During World War II a secret military operation, codenamed Habbakuk, set out to make a bulletproof aircraft carrier out of an unusual material: ice. This was no ordinary ice though; pykrete (named after the man who first conceived of Project Habbakuk, Geoffrey Pyke) is 10-20 per cent wood/paper pulp with the remainder made up of water, and incredibly it is bulletproof.
The ability of materials to resist gunfire is related to how effectively they absorb and distribute the energy of the impact. Ice is brittle and shatters when struck, however the flexible cellulose fibres in wood allow pykrete to soak up force much more effectively. As well as bullets, it can also deflect other impacts with a strength comparable to concrete.
As a building material, pykrete is very versatile. It can be machined to form shapes or cast like metal – and because it’s less dense than water it floats. However, like ice, if large structures are constructed from pykrete, they are susceptible to ‘creep’. Ice is held together by weak hydrogen bonds between stacked water molecules. If the block of ice is large, pressure can break the hydrogen bonds causing the molecules to slide over one another, leading to gradual deformation. Cooling pykrete to -16 degrees Celsius (3.2 degrees Fahrenheit), or using steel supports, helps to prevent creep though.
Pykrete has a higher melting temperature than water ice because the wood decreases its ability to conduct heat. Also, when it starts to melt, the wood pulp forms an insulating shell around the frozen core, slowing down the thawing process further.