The trick is slow freezing to allow air to escape in one direction.[0]

This means insulating the container so that it freezes slower in all but one direction.

Note that water gives off heat when it phase changes from liquid to solid. It's visible under FLIR when pouring supercooled water onto a nucleation point.

0. https://www.youtube.com/shorts/odhf1d7RBL4

> It also carves more cleanly—perfect for shaved ice (hello, kakigori fans)

There are places in Japan where you can enjoy kakigōri made from blocks of ice taken from a frozen lake. That has the clarity and density for good shaving.

Interesting physics to know.

Most of the places that care about the clear ice around where I live purchase pre-frozen cubes or spheres.

They come from the vendor with each one individually vacuum sealed in a long strip. Cut one off the strip, open it, and slip it into the glass.

Probably not worth the labor to produce the cubes themselves when there is a simple, deliverable, solution available.

Pretty great video about a company that makes ice for bars in NYC

https://youtu.be/ET8mqVGDQ1s