Dolomite in archaeological plaster: An FTIR study of the plaster floors at Neolithic Motza, Israel

Material studies of ancient plaster can provide invaluable information on pyro-technological advancements, living practices, stylistic preferences and possibly the cultural organization needed to produce the plaster. Past studies have established methods of analysis for calcite and gypsum-based plaster, but studies of dolomite-rich plaster can be more complicated. In particular, the useful FTIR-based method for determining the structural organization of calcite, which differentiates pyrogenic and geological calcite, is hindered by the overlapping calcite and dolomite peaks. Therefore, a new FTIR-based calibration is presented for quantifying the dolomite percent of the carbonates. This was tested both on known mixtures and in comparison to XRD analyses of ancient plaster. Weighted mixtures of calcite and dolomite were used to demonstrate the problem that dolomite causes when using FTIR to study calcite's structural order. Limits were established for when dolomite can be considered a small error versus when additional steps must be taken, such as a density separation step to separate disordered calcite from dolomite-rich samples. These methods were applied to a case study of red-painted plaster floors from PPNB Motza. Two types of plaster were found: the plaster preparation layers which contained large aggregates and, based on the new calibration, a high percent of dolomite and some sediment, while the finishing topcoat was almost pure calcite with finer aggregates. The same technology persisted across the examined PPNB building phases. Additional examination by light microscopy was able to clarify the outlier results and provide possible insight on the use of a sunken floor or basin. These methods can now be applied for comparison studies of plaster across sites and time periods, and could also be useful in geological studies where mixtures of calcite and dolomite are present.