This is actually really easy. You’ll just need a small cooler, a bread knife and a mallet.
As ice freezes it pushes out gasses. In an ice cube tray the gasses have nowhere to go and they get trapped in your cubes. By using an insulated cooler with the top open, you can force the ice to freeze from the top down.
Doing this creates one huge block with a perfectly clear top layer. It’s easier to work with if you pull it out of the freezer when only the top layer is frozen. However, if you let it freeze all the way through you can use the imperfect ice for shaking and stirring.
I have a 7 quart (~6.5L) cooler that’s a great size. One with a removable lid would be better. But really, anything you can fit in your freezer should work.
- Fill your cooler with tap water and put it in the freezer with the top open for 24–48 hours (time will vary depending on cooler size and freezer temperature).
- Pull it out and let it sit for at least 10min then tip the cooler upside down over a sink and get the block out. Trim the roughness off the bottom until you have one solid block.
- Let it sit to temper. If you don’t let the ice temper it will shatter instead of breaking cleanly when you try and cut it. The bigger the block the longer tempering takes. If you just froze the top 2” it’d be ready to cut in about 15–20min but you can leave a large block out for hours. If it’s shattering you didn’t leave it long enough.
- Score the ice with a with a bread knife along one edge, then hit the back of the knife with a mallet gently and repeatedly. It should crack along the score line and break into smaller blocks.
- Repeat cutting until you have the various shapes and sizes you need. 2” (50mm) cubes are great for a double rocks glass.
- Shake the excess water off each block and store in the freezer in a ziploc bag.
Pouring a drink over a cube that’s been pulled straight out of the freezer is likely to make it crack. A 2” cube only takes a couple minutes to temper which is, conveniently, about as long as it takes to make a drink.
This video gives a really good overview of the process but unfortunately it skips over tempering.