According to a new study published in the journal Physical Review Letters, ancient Egyptians used a simple trick to make it easier to transport heavy colossi and pyramid stones by sledge – they moistened the sand.
For the construction of the pyramids, ancient Egyptians had to transport heavy blocks of stone and large statues across the desert.
They placed the heavy objects on a sledge that workers pulled over the sand.
A multinational group of physicists led by Prof Daniel Bonn from the University of Amsterdam has hypothesized that Egyptians probably made the desert sand in front of the sledge wet.
To test their hypothesis, members of the group placed a lab version of the Egyptian sledge in a tray of sand.
They determined both the required pulling force and the stiffness of the sand as a function of the quantity of water in the sand. To determine the stiffness they used a rheometer, which shows how much force is needed to deform a certain volume of sand.
The results show that the required pulling force decreased proportional to the stiffness of the sand. Capillary bridges arise when water is added to the sand. These are small water droplets that bind the sand grains together.
“In the presence of the correct quantity of water, wet desert sand is about twice as stiff as dry sand. 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,” the scientists said.
The Egyptians were probably aware of this handy trick. A wall painting in the tomb of Djehutihotep clearly shows a person standing on the front of the pulled sledge and pouring water over the sand just in front of it.
“Besides revealing something about the ancient Egyptians, the results are also interesting for modern-day applications. We still do not fully understand the behavior of granular material like sand. Granular materials are, however, very common. Other examples are asphalt, concrete and coal.”
“The research results could therefore be useful for examining how to optimize the transport and processing of granular material, which at present accounts for about ten percent of the worldwide energy consumption.”
A.Fall et al. 2014. Sliding Friction on Wet and Dry Sand. Phys. Rev. Lett. 112, 175502; doi: 10.1103/PhysRevLett.112.175502