The welded tuffs of pumice-flow origin are made of deformed and consolidated glassy fragments. On the other hand, viscous glass particles at high-temperature and at high-pressure deform and comsolidate, and then they grow into dense compact body. The sintering of glass proceeds by the mechanism of viscous flow. At the early stage, we get nearly the same density from both the equations of particle-deformation model and pore-closing model for the same pt/η value. Slightly welded specimens (Shikotsu welded tuff) were experimented under various load-pressures and temperatures. These experiments indicated that the degree of sintering is controled by pt/η value. The density-increase of glassy fragments is sepresented by the following equation : dD/dt=3/4・p/η・(1-D).
