- On January 15th, 1919,
an over 2 million gallon tank of molasses
ripped apart in Boston's North End.
The disaster launched a 25 foot tall tsunami
traveling 35 miles per hour.
The wave of molasses went in every direction.
It collapsed buildings,
flooded homes and trapped people in their steps instantly.
When it was all over 21 people died
and another 150 were injured.
So why was there even a giant tank of molasses
in the city to begin with?
And how did this slow, dense,
and honestly, boring liquid cause so much destruction?
(gentle music)
The North End is Boston's oldest neighborhood.
It's famous for its historical landmarks,
Italian community and food.
And in 1919, it was also home
to the Purity Distilling Company
owned by US Industrial Alcohol.
Purity Distilling was in the business
of turning molasses into alcohol, specifically rum.
But booze wasn't the only thing on their mind.
Most of that alcohol was actually going to be used
to make explosives, but more on that later.
All right, so you probably know molasses
as one of the ingredients
in your grandmother's famous cookies,
but do you know where it comes from?
It's actually the liquid byproduct
of making granulated sugar from sugar cane.
This involves pressing sugar cane stalks
to extract the juice
and then boiling it down until crystallized sugar forms.
Once the sugar is removed,
the leftover liquid is a light molasses,
which is then boiled down again
to give it its signature thick dark brown look.
The heat causes the sugars inside
to decompose and caramelize.
This produces complex mixtures of shorter
and longer chains of sugar molecules,
which give molasses it's dark brown color.
Now molasses is mostly sugar and water
with traces of other stuff like amino acids,
which makes it the perfect food for bacteria
and fungi to ferment into alcohol.
To create alcohol distilleries add microorganisms
like yeast, which is actually a fungus, to the molasses.
Then the yeast converts the sugars into alcohol
and carbon dioxide through fermentation.
For the most common sugars like sucrose and glucose,
the fermentation reaction looks something like this.
The yeast breaks the sugar up into two ethanol molecules
and two carbon dioxide molecules.
The carbon dioxide is formed as a gas
and bubbles out of the mixture.
So this reaction looks neat and tidy, right?
Just ethanol and CO2.
Well, in reality,
this reaction is happening in a big pot full of molasses,
a complex mixture with a whole lot of other stuff in it.
And the yeast can only produce so much alcohol
before it dies off.
At around five to 20% concentration
the ethanol begins to strip the cell membranes off
of the yeast, effectively killing it.
When the fermentation reaction is complete,
you have a lot of stuff left over from the molasses
and the yeast that you don't want.
What you do want is pure ethanol.
This is the alcohol that beverages like beer, wine, vodka,
and moonshine, all have in common,
but to get the pure ethanol,
you have to distill it from the rest
of the fermentation mixture.
To do this you heat up the fermented mixture
to around 80 degrees Celsius,
which is just above the boiling point of ethanol.
At this point, the alcohol evaporates
from the mixture turning into a gas.
The water and much of the other stuff
we don't want in our alcohol is left behind
because its boiling point is much higher
at 100 degrees Celsius.
The ethanol vapor rises above the liquid
and then condenses in a series of tubes where it cools
and once again becomes a liquid.
A company might distill the condensed vapor several times
to purify the liquid even further
until they have a product that is almost entirely ethanol.
Another product of fermentation is methanol
and methanol can be deadly.
Luckily methanol has a boiling point
that's significantly lower than ethanol,
meaning it will evaporate
and distill out of the mixture first.
And it's only produced in really small amounts
in typical fermentation.
But manufacturers have to be careful
to remove it from the final product
because drinking even small amounts
can make you go blind or even kill you.
Now a portion of the Purity Distilling Company's alcohol
was bottled as rum.
And people were stocking up.
The 18th amendment,
which began the era of Prohibition
was set to go into effect in 1920,
just one year after the molasses flood.
But the majority of the company's alcohol
was used to make explosives like dynamite
and smokeless powder.
This is just what it sounds like an explosive powder
like gunpowder that produces almost no smoke
and they needed a lot of alcohol to make it.
It took one liter of alcohol
to manufacture every pound of smokeless powder.
And the allies went through millions of pounds of explosives
during World War I.
And to satisfy the demand for lots of explosives,
the Purity Distilling Company needed a lot of molasses
to make a lot of alcohol.
So in 1915 they built a gigantic tank in Boston,
50 feet high and 90 feet across.
It could hold more than two million gallons of molasses.
Ships from Cuba, Puerto Rico, and the West Indies
filled the tank with molasses,
which would then be shipped by rail cars
to the nearby processing plant for fermentation.
Now the tank's construction had a lot of problems.
Molasses could actually be seen leaking through its seams,
but instead of fixing the problem,
the tank was simply painted a rust Brown
to match the color of the leaking molasses.
And the company kept using the tank
in spite of these sticky warning signs,
caulking leaks, and carrying on as though nothing was wrong.
But then in January 1919 things went wrong.
Over half a million gallons of molasses
from the Caribbean were pumped into the tank,
a tank that already contained over 1.5 million gallons
that had been sitting through the Boston winter,
filling it to its maximum capacity.
We still can't know for sure exactly what happened
to cause the tank to fail that day.
But an MIT professor who studied the wreckage
shortly after the disaster
found that the tank was simply too weak
to hold the full load.
In fact, the steel used to construct tank
was only half the thickness it should have been
for a tank that size.
When the tank burst on January 15, 1919,
steel and rivets were launched into nearby buildings,
but much more destructive was the nearly 26 million pounds
of molasses rushing into the streets of Boston.
Estimates put the initial wave at 25 feet high
traveling 35 miles per hour in every direction.
