The Laurion mines of southern Attica, Greece, were mined for silver for Athenian coins, and were exploited again in the late 19th and 20th centuries. Ore processing workshops from the fourth century B.C. contain plaster washing basins lined with an enigmatic brown layer designed to prevent water loss in the arid, water poor environment. Analyses were undertaken to understand the technology used to create this lining material. The ternary system CaO-PbO-SiO* l2*s is basic to all syntheses involving plaster production at Laurion because of the ubiquity and close association of marble, quartz, and lead compounds. The brown layer is not a simple plaster since calcination of the three ingredients together would have produced a crystalline lead silicate. Its main constituent is calcite, with quartz and high levels of lead inhomogeneously distributed. Although low levels of lead silicate are present, the predominant reaction product at annealing temperatures is Ca₂PbO₄. Thus, it appears that the waterproof coating was made by heating the lead containing compounds to high temperatures, sometimes to fusion, then adding lime. The challenge of limited water supply thus led to a unique plaster technology.