The behavior of the Cu-Al-Si-O system was investigated in the temperature range of 800degrees-1150degreesC. X-ray diffraction, infrared spectroscopy, diffusive reflectance, and electron microscopy were applied to obtain information about the influence of sintering, crystallization, and chemical reaction, each of which determines the color of the material. Definition of the artificial term "engobe," opposite to the term "glaze," was possible using copper(II) oxide (CuO) as its indicator. The detected chemical processes render possible explanations about the traditional way of ceramic painting during the 16th century and support the knowledge transfer from the viewpoint of history and that of natural science, as attempted by "archaeometry."