The present invention relates to silicon nitride mould parts, particularly crucibles for use in connection with directional solidification and pulling of silicon single crystals. The mould parts consist of Si.sub.3N.sub.4 having a total open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the mould parts have a size which is larger than the means size of the Si.sub.3N.sub.4 particles. The invention further relates to a method for producing the silicon nitride mould parts.