Ice made in domestic freezers is inevitably cloudy because of the dissolved air that tap water contains, around 0.003 per cent by weight.
As the water in the ice trays drops below freezing point, crystals form around the edges of the compartments.
These are pure ice and they contain very little air because the solubility of air in ice is very low and the liquid left behind can still hold it in solution.
Once the concentration of air in the liquid reaches 0.0038 per cent by weight and the temperature has dropped to -0.0024 °C, the liquid can contain no more air and a new reaction begins.
As the water freezes the air is forced out of solution.
The natural state of air at the temperature and pressure involved here is a gas, so it forms bubbles in the ice.
Commercial ice machines produce attractive, clear ice by passing a constant stream of water past freezing metal fingers, or over freezing metal trays.
This freezes out a fraction of the water while the rest of it is discarded before the concentration of air gets too high. When the ice is thick enough the metal fingers or tray are warmed to release their crystal-clear ice that is good enough to film.
Alas, without an ice machine, the questioner may have to make do with cloudy ice cubes.
Water has its highest density at around 4 °C. Below that the water gets less dense as it approaches freezing point.
Air bubbles form in the ice when the cooling of the water is too rapid, which causes one part of the water to be at a different temperature to other parts. Ice is usually formed at the top of the water first because the warmer and denser water sinks beneath the ice layer that begins to form there.
Additionally, the top layer is usually the part that is in contact with the cold environment.
This is similar to what happens in a frozen lake. The various expansion rates of different parts of the water will inevitably create air bubbles that cannot escape because of the ice sheet above.
To avoid ice bubbles, the trick is to cool the water very slowly so that there is no large temperature gradient which can cause widely different expansion. Cooling it slowly also allows air to have sufficient time to move through the liquid and evaporate before it is trapped by the solid ice.
Failure to achieve clear ice even after boiling the water may depend on what precautions you take in addition to filtration and boiling. For instance, gas can still be dissolved if water is poured after boiling has finished and hard water may also need deionising to rid it of gas.
It also helps to exclude air while the water is cooling, perhaps with clingfilm. Try to achieve zone freezing by cooling the water slowly from the top down, maybe in a polystyrene container with only clingfilm over the top.
Do not use a vacuum flask because the glass is far too fragile.
Although the amount of gas that freezes out is unaffected by the method of freezing, zone freezing starts by giving a nice, thick chunk of clear ice. As freezing progresses murky ice appears and then you can stop the process.
Water contains dissolved gases. When it freezes, the gas is forced out of it to form bubbles that are then trapped in the ice, making it look opaque.
In order to get transparent ice you should use warm rather than cold water, since warm water contains less dissolved gas. Also, try to reduce the power of your freezer in order to allow time for the gases to diffuse out just before freezing.
This method works very well.
It is easy to be seduced by a professional photographer who uses hand-carved perspex “ice” cubes in ads for Scotch because they don’t melt under the studio lights.
If you look very carefully, you might also see the tiny glass bubbles on the meniscus of other drinks, these won’t disappear at the wrong moment either.
