When I look for ice cubes for an evening gin and tonic, I often find they have shattered in the mould. What causes this?
Ron Dippold
San Diego, California, US
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I had this problem just last month. Shattered ice waters the gin and tonic down too quickly. It is an affront to civilisation, so I did a bit of research and experimentation!
First, it is helpful to consider how ice forms in a mould. Water freezes from the outside in, and most importantly, it expands by about 10 per cent when going from liquid to solid. So, at some point, the ice in a hard mould starts expanding upwards since there is nowhere else to go. As a result, if this expansion gets impeded, the ice is likely to crack. It can also crack when you attempt to remove it from a mould. Ice is semi-plastic, so it can change shape without breaking, but if it deforms too fast, that will also make it crack.
So what makes cracking more likely? Assuming you have a normally operating freezer, there are three common reasons. The first is minerals in the water causing weaknesses in the ice. The second, and most common, reason is a build-up of minerals on the mould walls. The third is oxygen in the water.
Mineral build-up on the walls of a mould grabs the ice and keeps it from moving upwards when it wants to expand. Excessive minerals in the water cause 鈥済rain boundary cracks鈥� 鈥� where there are so many grains in the ice that it finds it easier to fracture than expand. Oxygen in the water similarly creates weak spots in the ice. In retrospect, I have seen this problem get better or worse as my water supply changes.
Luckily, there are several solutions. The easy way is just to use a soft silicone mould 鈥� if the ice can just expand evenly, the pressure won鈥檛 be a problem. The second solution is to occasionally wash your existing mould with hot water and vinegar to remove mineral build-up. That worked for me. The next thing to do would be to filter and boil your water to remove minerals and oxygen. I hope this works for you. Cheers!
Robin Maguire
Hobart, Tasmania, Australia
Ice cubes shatter in the mould because, unlike most substances, water at 0掳C (32掳F) expands as it transitions from liquid to solid 鈥� which is why ice floats on ponds. As the water in the mould cools, it will lose heat through the outer surface, which will freeze first. As the liquid interior subsequently freezes, it will expand, placing the prefrozen exterior in tension and rendering it liable to shatter. As with all brittle materials, anything that can initiate a tiny crack could trigger this 鈥� for example, a speck of impurity or the act of removal from the mould.
Tempered glass uses the same effect in reverse. It is made by spraying hot glass with a fluid, causing the outer surfaces to cool and solidify quickly. As the hotter interior then cools and sets, it shrinks. This places the exterior into compression, a state in which cracks don鈥檛 propagate, so shattering will be strongly resisted.
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