I mean the kind from ice machines. My fridge at home makes white ice. I don’t think it’s impurities because I have a filter. But ice machines in hotels, bars, and restaurants all turn out these crystal clear cubes.
How can I get glassy ice instead of packed snow?
Some years ago I saw a short film answering that very question. The answer was that the commercial ice machines do it by freezing it slowly. The white is caused by tiny air bubbles trapped in the ice. Freezing it slowly (which means gradually decreasing the temperature over time) gives the bubbles time to escape before it freezes.
I bought a ball ice tray. It’s that rubbery silicon stuff. It freezes the ice balls completely clear.
My under counter ice cube make has clear sometimes hollowed cubes. I really like it in my tea.
I have an icemaker in the fridge but we rarely use it for drinks.
It has to be the freezing time that makes the difference.
To follow up on what Doug K said, I’ve seen it explained that you get clear ice by freezing from the bottom up which allows/drives the dissolved air off before it’s trapped. The air is what makes the cubes “white”.
Some of the information says to use distilled or filtered/boiled water. That removes impurities and air.
I will personally vouch for this ice tray for making clear ice. Water can be straight from the tap and it makes large crystal clear cubes. And the concept is called directional freezing.
The impurities in the water is removed in the making of ice. Water is pumped over the freezing plates. As some of the water freezes the impurities stay in the water flowing over the freezing plates. During the harvest cycle the pump shuts off and the high mineral water left is dumped to the drain, and fresh water is taken on for next cycle.
Ike and Beck,
The rubber and/or insulated trays must be slowing down the freezing process, giving clear ice cubes.
It also occurred to me that a freezer that isn’t very powerful (i.e. old) might freeze water in an ice tray more slowly than a more powerful unit, resulting in clear ice regardless of the ice tray type.
Cooks Illustrated had an article a few months ago about making clear ice cubes.
This might be behind a paywall: What Is Clear Ice, and How Can I Make It at Home? | Cook's Illustrated
Their process is to start with distilled water, boil it to remove some of the dissolved air, then put it in ice cubes trays in the freezer on top of a stack of folded towels to help it freeze from the top down. This pushes the impurities to the bottom of the cube rather than the center. I tried it once and got pretty good results.
It seems to me that boiling the water doesn’t do anything; you’ll reduce the dissolved gases while it’s hot but as it cools back down they just dissolve back in… how could you prevent gases from dissolving back into the water unless you vacuum sealed it?
The main factor is freezing the water in layers and/or having a drain to remove the impurities & bubbles. In a normal ice cube tray the water starts freezing first at the surface then around all the sides and bottom of the tray, trapping the air bubbles in the center. Clear ice makers avoid that trapping.
Tooting my horn a bit here, several years ago I put together a video on the subject. The goal was clear ice for my homemade ice ball maker.
In short, the aforementioned directional freezing is the solution. Boiling, filtering, distilled water, etc. is worthless. Any old tap water works fine in a directional freezing apparatus.
Interestingly, I once tried to make colored ice with food coloring. It failed completely: directional freezing is so good at displacing impurities that it displaced the food coloring as well.
That is really interesting and a tad unexpected! I wonder if this method of shoving impurities away (down?) could be used as a means of purifying water. Would there be cases where ice found in nature is “cleaner” than the water it sits upon?
That’s similar to the directional freezing videos I came up with while researching how to make clear “rocks” ice cubes for my drinks. What I found worked pretty well was using a stack of 750ml Tupperware containers with the top one filled with water. The multiple layers of Tupperware and the trapped air insulated enough so that I could create a 2-3 in block of clear ice (which I then could chop into 2"x2" cubes.
I’m not sure it’s used in any industrial setting, but I did find this patent, which appears to describe a type of directional freezing system, and mentioning entrapment of brine between ice crystals when frozen uniformly.
I do think that standard lake ice shows just this purification effect, being frozen from the top down and liquid on the bottom.
Neat–there are probably a bunch of techniques that would work. My video is pretty old and at the time there were way more people describing bad techniques than good ones. I did eventually find one guy recommending putting your ice in a cooler in the freezer, and this in fact worked. My improvement was just to use expanding foam to create a custom insulated cavity. But really anything that insulates all sides but the top would work. Air is a fine insulator as long as you can reduce any convective motion, as you did.