The truth, researchers at the University of Amsterdam announced this week in a study published in the journal Physical Review Letters, may actually be quite simple. It has long been believed that Egyptians used wooden sleds to haul the stone, but until now it hasn’t been entirely understood how they overcame the problem of friction. It amounts to nothing more, scientists say, than a “clever trick.”
They likely wet the sand. “For the construction of the pyramids, the ancient Egyptians had to transport heavy blocks of stone and large statues across the desert,” the university said. “The Egyptians therefore placed the heavy objects on a sledge that workers pulled over the sand. Research … revealed that the Egyptians probably made the desert sand in front of the sledge wet.”
It has to do with physics. The sort of sledges the Egyptians used to transport the two-ton loads of stone were pretty rudimentary. They were wooden planks with upturned edges. Dragging something that heavy through hot sand would — unsurprisingly — dig into the grains, creating a sand berm that would make progress nearly impossible. It “was perhaps observed by the Egyptians that in [a] dry case, a heap of sand forms in front of the sled before it can really start to move,” says the study, authored by a team of eight researchers led by Daniel Bonn.
The only way around that problem would be to constantly clear the sand out of the way, making a tedious process even more tedious.
Damp sand, however, operates very differently. According to the research, “sliding friction on sand is greatly reduced by the addition of some — but not that much — water.” So this time, researchers placed a laboratory version of an Egyptian sledge in a bin of sand that had been dried in the oven. Then they threw down some water, and measured the grains’ stiffness. If the water had the appropriate level of wetness, something called “capillary bridges” — extremely small droplets of water that glue together individual grains of sand — would form.
These bridges not only stopped the sled from forming sand berms but also cut by half the amount of force required to move the cart. “I was very surprised by the amount the pulling force could be reduced — by as much as 50 percent — meaning that the Egyptians needed only half the men to pull over wet sand as compared to dry,” Bonn told The Washington Post.
Indeed, he says the experiments showed the required force decreased in proportion to the sand’s stiffness. “In the presence of the correct quantity of water, wet desert sand is about twice as stiff as dry sand,” the university says. “A sledge glides far more easily over firm desert sand simply because the sand does not pile up in front of the sledge as it does in the case of dry sand.”
Too much water, however, would create separate problems. “The static friction progressively decreases in amplitude when more water is added to the system,” the study says.
Adding more evidence to the conclusion that Egyptians used water is a wall painting in the tomb of Djehutihotep. A splash of orange and gray, it appears to show a person standing at the front of a massive sledge, pouring water onto the sand just in front of the progressing sled. What this man was doing has been a matter of great debate and discussion.
“This was the question,” Bonn wrote in an e-mail to The Post. “In fact, Egyptologists had been interpreting the water as part of a purification ritual, and had never sought a scientific explanation. And friction is a terribly complicated problem; even if you realize that wet sand is harder — as in a sandcastle, you cannot build on dry sand — the consequences of that for friction are hard to predict.”
He said the experiment not only solved “the Egyptian mystery, but also shows, interestingly, that the stiffness of sand is directly related to the friction force.”
In all, the scientists say, “the Egyptians were probably aware of this handy trick.”