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Pykrete is a composite material made of 14% wood pulp and ice, invented by Geoffrey Nathaniel Pyke and proposed during World War II to the Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier (Project Habbakuk), actually more of a floating island than a ship in the traditional sense. Pykrete has some interesting properties, notably its relatively slow melting rate (due to low thermal conductivity), and its vastly improved strength and toughness over pure ice, actually closer to concrete. Pykrete is slightly harder to form than concrete, as it expands whilst freezing, but can be repaired and maintained from the sea's most abundant raw material.

Pyke managed to convince Lord Mountbatten of the worth of his project some time around 1942, and trials were made in two locations in Alberta in Canada. Blocks of Pykrete were attacked with various explosives and it was found that a charge corresponding to a torpedo warhead would have made only a minor dent in the planned Habbakuk carrier.

At the Quebec Conference of 1943 Mountbatten brought a block of Pykrete along to demonstrate its potential to the bevy of admirals and generals who had come along with Winston Churchill and Franklin D. Roosevelt. It is reported that he first asked a US admiral to test the solidity of Pykrete by trying to break it with an axe. The axe bounced off the block and nearly chopped off the admiral's foot. Mountbatten then took out his service pistol and shot at the block, to give an idea of the resistance of this kind of ice to projectiles. The bullet ricocheted off the block and hit one of the walls, a few inches from the head of a US general. The comments of the aforementioned admiral and general (or their colleagues) were not recorded for posterity.

As a bit of fun, Pykrete can easily be made in small quantities by shredding bathroom tissue in water (or you can use sawdust) and molding it in (say) an ice cube tray. As an experiment, vary the volume fraction of pulp to water, and freeze it. One could bang on it with a hammer to see how resistant it is to impact loading.