It was a mixture of straw/wood pulp and water frozen, that was an idea. Never used. Pykrete, a mixture of water and woodpulp that when frozen was stronger than plain ice, was slower-melting and would not sink. Pykrete could be machined like wood and cast into shapes like metal, and when immersed in water formed an insulating shell of wet wood pulp on its surface that protected its interior from further melting. However, Perutz found a problem: ice flows slowly, in what is known as plastic flow, and his tests showed that a pykrete ship would slowly sag unless it was cooled to −16 °C (3 °F). The enormous amount of steel needed for the refrigeration plant that was to freeze the pykrete was greater than that needed to build the entire carrier of steel, but the crucial argument was that the rapidly increasing range of land-based aircraft rendered floating islands unnecessary.