How do you make ice cubes clear?

We put this question on ice for Naked Scientist Chris Smith...

Chris - I'm presuming he's having a cocktail party and I'm looking forward to my invitation if I can solve this one for you. The answer to why ice is not clear first of all helps you to understand what you need to have clear ice. Snow is ice but it of course looks white, but ice on a pond is transparent. Why the difference? Well, if you look at the difference between snow in ice and a pond, snow particles are lots of tiny ice crystals. The crystal that you see on the surface of a pond that you can see through is almost a single crystal of ice. If you have lots of tiny crystals, when light goes into the crystals then it gets reflected and it gets bounced about all over the place and all of the different colours of light come back towards you and that's why actually, it looks white because when you mix all those colours together, you get white light. So, when you have an ice cube that's crazed which is not clear, the reason it's not clear is because there are lots of little, either fractures in the ice cube or more commonly, lots of little crystals that have all formed to produce one giant ice cube. How then do you end up with a single ice cube that's a single crystal which is what you need in order to not disrupt the path of the light, like snow, does so it's clear? You need to do what's called nucleate the formation of the ice cube from one position only. In fact, you might not believe it, but when you're making a jet engine, you need to do exactly the same thing because when you make the metal parts of the engine, the strength is in growing your engine parts as one single metal crystal. So, you pour molten metal into a mould and then you drop in, or you initiate, one tiny form of crystal into one place and the whole thing then crystallises following the same crystal structure. So, what a pub will do or someone who's got an ice making machine, usually, you've got a stainless steel tube or something, which will conduct heat really well, you make that cold. You have a central core which is also cooled and this way, you've got lots of surface area in contact with ice. You drop the temperature really fast, but it's going to start the freezing process around that central cooling probe and then the crystal will grow out from there towards the margins - the edge - and therefore, you're more likely to have single crystal, and therefore, more likely to have nice clear ice.

Kat - So, this is why the ice cubes in pub drinks, those kind of their sort of circular shaped. They've got a hole in the middle.

Chris - Yeah, because that way, you've got as much contact of the thing that can take the energy away from the water, the metal, as possible and this drops the temperature nice and quick and that's how you make ice cubes fast but the by-product of that is you generally get them starting the freezing process in one place. They nucleate from one place, you get one crystal, nice clear ice.

Kat - So Kevin's answer is basically buy an ice maker or buy a pub.

Chris - No, just buy a pub.