Stuff You Should Know - Short Stuff: Yakhchāls - Ancient Fridges

Episode Date: February 12, 2025

If you lived in ancient Persia, you could do a lot worse in trying to cool things down than by building a yakhchāl. Today we break down how the early fridges worked.See omnystudio.com/listener for privacy information.

Do you want to see into the future? Do you want to understand an invisible force that's shaping your life? Do you want to experience the frontiers of what makes us human? On Tech Stuff, we travel from the minds of Congo to the surface of Mars, from conversations with Nobel Prize winners

to the depths of TikTok, to ask burning questions about technology, from high tech to low culture and everywhere in between. Join us. Listen to Tech Stuff on the iHeartRadio app, Apple podcasts or wherever you get your podcasts. Hey and welcome to The Short Stuff. I'm Josh and there's Chuck, Jerry sitting in for Dave. And so this is Short Stuff, the How Do You Say This Again edition.

Yeah, I'm going gonna say it's pronounced, spelled Y-A-K-H, you know, as most words are. C-H-A-A-L. I'm gonna say Yak-Als. I'm going with the straight ahead Yak-Chalz. All right. Okay, so you say it your way throughout, I'll say it my way, and I'm sure I'll inevitably unconsciously

start saying it your way. We'll see. But what we're talking about is the promise from our refrigeration episode a little bit more on these ancient, basically ancient refrigerators or cooling systems. They were found across ancient Persia, at least as old as 400 BC. This is modern-day Iran. And these are places where, believe it or not, the climate enables freezing of ice when you would not think you should be able to freeze ice.

Yeah, it's pretty amazing. And apparently still today in Iran, Afghanistan, and Tajikistan, yeah, I said it right the first time, they call their refrigerators Yak-Chal's, which is how I would say it if I were in Iran. But that's the name for their fridge, which means that at some point someone in Iran has gone into a store and said, you got a smeg Yak-Chal? Yeah, exactly. And these have been the fascination of like everyone from engineers to historians

to physicists over the years because they're just so kind of confusing in how they actually work. And I'm still not entirely sure how it works. It seems to be a little magic involved. One thing I know goes a long way toward keeping this ice, and we got to say some of this ice is mined from the mountains and brought down and preserved. Some is made on site, we'll get to that. But one big factor is the insulation of the structure itself, which is made from a mortar called Sourouge. Is that how you're saying it? Yeah, sand, clay, egg whites,

lime, goat hair, and ash. Quite a mixture. Some of these were several meters thick, some of these yak chals were. There was a study, we got some of this information from the engineering firm Max Fordham, and they did an analysis from 2018 of yak chiles and just how effective they might have been and they found that the walls of a yak chile had the same

insulative properties as a wall of concrete three inches thick surrounded by a one foot thick wrapping of styrofoam insulation. That's how effective these things were. Sand, clay, egg whites, lime, and goat hair, and ash. I was gonna say like the secret is egg whites, but I don't know, you throw goat hair in there. Yeah. Who knows what the secret, I think the secret is the whole thing together,

the whole se-roo-ge mixture. Yeah, probably so. So I mentioned that sometimes the ice was brought in from the mountains and kept there throughout the year, but usually what would happen is they would make ice, they would bring in water, they would divert water from an aqueduct through these underground water channels called canots, and they would channel them to the north side of this wall. It's another thing we haven't mentioned yet is they have these very, very high walls that act as shade for these channels to keep, you know, the wind off of it because stuff isn't going to freeze as fast if it's moving.

So to keep the water still and to keep it cooler away from that sun. Yeah, and this, so the channel has a little diversion into a trench or a pit or like a very shallow like rectangular pond usually, and it, they'll divert water in there to fill it up and then they let it freeze overnight. Over the course of a few nights, it'll continue to freeze and freeze and freeze in layers. And what they're taking advantage of, you know, like when it snows and then the temperatures heat up and all the snow melts, but there's a little pile of snow

like in a shaded corner of your yard that never gets direct sunlight and it just takes forever to melt. They're taking advantage of the same thing. They're building that big old wall to keep it shaded and to just let this ice, um, grow and grow and grow. And then once it reaches, I think, 50 centimeters, which is like about a half a meter, um, thick, then they'll cut it into blocks and they put it in the Yak

chow and they store it through summer. Like this stuff will stay frozen for an entire summer. So in that sense, these yak chals are built to store cold throughout the course of a year. Even when the summer comes around, still cold. Yeah, another way that helps us out is that dome shape. It's not domed because they like domes, even though domes are nice.

