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Comment by leblancfg | Hacker News Reader

Former bartender here. Having nice cocktail ice chunks at home isn't as complex as this might make it seem. No need for a clunky box in your freezer or costly gizmos.

I keep a travel-sized coffee thermos in my freezer with about 2-3 inches of water at the bottom. Takes 18-24 hours to create one chunk. If you time it right, 90% of the water freezes, and the water impurities pool at the bottom – pop out the frozen chunk and the result is clear ice. I store 4-5 of these in a zip-top bag until I have friends over and serve drinks.

Best to use a thermos with a slight conical interior so it's easy to pop out the ice. Makes a nice plug-shaped, clear ice chunk. Then refill and restart the process.

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In the lab one of the cool things is to take a 100 mL pyrex tall-form (Berzelius) beaker about half full of DI water and set it on top of a little slab of dry ice which is at the bottom of a small bowl.

Then pack some crushed dry ice around the beaker and the water starts to freeze quite quickly from the outside in.

When the frozen water is about 5mm thick, remove the beaker, pour out the liquid water and return the beaker to the dry ice nest until it's quite a bit colder.

The ice in the beaker continues to shrink and can then be removed from the beaker "mold" as a shot glass made of ice.

Caution: do not handle it with bare hands, nor consume any contents at this point until it is no longer very far below 0.

Best to allow these ice glasses to "warm up" to a comfortable temperature in an ordinary freezer before use.

It will then withstand pre-chilled liquor for a number of minutes after serving.

>If you time it right, 90% of the water freezes, and the water impurities pool at the bottom

And even if you don't, worst case you end up with a cloudy end on your clear ice that can be easily removed by rubbing it on an aluminum sheet pan

I'm not sure it will be so easy to get the ice out if you let it freeze solid.

Well luckily there’s an easy solution for that as well: letting it sit for awhile.

Aluminum is a fantastic heat conductor. You can slice ice using a non-sharpened aluminum flat, easily.

Or just passing the cloudy bit under hot water. Melts like a charm.

An even lower equipment option I've had some success with: Just use any food container, such as an ice-cream box or tupperware. Part fill it and put in the freezer with no lid. It will start freezing from the outside in. When it is partly frozen but before it goes cloudy, break a hole in the top of the ice to allow the trapped liquid inside to escape, and put it back in for a few more hours until it is nearly all frozen. This way you can get a big mostly clear block with a cloudy bit in the middle (you can repeat the break and freeze to get it even smaller), which you can cut up and melt off any cloudy bits with warm water.

Water expands when it freezes, in my head thermos is an extremely rigid structure, how do you stop it from rapturing?

Simply don't fill it to the brim, allowing room for expansion.

The thermos is mostly full of compressible air

I think the idea is leaving it open so it freezes from the top down. From what I've read there, the trick of clear ice is directional freezing.

I've seen some other ideas like freezing water in a camping cooler with its top open.

That's what I thought the idea was, too.

But then, why aren't my ice-cubes made in the regular fridge tray clear? The same principle should apply: they should be clear at the top, where the tray is open.

The thermos wall is an insulator, unlike the tray.

Ice in a tray freezes from all sides since it's not insulated. And indeed, the ice from your tray is crystal clear on the sides, with the cloudy ice at the center of the cube.

In an insulated vessel, the sides and bottom remain liquid until the frozen portion grows down from the top, pushing the cloudy bit to the bottom, which you can then just lop off.

I'm not sure if your end of the world planning needs to include your thermos.

Only if its completely full. A travel thermos is ~6 inches deep and OP said to fill it to 2-3 inches.

> how do you stop it from rapturing?

From the post you’re replying to: 2 to 3 inches of water at the bottom … expands to fill the thermos

Air compresses. A lot.

Not an issue. It's only partially filled, less than half-way. As it forms, the ice floats and glides up the sides until it's like, 1mm higher than where it started.

Well you make sure it isnt baptized.

But also as the ice freezes I believe it becomes more buoyant and that rigidity causes the ice to be lifted up when it exerts force on the thermos interior.

Pun of the year.

Will be trying this. Thanks for the tip!

Have a particular one to recommend?

Whatever's cheap and slightly conical – most will do. Worst case you let it sit for ~1h so it melts a little.

From 10 seconds of Googling, something like this: https://browzefactory.com/products/tumbler-stainless-steel-2...

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