Reading practice 54
NATURAL SCIENCE: The Great Pyramid at Giza: Its Composition and Structure
The Great Pyramid at Giza is arguably one of the most fascinating and contentious1 pieces of architecture in the world. In the 1980s, researchers began focusing on studying the mortar from the pyramid, hoping it would reveal important clues about the pyramid's age and construction. Instead of clarifying or expunging older theories about the Great Pyramid's age, the results of the study left the researchers mystified.
Robert J. Wenke from the University of Washington received authorization to collect mortar samples from some of the famous ancient construction sites. Among these sites was the Great Pyramid. The mortar that Wenke discovered was formed by particles of pollen, charcoal, and other organic matter2. By using radiocarbon dating, scientists were able to make some disconcerting discoveries. After adjusting the data, the mortar revealed that the pyramid must have been built between 3100 BC and 2850 BC with an average date of 2977 BC. This discovery was controversial because these dates claimed that the structure was built over 400 years earlier than most archaeologists originally believed it had been constructed.
Furthermore, archaeologists discovered something even more anomalous. Most of the mortar samples collected appeared to be little more than processed gypsum with traces of sand and limestone. The sand and limestone found in the gypsum were not added but were actually contaminants of the processed gypsum. The mortar used to build the Great Pyramid is of an unknown origin. It has been analyzed repeatedly and its chemical composition has been established. However, even using modern techniques, scientists have been unable to reproduce it. The gypsum mortar is stronger than the stone on the pyramid and the mortar is still intact today, thousands of years after the pyramid was built. This mortar was not used to bond the heavy stone blocks together like cement mortar does with modern bricks. Instead, the gypsum mortar's role was to buffer the joints and to reduce friction4 as the enormous blocks were put into place.
Examining the mortar from the Great Pyramid assists scientists in making inferences5 about Egypt's past. Researchers questioned why the Egyptian builders would choose to use gypsum mortar over lime mortar. Egypt had numerous limestone mines that could have been used to create a more durable lime mortar. Despite the abundance of lime, there is no evidence of lime mortar being used in Egypt until 2500 years after the pyramids were built. Researchers then began to determine why the more water-soluble gypsum would have been preferred. They discovered that gypsum would have been easier to mine than limestone. In addition, the Egyptian builders discovered that when gypsum is heated to approximately 265°F, some of the moisture is excluded. When the processed gypsum is mixed with water again, the resulting substance is used for the mortar.
Despite having significant mineral resources, Egypt has few natural fuels available6. The 265°F temperatures needed to process the gypsum and turn it into mortar can be achieved with the heat of an open fire. On the other hand, to make lime mortar, extremely high temperatures of around 1800 degrees Fahrenheit are needed. Most historians conjecture that the high heat needed to process limestone is the reason lime mortar was not used. The shortage of natural fuel sources would most likely have made the creation of lime mortar highly uneconomical.
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