Dating Ancient Mortar
Although radiocarbon dating is usually applied to organic remains, recent work shows that it can also reveal the age of some inorganic building materials
Even more than digging implements, archaeologists need tools for
finding the age of the objects they study. After all, many sites and
remains—in caves, in deserts, on the sea floor—require
no excavation, but all must be dated. When archaeologists of the
future write the history of their discipline, the second half of the
20th century will stand out for the development of many scientific
methods for ascertaining the age of artifacts. This article is an
account of how our Scandinavian-American team, which includes a
nuclear physicist, a geologist, an art historian and two
archaeologists, developed the means for dating ancient building
materials that contain lime mortar.
In the early days, archaeologists trying to make age determinations
often depended on information supplied by others. Principally, they
relied on historians, who knew the chronologies of literate
societies of the past five millennia, with their written
inscriptions on seals, records, tombs, monuments and coins.
Archaeologists also relied on geologists, who could sometimes make
age determinations based on the association of human remains with
geological features of known age.
Unfortunately, this dependence on historical dates and geological
associations left large areas of the human past untouched. But
beginning in the late 1940s, a new world opened with the development
of radiocarbon dating for organic remains, tree-ring dating for
wood, thermoluminescence for fired clay and potassium-argon dating
for volcanic materials. Of these, radiocarbon dating had the most
universal importance for archaeology. So vital was its discovery
that the pioneer of the field, Willard F. Libby, was awarded the
Nobel Prize for Chemistry in 1960.