Abstract: In manufacturing a silicon nitride product of complex shape and uniform density, a mixture of silicon nitride in powder form together with a fluxing agent and a temporary binder is first produced. From the mixture a preform of substantially uniform density less than the density of the required product is then formed, the dimensions of the preform in the direction of subsequent hot pressing to the dimensions of the final product in the same direction being arranged to be in the ratio of the preform density to the required final density. The preform is then heated to remove the temporary binder and is hot pressed in a die carrying punches which together form a cavity defining the shape of the required product, the preform shape not conforming to the die cavity.