This video is sponsored by Squarespace. If you've ever been lucky enough to have some
fancy, expensive flaky sea salt, you might've noticed a curious crystal shape in the mix.
It's a hollow pyramid. This brand of sea salt from England is renowned for its particularly
huge hollow pyramids. It looks like they made them in a mold, but they didn't. That crystal
grew that way naturally. Table salt isn't the only mineral that does this. Under the
right conditions, tons of rocks will crystallize that way. Scientists call it a hopper crystal.
A hopper is a big box in a factory that tapers down to a narrow point underneath. It's used
for funneling stuff into something else. That's a hopper. The crystal looks like a hopper,
so they call it a hopper crystal. A lot of people, myself included, consider this a highly
desirable shape for a finishing salt — that is, a salt that you put on right at the end.
If the salt is going to be dissolved in the food, then that's a waste of a good crystal.
I'm talking about salt that you put in right at the end so that it will be intact on the
top as you start to eat. Because the crystal is big, the saltiness is concentrated rather
than being dispersed evenly across the food. This results in a heterogeneous eating experience.
Different areas of the food taste different and combine in interesting ways in your mouth
as you chew. And because those pyramidal crystals are hollow, they crunch as you bite through
them. But, as salt goes, salt products that have this shape in them tend to be very expensive,
because the traditional processes that give you this crystal shape are not quick. There
may be less traditional ways of growing them a little bit faster, but all of my experiments
at home have failed. And I've tried a million things.
This is actually an area of active proprietary research by snack food companies. Hollow salt
crystals have more surface relative to their total mass, so they dissolve faster in your
mouth and therefore they taste saltier. If snack makers can grow tiny hollow salt crystals,
they can give us products that taste as salty as those covered in solid crystals, but with
less salt. And then they can sell us a bag that says, 'Same great taste, now with less
sodium!' Those companies are not going to share their research with me. But this guy
will share his research with us.
"My name is Pietro Fontana. I'm a chemical engineer and retired since 10 years. My hobby
are crystals and crystal growth."
Dr. Fontana has published some awesome work on salt hopper crystals in recent years and
he's going to explain to us why they happen. Quick note, Dr. Fontana is Swiss. He's not
super confident in his English. So when he and I spoke, he talked a lot of the time from
prepared remarks that he had scripted in advance. I think that's totally fine. That's just why
it sounds the way that it does. He also made a PowerPoint presentation. I'm going to show
you those slides as we go.
"The common salt has a chemical formula of NaCl. It is composed of positively charged
sodium and negatively charged chloride ions."
Okay. So when you dissolve salt in water, those ions — those individual electrically
charged sodium and chloride atoms — when they dissolve in water, you split them apart.
Once fully dissolved, there are no sodium chloride molecules swimming around in here
anymore. The water has broken the ionic bonds between the sodium and the chloride. Each
individual atom is now surrounded by water. Technically there is no salt in this pot anymore.
It's a new substance commonly called brine. But as you evaporate the water out, you eventually
reach a point where there just simply isn't enough water left to wrap around each of those
atomic components of salt and keep them separate from each other. You end up having sodium
and chloride ions that are able to bump up together again and start bonding into salt.
You can already see it happening on the surface there — solid salt rising up out of solution
and floating on top.
"You see, here in the model of the salt structure with violet sodium and green chloride ions,
they are arranged in a three-dimensional periodical way, whereas sodium ion is surrounded by six
chloride ions and the chloride ions is surrounded by six sodium ions. The smallest possible
unit with the sodium chloride structure is a cube."
A cube. These attractive forces caused those ions to bunch up and squish in together. And
their natural tendency will be to bunch up in as compact a form as possible. And the
resulting shape is going to be a cube. You might also get some other very simple geometric
shapes depending exactly on how the first few ions actually start to come together.
But the most common shape is going to be a cube.
"Normally a crystal grows layer by layer."
And that's what Dr. Fontana is showing us on this slide here. As more ions come out
of the solution and link up with the others, they basically replicate that cubic shape
over and over again. That's the "periodical array" that Dr. Fontana was talking about.
The period is the cubic shape. The array is that cubic shape self-replicating over and
over and over again into a highly ordered structure that they call a crystal.
"As a consequence, salt crystals remain the shape of cubes from micrometer to centimeter
sizes."
But that's with all things being equal, when the attractions between these ions are allowed
to do what they 'want' to do without other things getting in the way. In the real world,
there are other factors at play — impurities in the water getting in between all the stuff,
convection currents pushing things around, tiny variations in temperature that result
in one area of the brine being more concentrated than another. All these factors and more result
in tons of different shapes of crystals. In this spoonful of brine that I let evaporate
in my kitchen, I've got some hoppers. I got some cubes. I got some bars. Lots of different
shapes going. Check this out. When I slowly simmered all the water out of this brine,
I got salt crystals growing up in these spindly tendrils. Some of them like a centimeter long.
This is apparently just how salt molecules link up when being pushed around by the particular
soup of forces at play in a simmering pot.
I don't claim to understand it, but there it is. In the case of hopper crystals, these
hollow pyramids, that particular shape happens when the normal cubic growth of salt is interrupted
by a few factors, one being speed. This block by block, layer by layer cubic growth — this
happens when conditions in the brine are such that the crystal grows slowly.
"When however, the crystallization rate is so that the growing layer is not competed
before new ions arrive, hollow forms are developed."
