section_path: Casing
section_name: Casing
paragraph_text: Casing
  
  At completion, the Great Pyramid was cased entirely in white limestone. Precisely worked blocks were placed in horizontal layers and carefully fitted together with mortar, their outward faces cut at a slope and smoothed to a high degree. Together they created four uniform surfaces, angled at 51°50'40" (a seked of 5 + 1 / 2 palms). Unfinished casing blocks of the pyramids of Menkaure and Henutsen at Giza suggest that the front faces were smoothed only after the stones were laid, with chiseled seams marking correct positioning and where the superfluous rock would have to be trimmed off.
  
  The height of the horizontal layers is not uniform but varies considerably. The highest of the 203 remaining courses are towards the bottom, the first layer being the tallest at 1.49 metres (4.9 ft). Towards the top, layers tend to be only slightly over 1 royal cubit (0.5 m; 1.7 ft) in height, with stones weighing around 500 kilograms (1,100 lb). An irregular pattern is noticeable when looking at the sizes in sequence, where layer height declines steadily only to rise sharply again.
  
  So-called "backing stones" supported the casing, which were (unlike core blocks), precisely dressed as well and bound to the casing with mortar. Now, these stones give the structure its visible appearance, following the partial dismantling of the pyramid in the Middle Ages. Amidst earthquakes in northern Egypt, workers (perhaps the descendants of those who served al-Ma'mun) stripped away many of the outer casing stones, which were said to have been carted away by Bahri Sultan An-Nasir Nasir-ad-Din al-Hasan in 1356 for use in nearby Cairo.
  
  Later explorers reported massive piles of rubble at the base of the pyramids left over from the continuing collapse of the casing stones, which were subsequently cleared away during continuing excavations of the site. Today a few of the casing stones from the lowest course can be seen in situ on each side, with the best preserved on the north below the entrances, excavated by Vyse in 1837.
  
  The mortar was chemically analyzed and contains organic inclusions (mostly charcoal), samples of which were radiocarbon dated to 2871–2604 BC. It has been theorized that the mortar enabled the masons to set the stones exactly by providing a level bed.
  
  Although it has been suggested that some or all of the casing stones were made from a type of concrete that was cast in place, rather than quarried and moved, archaeological evidence and petrographic analysis indicate this was not the case.
  
  Petrie noted in 1880 the four sides of the pyramid to be "very distinctly hollowed" and that "each side has a sort of groove specially down the middle of the face", which he reasoned was a result of increased casing thickness in these areas. Under certain lighting conditions and with image enhancement the faces can appear to be split, leading to speculation that the pyramid had been intentionally constructed eight-sided. Laser scanning and photogrammetrical surveys concluded the concavities of the four sides to be the result of the removal of the casing stones, which damaged the underlying blocks that form the outer surface today.