It's conical because that optimizes what's called the solar chimney effect. That's when you create a convection current, letting that heat go up, up, up, and out through these openings at the top and bringing in that cool air from the bottom. They also have wind catchers that they call badgers. And they actually take wind and direct it downward

into the Yakchal Dome. And so the air that hits it is cooled by that ice and the air that's not cooled by it or cooled enough, like you said, that chimney effect takes it up along the curved sides toward the hole in the top and it says, see you later, don't come back. That's right. And that feels like a good time for a break.

And we'll come back and talk about what the heck they're doing with all this ice right after this. Do you want to understand an invisible force that's shaping your life? I'm Osvaldo Lossian, one of the new hosts of the long-running podcast Tech Stuff. I'm slightly skeptical but obsessively intrigued. And I'm Kara Price, the other new host. And I'm ready to adopt early and often.

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Distracted driving is what's really scary and even deadly. Eyes forward, don't drive distracted. Brought to you by NHTSA and the Ad Council. All right, so they're making ice, they're preserving ice. What are they doing with all this ice? One of the things they're doing is using it as a refrigerator. You know, they'll store food in there that they don't want to go bad and just using it as a refrigerator. You know, they'll store food in there that they don't want to go bad and just using it as a cold house. They will also just use the ice to eat in a treat. They have something called falooda over there. It's a Persian traditional dessert.

Has thin vermicelli noodles made from cornstarch. And then you mix that up with a little semi-frozen syrup of sugar and rose water, and then serve it up with a little lime juice and maybe some ground pistachios. And it's like a little Persian icy. That has to be better than it sounds, don't you think? Oh, I think it sounds great. I think the pistachios are where they're throwing me off.

Oh yeah? You don't like pistachios or? I do like pistachios. It just, they don't seem to go with the rest of the ingredients. But like I said, I'm sure it's delicious. I mean, it's a traditional Persian dessert. It's got staying power. So who am I to question Faluda?

Exactly. I bet it's delicious. One of the cool things about this is that Yak Chals, I mean, and there's some still around. I think there's one in Kerman, Iran, that's about the size of a five-story building. So you would think, of course, obviously, this was reserved for royalty. You would be dead wrong. Because not only were Yakchals open and available to the public, there were some that people just built for their houses that were of private use as well. Yeah, for sure. They would make those delicious faluda. They would make sherbets and preserve

them. And that nuts? With fruit and ice. And they would put them on a donkey and go sell them at market and stuff like that. They would sell ice directly from those places sometimes. And, you know, one of the things they're also taking advantage of is the greenhouse effect. You know, the earth is going to stay fairly warm at night because of the greenhouse effect, trapping those gases in the atmosphere. But if it's low humidity and if it's a really clear night, that effect is going to be weaker and that heat can dissipate and disappear more readily.

And so that's when they discovered like, hey, we can make these little thin layers of ice and kind of build day by day, night by night on these clear low humidity nights eventually to get some pretty significant ice. And once they figure that out, someone said, go get the donkey. Exactly. It's time to sell some sherbet. And harvest some goat hair. One other thing I saw in that Max Fordham analysis,

they figured out that they could make about what would be equivalent to three million ice cubes a season. Wow. Yeah, which is a lot, but they were like, you'd think it'd be more. And I was like, it seems pretty good to me, especially in 400 BC, you know? Depends on how big those cubes were.

Were they big fancy cocktail cubes? That's a lot more ice than an ice nugget. That's right, you're absolutely right. I expect that they were probably all donkey head size. So three million donkey head size ice cubes. That's big. Think about the poor donkey that had to cart those around. I know, no fun.

You got anything else? Yeah, I mean, these went away, obviously, because of modern refrigeration came on the scene. And also, one thing that was happening was they were making this ice kind of out in the open, and a lot of it would get contaminated with dust, and it wasn't like the healthiest ice in the world. And so that combined with modern refrigeration coming along, they're like, maybe we should

just not have these much anymore. But like you mentioned, there is one still around at least in Kerman, and there are groups there that are trying to preserve this way of life and at least keep it, you know, not like a chief refrigerating method, but like, hey, we can't lose our culture, and so let's work to take some of these old ones and restore them at least, even if only for like museum and touring purposes. For sure, but also there's a lot to learn from them,

especially when we're trying to advance like passive cooling and other things that require less energy to cool things down, yakchals are something to turn to and say, how do we do this? And someone says, go get the goat here. That's right, get some egg whites too. No one likes those.

What do we do with all these yolks? Yeah. Well, I think that's it for short stuff, right Chuck? I think so. Well, that means short stuff is that. Stuff you should know is a production of iHeartRadio. Well, that means short stuff is that.