Black tea stain formed on the surface of teacups and pots. Part 1 – Study on the chemical composition and structure

Repeated use of teacups and teapots results in stains that are difficult to remove with normal dish-washing detergent and a soft sponge. The tea scum and tea cream formed in black tea infusion have been investigated previously. On the other hand, the composition and the formation mechanism of the stain are not well understood. The present study deals with phenomena occurring on the surface of porcelain tiles that were immersed in tea infusion brewed with Tokyo tap water. The stain was generated on the tile surface with the lowering of the waterline caused by natural evaporation. The FT-IR spectrum of the stain differed from that of the adsorbed materials below the waterline of the infusion, and the latter was easily removed by pouring distilled water. SEM-EDS analysis revealed the existence of islands of calcium silicates on the underlying organic material, which were probably the oxidative products of polyphenols. Those results suggested that the inorganic ions of calcium and silicate mainly from tap water play key roles together with oxygen in the formation and stabilization of such stains.