About Julian Vallis
Clear vs Cloudy ice
Most ice you make in standard ice trays and from American freezers are often cloudy in the centre of the cube.
The reason for this is that plain old tap water actually has a lot of minerals in it, and water also contains a fair amount of air in it too. You don't notice it when it's liquid because as water gets warmer, it can have dissolved substances within it - just like you find salt and sugar dissolves much better in hot water versus cold water.
When water freezes into ice, it forms a lovely crystal lattice, and any impurities in the water are pushed away from the ice being formed.
The issue is in a standard ice tray, the freezing direction starts from the outside of the cube, and progressively freezes towards the centre. This means all the impurities in water gather in the centre of the cube, and trapped by ice all around they simply have nowhere to go.
If there are too many impurities, the whole cube will actually crack itself open because pressure builds up in the centre and eventually causes the ice lattice to break.
The trick with clear ice therefore takes the phenomenon of a lake freezing over - only the very top layer of the lake actually freezes, and the ice gets progressively thicker. The reason is because the earth and rock holding the lack is insulating the water and doesn't freeze underneath, but the air on top is much colder and it freezes from top-down in a particular direction.
Hence clear ice is formed by forcing the direction of the freezing in one particular direction, and thus known as directional freezing.
How to make clear ice
To exploit directional freezing, you need to insulate 3 sides of your ice mould. If you took an ice tray and pierced some holes in the bottom of it, then suspended the ice tray over a cake tin, and finally insulated the cake tin, what you'll get is lovely clear ice cubes, with a massive ice cube underneath the tray in the cake tin containing all the cloudy impurities.
This is because the ice progressively froze from the top, and all the impurities went through the holes you pierced before the ice below the tray started freezing itself.
Another way to make big ice blocks therefore is to get an insulated picnic cooler and put the water in that, then put the entire thing in a freezer. As you don't have a tray separating the clear ice from the cloudy ice, what will happen is the top third of the ice block will be perfectly clear, while the bottom half will be all cloudy, and you'd need to saw it in half to get the clear ice.
There are plenty of 'heath robinson' devices that you can make involving picnic coolers and coathangers, but there are commercial ice makers as well.
If you're interested in making your own ice making kit, we suggest you read Camper English's blog at Alcademics, as he's written loads of interesting articles on directional freezing.
The best actual ice machines for bars are a Japanese company called Hoshizaki, but these are expensive professional catering devices.
Finally what we normal people suggest is looking at the Wintersmiths range of ice makers and moulds that you can put in a normal freezer.