This happens when the solution is really super-saturated — way more sodium and chloride ions in there
than the available water can keep apart. Ions show up and start linking up on the edge of
the crystal face before that new layer is even completed in the middle. This is known
as skeletal growth and the result is a hollow area in the center.
"It grows at the bases and during the growth, the summit looks down to the brine."
And once that happens, the salt crystal is basically a boat floating on the surface.
Its buoyancy and the surface tension of the water keep its open face stuck to the surface
of the water. Just like the surface of a boat, it's dry on the inside so there's no way for
new ions to get inside and start filling up the inside of the hopper. New ions just keep
linking up along the edges at the surface and the pyramid grows bigger and bigger and
points down deeper and deeper into the brine. I can actually show you this happening. This
is some brine that I have super-saturated by boiling. Hot water can hold way more salt
in solution than cool water can. I lay a film of that brine on a glass surface. It rapidly
starts to cool down. It can't hold all of that salt in solution anymore so crystals
start forming really fast. What I'm showing you under the microscope camera happened over
the course of just a few minutes. Look at those hoppers form.
"By growing, the weight increases. When the gravity force is higher than the force of
the surface tension and the force of buoyancy, the pyramid sinks to the bottom."
Why does it fall? Well, for the same reason that a bug can walk on water, but I can't.
Eventually you just get too heavy for the surface tension of water to be strong enough
to hold you aloft. These crystals just keep growing to get bigger and bigger. They get
so big and heavy that their buoyancy and the surface tension of the water is not strong
enough to hold them at the top so they sink down to the bottom. That's called sedimentation.
And once they're down there, you will get no further hopper growth. That's why in the
traditional fleur de sel process, they skim the crystals off the surface right before
they fall. It is extremely labor intensive. If the crystals fall, the hollow pyramids
might start to fill in, which I think is what happened to me in this particular experiment
that I did in the oven.
You can see that I've got a few little hollow crystals left there, but most of these have
grown into big solid blocks. You got to wonder then what would happen if you grew hopper
crystals in a place where they could not fall — a place where there is virtually no gravity
to pull them down into the brine. This is exactly what Dr. Fontana and the American
astronaut Don Pettit wanted to find out. So while Dr. Pettit was on the International
Space Station, he took out a little loop of wire and he dipped it into one of the packets
of salt water they use to season their food up there. I guess a salt shaker would be pretty
messy in microgravity.
"And the sodium chloride solution that we have in the galley is close to a saturated
solution. So once we draw one of these films, it doesn't take much evaporation before the
crystals start to grow."
Because the gravitational forces are basically symmetrical up there, you don't get pyramids.
You get hopper cubes — six different hoppers, one on each side of the cube, all growing
out in six different directions, six different hollows on each side of the cube. And because
it will never sink, the sky's the limit on size.
"If you have a lot of material available and a lot of space, it is no reason why it is
not possible to make huge salt crystals in space."
Of course, to do that you'd have to keep isolating the crystals — pull out a good one and get
it growing by itself in a fresh film of brine. This is true on earth as well. If you let
multiple crystals keep growing near each other, they may eventually agglomerate, or cake together.
That's what happened in my spoon experiment. The crystals bond together in what's called
a raft and thus their growth is restricted. In every attempt that I made to grow big hopper
crystals here at home, agglomeration was the main limiting factor. I posted this on my
Instagram. I tried tons of different super saturation levels, lots of different evaporation
temperatures and humidities. My crystals always caked together before the hoppers could grow
to any decent size. What happened here though? Well, in that one, I boiled my salt in a one-to-one
mixture of water and ethanol, booze.
Salt doesn't dissolve an alcohol, so I figured that maybe it would act as a buffer. It would
get in between the growing crystals and keep them from agglomerating together before they
had a chance to grow to a substantial size. And I did end up getting very clean, distinct
crystals. This is the biggest one that I got, but as you can see, it's not a hopper crystal.
It seems to have the internal structure of a hopper crystal, but the hollows are filled
in. Dr. Fontana applauded my industry here, but he couldn't really explain what was going
on with the alcohol and he didn't really have any tips for me about how I could grow big
hopper crystals myself at home. As far as I can tell, nobody at least on the internet
has cracked this particular code. There's lots of guides online for how to make your
own flaky salt at home.
But if you look at those, they've got these rafts forming at the top of their brine and
the crystals are agglomerating long before you get big distinct hopper crystals. The
doc pointed out to me that all the traditional processes are very particular. They have to
be done in big pools so the crystals have lots of space. They require just the right
rate of evaporation, just the right temperature, just the right humidity. Basically the conditions
you get on the Mediterranean coast where they make fleur de sel. But even fleur de sell
usually doesn't have very big hopper crystals. I've seen some big ones that remain on Cyprus,
but the biggest ones I've ever seen are Maldon sea salt, this very expensive salt made on
the southeast coast of England. I've emailed the Maldon company several times, and apparently
they're not interested in sharing their centuries long trade secrets with me.
Totally understand that. There is this promotional video that they made that I've linked in the
description. Judging on what I can see in there, I'm guessing that maybe it's their
agitation of the brine that keeps the crystals from agglomerating before they can grow to
a decent size. But that is just a guess. If anyone has any ideas on how to easily make
hopper crystals at home, drop them in the comments. But if you need to easily make a
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to be my favorite inorganic substance in this universe.