Iron Age hydraulic plaster from Tell es-Safi/Gath, Israel

Hydraulic plasters or mortars prior to the Roman period are rare. Here, we report the identification and characterization of 3000 year old (Iron Age) hydraulic plaster surfaces from the site of Tell es-Safi/Gath. This site, located in central Israel, was occupied almost continuously from prehistoric through modern times, and is identified as the Canaanite and Philistine city of Gath. A survey using an on-site Fourier transform infrared spectrometer (FTIR) identified the presence of amorphous silicates, in addition to calcite, in each of two superimposed plaster layers. This suite of minerals is characteristic of hydraulic plaster. An in-depth characterization of the plasters using FTIR, acid dissolution, X-ray fluorescence (XRF), X-ray powder diffractometry (pXRD), heating experiments and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (SEM-EDS), shows that special silicate-containing minerals were brought from some distance to the site in order to produce these plaster surfaces. We therefore conclude that the plasters were deliberately produced, and were not the result of a fortuitous addition of local silicate minerals. A layer of around 150μm thick enriched in carbonate hydroxylapatite was found in both plaster surfaces, and the same mineral was identified by infrared spectrometry in the surrounding sediments. This suggests that organic materials were used on these surfaces. These surfaces are among the oldest hydraulic plasters known.