Note 15 — Chapter XIII · POTASSIUM, RUBIDIUM, CÆSIUM, AND LITHIUM. SPECTRUM ANALYSIS
George Kamensky translation
Fused nitre, with a further rise of temperature, disengages oxygen and then nitrogen. The nitrite KNO2 is first formed and then potassium oxide. The admixture of certain metals—for example, of finely-divided copper—aids the last decomposition. The oxygen in this case naturally passes over to the metal.