Oxygen and nitrogen isotopic constraints to the origin of saltpetre in historic gunpowder prevailed during the 19th century in Japan

Saltpetre (KNO₃) is the major ingredient of gunpowder, which played a driving force in political changes during the late medieval to middle modern times in the world. The intrinsic isotopic parameters of the mineral are useful for elucidating the origin and following its marketing route. Dual isotopic analysis (oxygen together with nitrogen, as expressed by δ¹⁸O and δ¹⁵N notations, respectively) was made for nitrates from diverse gunpowder samples which prevailed during the 19th century in Japan. Three distinctive sources which represent Japanese domestic (characterized by low δ¹⁸O-low δ¹⁵N values), British Indian (characterized by clearly higher δ¹⁸O-higher δ¹⁵N values) and Atacama Desert nitrates (characterized by very high δ¹⁸O up to +50‰ - δ¹⁵N = around 0‰) were determined by binary plotting. Some samples recovered from portable powder reservoirs for less developed guns tend to exhibit a small shift in δ¹⁵N value toward a British Indian origin, reflecting the positive mixing of the imported source saltpetre from British Indian during preparation. Very unique gunpowder was discovered from the powder reservoirs for early models of the Enfield rifle in northeastern (Kiyokawa Village and Tsuruoka City in Yamagata Prefecture) and southwestern Japan (Miyoshi City in the inland of Hiroshima Prefecture) and metallic cartridges applicable to Lefaucheux and Smith & Wesson revolvers, all fall close to the alley of nitrates converted from the Atacama Desert nitrate formed by non-mass-dependent fractionation mixed with varying amounts of the British Indian nitrates. The evidence implies the prevalence of a unique marketing route. The saltpetre represents industrial conversion of the Atacama Desert nitratine (NaNO₃) by reaction with the mined potassium chloride in the middle 19th to early 20th centuries in Prussia (northern territory of Germany) and Great Britain. Dual isotopic analysis for nitrate in historic gunpowder provides direct evidence for the origin and subsequent marketing route during the 19th century in Japan. Socio-historic consequences of the use of Atacama Desert nitrate in manufacturing the gunpowder are reviewed and discussed, basing on the old documents.