Smalt is a pigment that consists of ground blue potassium glass with cobalt as coloring agent. It has been recognized that some smalt in paints had the tendency to discolor, but the mechanism of the discoloration has remained unknown. In this paper, we investigated whether five kinds of smalt made by the authors (self-made smalt) with various compositions of cobalt and potassium would show the different influence on drying and discoloration of the drying oil (linseed oil) with smalt. Linseed oil samples with five kinds of self-made smalt were heated at 110°C for up to 336 hours respectively and the amount of fatty acid component in the oil samples and the visual color changes of them were measured. Higher content of cobalt in smalt accelerated the drying of the oil while higher content of potassium in smalt slowed it. On the other hand, visible absorption spectra showed that higher content of potassium and lower content of cobalt in smalt caused greater discoloration of the oil. Thus, it was clear that the extent of drying and discoloration of the oil with smalt is influenced by the content of cobalt and potassium in smalt.