Pykrete is a composite material made of approximately 45 percent sawdust or some other form of wood pulp (such as paper) and 55 percent ice by weight. The properties of such a composite were apparently first noted by a couple of researchers at Polytechnic University of New York, and were investigated more thoroughly by Max Perutz. Its use was proposed during World War II by Geoffrey Pyke to the Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier. Pykrete has some interesting properties, notably its relatively slow melting rate (due to low thermal conductivity), and its vastly improved strength and toughness over unmodified (crystalline) ice, actually closer to concrete. Pykrete is slightly more difficult to form than concrete, as it expands during the freezing process, but can be repaired and maintained from the sea's most abundant raw material: water.
Plain ice proved to be insufficiently strong. Pyke learned from a report by Herman Mark and his assistant Walter Hohenstein that ice made from water mixed with wood fibres formed a strong solid mass – very much stronger than pure water ice. Max Perutz later recalled:
Then, one day, Pyke handed me a report that he said he found hard to understand. It was by Herman Mark, my former professor of physical chemistry in Vienna, who had lost his post there when the Nazis overran Austria, and found a haven at the Polytechnic Institute of Brooklyn. As an expert on plastics, he knew that many of them were brittle when pure, but could be toughened by embedding fibres such as cellulose in them, just as concrete can be reinforced with steel wires. Mark and his assistant, Walter P. Hohenstein, stirred a little cotton wool or wood pulp - the raw material of newsprint - into water before they froze it, and found that these additions strengthened the ice dramatically.
When I had read their report, I advised my superiors to scrap our experiments with pure ice and set up a laboratory for the manufacture and testing of reinforced ice. Combined Operations requisitioned a large meat store five floors underground beneath Smithfield Market, which lies within sight of St. Paul's Cathedral, and ordered some electrically heated suits, of the type issued to airmen, to keep us warm at
Perutz would later learn that Project Habakkuk was the plan to build an enormous aircraft carrier, actually more of a floating island than a ship in the traditional sense. The experiments of Perutz and his collaborators in Smithfield Meat Market in the City of London took place in great secrecy behind a screen of animal carcasses. The tests confirmed that pykrete is much stronger than pure ice and, unlike the latter, does not shatter, but also that it sags under its own weight at temperatures higher than -15 C.
Mountbatten’s reaction to the breakthrough is recorded by Pykes biographer David Lampe:What happened next was explained several years after the war by Lord Mountbatten in a widely-quoted after-dinner speech. "I was sent to Chequers to see the Prime Minister and was told he was in his bath. I said, 'Good, that's exactly where I want him to be.' I nipped up the stairs and called out to him, 'I have a block of a new material which I would like to put in your bath.' After that he suggested that I should take it to the Quebec Conference."
The demonstration in Churchill's streaming bath had been most dramatic. After the outer film of ice on the small pykrete cube had melted, the freshly exposed wood pulp kept the remainder of the block from thawing.
As an after dinner speech, Mountbatten’s story may have been elaborated somewhat to entertain his audience — the mental image of Churchill in his bath perhaps smoking one of his trademark cigars is rather amusing — and the incident may have been entirely imaginary. The story does, however, convey the sense of excitement generated by the discovery.
Despite these tests, the main Project Habakkuk was never put into action. The funds simply were not available due to other WWII projects, as well as the belief that the tides of the war were beginning to turn in favour of the Allies using more conventional methods..
According to the memoirs of British General Ismay:A good deal of consideration, much of it highly technical, was also given to the feasibility of building floating platforms which could either be used by fighters to support opposed landings until such time as airfields ashore were available, or act as staging points for ferrying aircraft over long distances. The idea as originally conceived by a member of Combined Operations staff, and vehemently supported by Mountbatten, was that these floating platforms should be constructed out of icebergs. They would be provided with engines which would enable them to steam at slow speed, and with refrigeration plants to prevent them melting. They would be unsinkable. The whole thing seemed completely fantastic, but the idea was not abandoned without a great deal of investigation. Various alternative methods of construction were then considered by the United States naval authorities, but in the end there was general agreement that carriers and auxiliary carriers would serve the same purpose more effectively.
New concepts for pykrete however crop up occasionally among architects, engineers and futurists, usually regarding its potential for mammoth offshore construction or its improvement by applying super-strong materials such as synthetic composites or Kevlar.
DurabilityThe durability of Pykrete is up for debate. It is often asserted that Pykrete has a crush resistance of greater than 3,000 pounds per square inch (21 MPa) so a very short 1 inch (25 mm) column could support the weight of a typical car. However, no published figures support this. Perutz himself gives a value of around 1100 lbs/sq. in..
A September 1943 proposal for making smaller pykrete vessels included the following table of characteristics. Strengths are in units of pounds per square inch:
Comparative Properties of Materials Mechanical properties Ice Concrete Pykerete Crushing strength 500 2,500 1,100 Tensile 160 250 700 Density 0.91 2.5 0.98
A British service rifle bullet (a .303 caliber) will penetrate only 16.5 cm (6.5 inches) of pykrete. Of course ice always slowly deforms under pressure anyway, and this isn't really affected by the presence of the pulp.
Here is a modern demonstration, for example:
The above image shows a 25mm (1 inch) thick 50% mixture (by volume using shredded wood mulch) hit by a single 7.62 x 39 mm rifle round fired from 30 feet (10 m) which bounced off the surface. It took an additional 15 rounds of .223 fired from 5m (15 ft) to crack the block. The above image shows a 50 mm (2 inch) thick 50% mixture (by volume using shredded wood mulch) hit by a single 7.62 x 39 mm rifle round (lower impact mark) fired from 10 m (30 ft) which bounced off the surface. It took an additional 7 rounds (upper penetration mark) of 7.62 x 39 mm fired from 5 m (15 ft) to penetrate the block.
ManufacturePykrete can be easily formed using water and some form of wood pulp, even newspaper or paper towels. (Other fibrous materials might also be suitable, but there seems to have been little investigation of alternatives.) The mixture can be molded into any shape and frozen, and it will be extremely tough and durable, as long as it is kept at or below freezing.
TriviaTradition asserts that Mountbatten managed to convince Winston Churchill of its usefulness by bursting in on Churchill when he was having a bath, and plunging a block of pykrete into the bathwater. This may or may not have happened. Another tale is that at the Quebec Conference of 1943 Mountbatten brought a block of Pykrete along to demonstrate its potential to the entourage of admirals and generals who had come along with Winston Churchill and Franklin D. Roosevelt. Mountbatten entered the project meeting with two blocks and placed them on the ground. One was a normal ice block and the other was Pykrete. He then drew his service pistol and shot at the first block. It shattered and splintered. Next, he fired at the Pykrete to give an idea of the resistance of that kind of ice to projectiles. The bullet ricocheted off the block, grazing the trouser leg of Admiral Ernest King and ending up in the wall. The Admiral may or may not have been impressed by Mountbatten's unorthodox demonstration. According to Perutz's own account, however, the incident of a ricochetting bullet hitting an Admiral actually happened much earlier in London and the gun was fired by someone on the project — not Mountbatten.
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- Perutz, Max (2002 - paperback). I Wish I Made You Angry Earlier. Oxford University Press.
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