technical, viscosity is how easily a fluid can flow. Water has a very low viscosity and can flow very easily. Molasses and syrup have a much higher viscosity and don’t flow as easily. Temperature can change the viscosity of a substance. Usually, the hotter a fluid is, the lower the viscosity. This will be important.

The second thing is to understand Newtonian fluids. Water and motor oil are examples of Newtonian fluids and it is what we are used to dealing with in our day to day lives. Newtonian fluids do not change their viscosity regardless of the amount of stress or pressure applied. 

Molasses is a non-Newtonian fluid. It does change its viscosity based on the stress applied. There are two types of these non-Newtonian fluids: sheer thickening or sheer thinning.

There are some great videos you can find online which show how shear thickening fluids work. If you mix corn starch and water, you can create a fluid you can actually walk on. The pressure from your step causes the fluid to get briefly firm, allowing you to walk on it, so long as walk quickly and don’t stop.

Molasses is a shear-thinning fluid. That means under great pressure, it actually becomes less viscous and flows more easily. Molasses will also decrease its viscosity and flow more easily when it is heated. Both of these properties of molasses, its increase in viscosity due to heat and pressure, would be critical to the events of January 15. 

On the day of the tragedy, two things happened which influenced events.

First, was that that ship arrived from Cuba and transferred over 600,000 gallons of molasses to the tank. The molasses was heated to make it less viscous, so it would be easier to transfer and the amount transferred almost filled the tank to the top. One of the few times the tank had been completely filled since it was constructed.

The other thing which happened was a warm front descended on Boston. Temperatures went from 2F or -17C to 40F or 4C in one day. 

It is believed the thermal expansion of the previously cold molasses due to the increase in temperature and the addition of warm molasses from the ship, put enough pressure on the tank to cause it to fail catastrophically. There also may have been fermentation going on inside the tank, which would produce CO2, which would also create pressure.

At 12:30 pm, the tank burst. 

Because of the increased temperature of the molasses and the incredible pressure, it was under in the full tank, the viscosity was quite low. When it burst out of the tank, it did so like water. It wasn’t slowly pouring out like you would see molasses come out of a bottle. 

Reports were that the wave of molasses was 25 feet high, or 6 meters, and traveled at 35 miles per hour down the street. The explosion of the tank burst with the energy of 850 sticks of dynamite. Rivets from the tank shot out like bullets and lodged in buildings.

The tank couldn’t have burst at a worse time. Children were walking home from school for lunch. Workers were out on the street. 

Buildings and elevated railways were hit by the flood and collapsed. 

Once the molasses burst forth from the tank, the non-Newtonian nature of the fluid began to kick in. Now that it was no longer under pressure and exposed to the outside air, the molasses viscosity increased, and its flow slowed dramatically. Once people were caught it in, it became the thick syrup that most people are accustom to, which made it very difficult to get out of. 

The initial death toll was only 10 people, however over the next several days and weeks, more bodies were found, with the final body count reaching 21. The last bodies were found 4 months after the flood because their bodies were swept out into Boston Harbor.

After the initial destruction in the immediate area surrounding the tank, the molasses keep flowing. Eventually, much of downtown Boston had 1 to 2 feet of molasses. 

The clean up was very difficult. Molasses in January doesn’t drain away. People from all over Boston and nearby cities came to the waterfront to help clean up, and to take home some free molasses. So many people came to the site of the flood, that streetcars and other communities around Boston became sticky and smelled of molasses for weeks. 

Boston harbor was stained with molasses until the summer, and many buildings had molasses stains that lasted for years. 

In the aftermath of the disaster, one of the first and largest class action lawsuits in American history was filed against the United States Industrial Alcohol Company. The trial lasted over three years and was the longest trial in Massachusetts history up to that point. There were so many lawyers involved, that they couldn’t fit them all in the courtroom.

The company claimed that the tank was attacked by anarchists who were protesting the use of the molasses to make explosives. There were actually two incidents similar to that which had happened during the war. However the evidence of negligence was overwhelming, and the company was eventually found guilty and had to pay $628,000 in damages, or the equivalent of $9.3 million dollars today. 

Today, the great molasses flood remains the second-worst disaster in the history of Boston. 

Many of the buildings and structures in the area of the flood have long since been torn down and replaced, but give the nature of the molasses getting into every nook and cranny, there are still people who claim that on a warm summer day, you still smell molasses in Boston.