Reworking saltpetre manufacture in Lao PRD: Implications for isotopic fractionation during the historic processes

Saltpetre (KNO3; potassium nitrate) is one of the major ingredients of gunpowder which often served as a driving force during the political changes in historical times. Before development of chemical synthesis which involves the fixation of atmospheric dinitrogen followed by oxidation of NH3 with atmospheric oxygen, natural occurrence of nitrate minerals in sediments and soils was the sole source of the saltpetre. Reworking historic saltpetre manufacture was conducted in five locations from Lao PDR, a landlocked country in the Indochina Peninsula. The overall manufacturing processes involved the extraction with water of economic nitrous-earth (so-called bat guano deposits) collected from limestone caves, followed by addition of potash (potassium carbonate in form of wood-, grass- and bottom-ashes) to convert native nitrates (mostly Ca-salts) into saltpetre, evaporative removal of excess water from the elutes by pro-longed boiling (evaporative concentration), removal of floating scum and finally differential crystallization from the concentrates. The process was tested by use of stable isotopic analyses of nitrogen and oxygen isotope of the relevant chemical constituents (ambient water and nitrate). The analytical results for δ15N-δ18O values of nitrates collected at each step of the manufacturing processes verify limited isotopic modification, even in the prevailing conditions under strong alkaline media (pH: up to 11) and repeated evaporative removal of the ambient water which accompanies selective removal of 16O from the fluids. Slight negative/positive changes up to 2‰ in nitrate isotopic signatures of saltpeter concentrates were observed during the repeated removal of scum due to co-precipitation of nitrates with associated insoluble sulfates. Significance of the stable isotopic fingerprinting for the source identification as applied for saltpetre in the historic gunpowder was emphasized.