The molasses ripped buildings from their foundations,
crushed cars and wagons, and trapped people
and animals in the aftermath.
So let's try and recreate this.
Here we have our tiny model of the Boston Harbor
and a jar of molasses.
This is a big mess,
but it doesn't look all that destructive, right?
Well, pictures and evidence of the aftermath
tell a different story.
So what made the tank of molasses so deadly
and how did such a simple liquid cause so much destruction?
People have talked about the disaster for years,
but it wasn't until recently
that anyone studied the fluid dynamics of the disaster.
But in 2016, Dr. Nicole Sharp
and her collaborators, Jordan Kennedy and
Dr. Schmuel Rubinstein decided to look
into the physics of it.
And they discovered
that the wave was a form of gravity current.
Gravity or density currents happen
when there are two fluids of different densities
next to each other,
both under the influence of gravity.
You can see that here with these two liquids.
On the left, we have plain water
and on the right, we have more dense sugar water.
If I remove the wall between these two,
the more dense sugar water exerts more pressure
on the newly created opening
than the plain water causing it to flow underneath
as it settles downwards due to gravity.
In the molasses flood,
the two fluids are the more dense molasses
and the much less dense air.
Sharp and Kennedy studied what these gravity flows look like
for molasses and how the viscosity of molasses
or its resistance to changing shape
is influenced by pressure.
For many fluids, their viscosity remains the same,
no matter how much force is applied.
These are Newtonian fluids.
Their viscosity can be changed however,
by things like temperature.
Traditional car oil is an example of a Newtonian fluid.
It might be hard to try and start your car
on a cold day
because the cold thick oil will be highly viscous,
but once the car warms up the oil,
it will flow more freely through your engine
as it becomes less viscous.
But temperature isn't the only thing
that can change the viscosity of some fluids.
These are non-Newtonian fluids and molasses is one of them.
Non-Newtonian fluids changed their viscosity
in response to applied force,
you might be familiar
with the non-Newtonian fluid Oobleck.
This is just corn starch and water.
It flows like a liquid when no stress is applied,
but it becomes much firmer
when I ramp up the force like this.
So we call it Oobleck a shear-thickening fluid.
There are a few types of non-Newtonian fluids
and molasses is a shear-thinning fluid.
Unlike Oobleck, shear-thinning fluids get less viscous
as more stress is applied to them.
This may have helped the molasses to rush out of the tank
and act like a much thinner liquid.
This type of behavior sometimes appears in fluids
that have lots of long molecules,
which get tangled together,
making the fluid viscous.
But once a force is applied,
those molecules start to untangle
and slide past one another lowering the viscosity.
The gigantic amount of molasses
in the tank created lots of pressure pushing down
on the molasses towards the bottom.
This could have contributed to shear-thinning
and faster flowing behavior.
But how much of an impact did this have on the flood?
Dr. Sharp and her colleagues calculated
that the temperatures in the Boston winter
likely had a bigger impact on the viscosity
than the shear-thinning behavior.
Lowering the temperature of molasses
from just 10 degrees Celsius to zero degrees Celsius
makes it three times more viscous.
And of course that means a raising the temperature
can make it much less viscous.
You can see that here,
this molasses on the left
is at room temperature around 25 degrees Celsius.
And this one on the right
that I just pulled out of the refrigerator
is around 20 degrees lower.
You can see just how much more viscous
and thick the colder one is as I try to pour it.
The freshly delivered molasses
in the tank was about five degrees warmer
than the surrounding air.
This warm molasses was less viscous
and more fluid than colder molasses would have been.
Another potential reason why this molasses
might've flown so freely from the burst tank.
But once it hit the cold Boston air,
it cooled and became way more viscous and hard.
People who were trapped in the molasses tide
suddenly found themselves stuck
and rescuers had no way to reach them.
And we haven't even talked about the enormous amount
of potential energy stored in that tank yet.
The four story tank with molasses stacked atop itself
was potential energy just waiting to be released.
In fact, the wave was so strong
it bent the steel beams holding up a train track,
luckily missing a train
that had just passed moments earlier.
We can estimate just how much potential energy
there was in that tank.
Let's split the tank up into one meter chunks
and calculate each one's potential energy individually.
For the top meter,
we can calculate the volume of molasses
in that layer by multiplying pi times the radius squared
times the height,
which gives us 590 meters cubed.
By multiplying that volume times the density of molasses
at room temp,
we can see that it weighed 826,000 kilograms,
a little over 910 tons.
We can then calculate the potential energy
by multiplying the mass by the acceleration of gravity,
by the change in height.
So at a height of around 14 meters off the ground,
the potential energy of this layer
was around 113 million joules.
If we do this for the entire tank,
we get a total potential energy
of around 850 million joules.
That's nearly as much energy
as there is in 850 sticks of dynamite
From the photos we can see
just how much energy all of that molasses had
when it was released.
It took several days just to recover the dead bodies
under the tons of spilled molasses.
And one wasn't found until four months later,
firefighters used seawater
to cut through the the hardened molasses,
which then ran back down into the harbor,
turning it brown for weeks.
The disaster resulted in little longest court case
at that point in Massachusetts history,
which seemed like a never ending circus.
US Industrial even tried to claim
that the disaster was the result
of an anarchist who blew up its tank.
But in the end,
the company was found liable for the collapse of the tank
and had to pay $628,000 in damages,
which would be over $9 million today.
After this case the United States set into motion
a number of new laws and construction standards
to prevent a similar disaster from happening in the future.
The disaster is so notorious in Boston lore
that some people will swear
you can still smell the faint aroma
of molasses on a hot summer day.
(jazzy music)