A magic eggshell: Cu2+ reacts with S2- to produce metal Cu, rather than CuS

In common sense, the chemical reaction between Cu2+ and S2- ions produces black precipitation of CuS. However, if they are separated by an eggshell, they produce copper with trace amount of CuS on the surface of the eggshell at the side of copper chloride solution within one week. Such an unexpected phenomenon strongly indicated that Cu2+ is able to be reduced into metal copper when S2- diffuses throughout the porous eggshell. The produced copper was verified by reacting with nitric acid to generate red-brown nitrogen dioxide at ambient temperature, as well as by the powder X-ray diffraction (XRD) and energy dispersive analysis of X-rays (EDS).