Japanese art tradition, contrary to the case of China, is characterized by an efficient and continued, although mostly undocumented, use of smalt from the late 16th century onward. Recent studies have successfully identified this pigment, the cobalt-colored glass that spread throughout the Old Continent during the Renaissance period, as the coloring agent employed for overglaze-blue enameling on Japanese porcelains produced at the kilns of Arita (the porcelain production center of Japan) from the early 1640s until the 20th century. Fragmentary evidence of the use of smalt in Japanese sacred art has also been reported, yet its earliest incorporation into such a type of traditional art form could not be identified. In order to resolve this crucial issue, portable EDXRF was employed for the non-destructive analyses of Japanese porcelains and sacred images bearing blue decoration. Scientific analysis allowed, for the first time ever, to establish a clear timeline of smalt use. Furthermore, this evidence and the literature data both agree, leading to the identification of the origin of the blue material used on both art productions.