Air-free ice dilutes drinks less. The naive way is to boil the water. But it seems like a waste of fuel. I have both a stove and microwave. How do I make air-free ice without using as much fuel?
Freeze it slowly
The problem with freezers today is that they're just too fast. Still water doesn't have time to let all the air out before it's trapped in ice. By slowing it down, you can get clearer ice.
One way to do this is by changing the setting on your freezer. Use something very close to freezing, say -1C or 30F. For most freezers, this will be the warmest setting. The downside to this is that it will affect everything else in the freezer as well. If you have food in there you'd like to keep colder, this probably isn't the method for you.
Another way is by using something insulated. You can probably find a plastic or styrofoam cooler that will fit an ice tray (and fit in your freezer). Put your tray in it, and toss it in the freezer.
Both of these things will cause it to take much longer to freeze. The one time I tried the freezer temperature method, it took about 24 hours for it to completely freeze two trays, but they were nice and clear.
The cooler method probably takes longer, but I haven't tried that. I read online that a good cooler can keep water unfrozen for a few days. If that's the case, try it with the lid cracked a bit. This should speed it up some while still providing a good bit of insulation.
Make a block of ice much larger than necessary cut it into pieces. Discard the ones from the centre. The freezing process pushes air towards the centre of the block so the outside of the ice ends up air-free enough to be optical quality.
There are several methods described on this website and summarized here.
Disclaimer, I've not tried any of these except using purified water, which does work well.
Use pure water. This method looks to get as much of the air and mineral impurities out of the water as possible before freezing, start with already distilled water. Filtered bottled water will work, or any water purified by reverse osmosis.
Top-Down Freezing Method
Use a cooler to will insulate your ice cubes, forcing them to freeze slowly from the top down.
Place your ice tray, mold or other freezing container at the bottom of the cooler, open to the top. Fill your tray or molds with water.
Pour water into the bottom of the cooler, filling in around your ice tray or molds. This water will seal off your ice cubes, keeping cold air from freezing the sides or bottom.
Place the cooler with the lid off in your freezer. Make sure your freezer is not set too cold—17-25°F should do it. Leave the cooler for 24 hours. away the ice around your tray or molds and remove your ice cubes.
High-Temperature Freezing Method
Set your freezer to just below freezing, around 30°F or -1°C. Fill an ice tray or mold with water and place put it in the freezer. Leave it to freeze for 24 hours. The slow freeze should force out any gasses and impurities, leaving you with perfectly clear ice cubes.
Bottom freezing method
Fill up a bowl with water then dump a lot of salt into it to prevent it from freezing then put it in the freezer. Be careful not to put too little water in the bowl or the freezing will release enough heat to warm up the salt water to 0°C before the ice cubes finish freezing. The colder the freezer is, the higher the concentration of salt needs to be to prevent the salt water from freezing. Leave the salt water in the freezer at least 3 hours to get really cold. Take the bowl of salt water out of the freezer to prevent the water in the ice cube tray from freezing from the top.
Fill up an ice cube tray with water then float it on the salt water in the freezer which is denser than fresh water. The result is bubble free ice that is super strong and crack free because it froze without any trapped regions of water ever forming during the process.
(added for the sake of completeness, I realize you don't want to boil water in your question).
Boil the water twice. Boiling removes air bubbles from the liquid, allowing the water molecules to stick together even harder in the freezer. After boiling the first time, allow the water to cool. Then boil again. Keep the cooling water covered to prevent any dust from collecting on the surface.
Cover the tray as it cools to keep particulate out.
There are lots of ways to make nearly-clear ice. I will not discuss those.
There are two basic methods to make perfectly clear (gas-free) ice. The easiest by far is to buy a small bar ice maker, also called a "clear ice" maker. Such ice makers are used by bars and restaurants. For about $200 you can get a small one that fits on a counter top. They work by using a cold plate at the bottom and circulating the water. This causes the ice to freeze from the bottom up. Air escapes out the top.
The do-it-yourself method is a lot more work. The idea is the reverse of the cold-plate method. Put a well-insulated box (like a small cooler or styrofoam box) in the freezer and make a cube tray that will just fit it in the bottom. The idea is that you are going to freeze the ice from the top down. The insulated box prevents any cold from reaching the water, except from the top. In the bottom of each cube cell make a hole. As the ice freezes downwards, it will push the air out the hole. You have to take the ice out as soon as it reaches the bottom of the tray, otherwise you will have bumps on the bottom of the ice if it is allowed to grow through the holes.
Use pure water.
This method looks to get as much of the air and mineral impurities out of the water as possible before freezing, start with already distilled water. Filtered bottled water will work, or any water purified by reverse osmosis.
Boil the water twice.
Boiling removes air bubbles from the liquid, allowing the water molecules to stick together even harder in the freezer.
After boiling the first time, allow the water to cool. Then boil again.
Keep the cooling water covered to prevent any dust from collecting on the surface.
Pour water into an ice tray or other mold and cover with a plastic wrap to keep out particles.
Make sure that you have let the water cool a little bit before pouring it in the tray so that it doesn't melt the plastic. If you really want to impress, try making extra large clear ice cubes and ice spheres. Nothing like drinking a cocktail on one very large rock.
Place the ice tray in the freezer.
Leave it for few hours to freeze.
Thats' It ... You have Done !!!