The iron slags found in Okayama and Hiroshima prefectures, central Japan, were analyzed by x-ray fluorescence analysis for aluminum, calcium, iron, magnesium, silicon, and titanium, and their molar ratios were calculated. The aluminum to silicon molar ratios in the slags between 6th and 17th centuries are in the range 0.12-0.14. The potassium to silicon ratio and calcium to silicon ratio also show the similar tendencies of converging to certain values. This indicates the stabilization of techniques and optimum compositions of furnace materials. Since the potassium to aluminum and aluminum to silicon ratios are particular for each iron smelting site, these values may be used to distinguish the smelting sites. -- AATA