An earthenware type reduction lustre glaze containing bismuth, cobalt, copper and lead has been fired in H-2/N-2 atmospheres to produce successful lustre effects on ceramic surfaces. The glazes produced at different stages of reduction have been studied using energy dispersive X-ray and X-ray photoelectron spectroscopy. Thermogravimetry has been used to study the kinetics of the hydrogen reduction process. The results have been interpreted in terms of diffusional models. It is suggested that hydrogen diffusion into the glaze at the reduction temperature is very rapid. The rate of formation of the metallic clusters which give the lustre effect may be dependent on the rate of diffusional loss of water from the solid phase. The metal clusters are formed at depths of more than 5 nm below the surface of the glaze.