Ballistic History: Bullet Resistant Barriers from Ice and Sawdust

Bullet Resistant Barriers

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During WWII Geoffrey Pyke–a somewhat eccentric British journalist and inventor–had what, at first glance, was a crazy scheme: To supplement the British Navy with floating, bulletproof iceberg islands.
Allied navies and merchants were sitting ducks for advanced German submarines. Pyke envisioned “berg-ships” up to 4,000 feet long, 600 feet wide, and 130 feet deep, made from forty-foot blocks of ice. They’d be slow-going ships, but their extremely thick, naturally buoyant hulls could endure lots of bullets and torpedoes. And, unlike steel vessels, they’d be floating in a sea of raw material ready for repairs or expansion.


Of course, there were a few hitches in this plan. Obviously, icebergs–even enormous ones–don’t hold up well as they travel south, and even a very large burg can be steadily chiseled away by a concerted barrage. Worse yet, ice is a very unpredictable building material; it fails at loads ranging from 5 to 35 kilograms per square centimeter (kg/sq-cm).
But Pyke had stumbled on a possible solution. He’d discovered that by adding impurities to water–such as sawdust–he could significantly improve the material qualities of the finished ice.


Through grueling hours working in an undercover meat-locker laboratory, Pyke and a team of military researchers discovered a mixture of water and sawdust that, when frozen, was as strong as concrete, could be preformed as easily as ice cubes, and could be worked with hand tools like blocks of wood. One scientist on the project, Max Perutz (who would later go on to win the Nobel Prize for Chemistry in 1962) wrote:
“Blocks of ice containing as little as four percent wood pulp were weight for weight as strong as concrete; in honor of the originator of the project, we called this reinforced ice ‘pykrete.’ When we fired a rifle bullet into an upright block of pure ice two feet square and one foot thick, the block shattered; in pykrete the bullet made a little crater and was embedded without doing any damage.“
Pyke ultimately settled on a mixture of 14 percent sawdust and 86 percent water (by weight). The sawdust significantly lowered the material’s thermal conductivity, which meant that once the outer layer melted slightly and went mushy, it functioned as an insulating jacket, helping to keep the pykrete core frozen solid. These opaque, tan blocks consistently held loads of 70 kg/sq-cm, and were almost uncrushable (a one-inch column could easily support an automobile). A bullet resistant barrier made from pykrete performed as well as brick against small arms fire, and even high-powered rifles. But the real selling point: Not only was pykrete easy to make and form with conventional tools, it was cheap.
Bulletproof glass history


As is the case with conventional bullet resistant barriers–like steel and DuPont™ Kevlar® fiber–the secret to pykrete’s power is in its tensile strength, which is nearly three times that of poured concrete, and five times that of conventional ice. A 7.69 mm rifle bullet fired at pure ice will shatter most blocks (due to the material’s brittleness). Even if you had one of Pyke’s 40-foot blocks, a rifle shot would still burrow more than a foot into the ice. That same bullet fired into pykrete won’t make it more than six inches.

A six-inch thick ice-wall might seem a little inconvenient–especially if you’re building a bank in Georgia–but it’s hard to beat the price. And don’t worry if you don’t happen to live near a lumber yard willing to part with hundreds of pounds of sawdust: In 2009 MythBusters Jamie Hyneman and Adam Savage experimented with “super pykrete” made with newspapers instead of wood pulp. It performed even better than Pyke’s final formula.