Here's how to make ice cubes grow long, natural spikes with distilled water, an ice cube tray, a freezer, and some luck.
Ice cubes are rarely meant to be decorative. Just as long as they're small enough to fit into a glass of iced tea and easier to bag and slap on a sprained than a package of frozen lasagna, whatever shape they come in is good enough. Most tea drinkers and injury sufferers, though, have occasionally noticed that the surface of certain ice cubes is warped. What should be a smooth-topped cube is rippled at the surface, as if the ice froze while the water was still being poured into it.
Occasionally that dimpling of the surface is more dramatic, with long spikes extending from the surface of the ice cube. Since water doesn't freeze instantly in a freezer, this has to be an occasional product of the freezing process.
Several different physical processes contribute to making the ice spikes. The first is the fact that when water is exposed to cold temperatures, it freezes from the outside in. That much has been observed by anyone who has seen frost on the top of a puddle. The cubes start freezing at the top, and where the sides touch the plastic trays, which gives up their heat much faster than the water does. At the first stage of the freezing process, the cube of water will be a crust of ice - thinnest at the very center and top of the cube, farthest away from the cold plastic sides – with a liquid center.
When water freezes, it expands. This is no problem for the cube when the top crust forms. It can expand upward. Once the crust has formed it traps the remaining water inside. As the sides and top of the cube continue to freeze, they squeeze the water inside the cube. The result is no different from a person opening the top of a soda can and slowly compressing the sides of the can with their hand. The liquid rushes up and pours out of the top. In the cube, the ‘open' top, is usually at the center of the cube, where the ice is thinnest.
This accounts for the uneven tops commonly seen in ice commonly seen in ice cubes, but spikes are far more dramatic, and are the result of a more rare process. As the water is slowly squeezed by the tube, it seeps out the top. Since the water that comes out of the top is now exposed to the cold, rather than insulated by other liquid water, the way it was before, it freezes quickly. Since the water bordering the hole touching ice, instead of just cold air, it freezes more quickly than the rest, making a ‘neck', a slightly-raised border of ice around the hole. As the water inside the cube continues to be squeezed by the sides, more water is pushed up through this neck and the outer border freezes again, building the neck higher and higher. It also builds the neck narrower and narrower, until it forms a spike.
Conditions have to be just right to build an ice spike, but one controllable factor is the purity level of the water. Higher impurities in water give it a lower freezing temperature. In order for a neck to form, the outer edge of the neck will have to form faster than the water forced up from underneath. If it doesn't, the water will just spread out over the cube and the neck won't build up into a spike. Distilled water removes the impurities and lets the water freeze fastest.
Top image by Seabrooke Leckie.