Hot isostatic pressing (HIP) is one of the industrial techniques for production of structural ceramics. A high quality of the as-HIPed surface is highly desirable, as it may make it possible to minimise or eliminate costly finishing machining. When a porous ceramic is HIPed, an encapsulation of glass can be applied on the component as a gas tight envelope. Silicon nitride ceramics work well with a borosilicate encapsulation glass but oxides, such as alumina and zirconia, react severely. A protective barrier between the oxides and the borosilicate glass has been developed earlier. With such a system of protective layers it was possible to demonstrate HIP of some oxides, with good mechanical as well as biocompatibility properties as a result. However, there is a difference between the bulk and the surface properties of these materials. A model system of silicon nitride and borosilicate glass was chosen for a more thorough study on interactions between the encapsulation glass and the porous ceramic green body during HIPing. Mechanical, chemical and compositional gradient from the surface of the ceramic into the bulk were studied. Theoretical calculations on chemical reactions in the system Si3N4 - B2O3 were done accompanied by practical experiments. Special attention has been given the part of the HIP-cycle when the pressure is applied and the glass is pushed against the still porous ceramic component. Possible viscous flow of the glass into the ceramic was analysed. TEM-studies of the densified silicon nitride revealed two different types of hexagonal boron nitride formed in the surface region and their role in the encapsulation is discussed. The increased knowledge can beused in work for improved surface quality and for development of intermediate protective layers.