A method for producing a solar crucible with a rectangular shape for use in a crystal growing process for silicon, the method comprising: providing a crucible base body of transparent or opaque fused silica comprising an inner wall, providing a dispersion containing amorphous SiO 2 particles; applying a SiO 2 -containing slip layer with a layer thickness of at least 0.1 mm to at least a part of the inner wall by using the dispersion; drying the slip layer so as to form a SiO 2 -containing grain layer and thermally densifying the SiO 2 -containing grain layer so as to form a diffusion barrier layer, characterized in that (a) the dispersion contains a dispersion liquid and the amorphous SiO 2 particles that form a coarse fraction with particle sizes in the range between 1 μm and 50 μm and a fine fraction with SiO 2 nanoparticles with particle sizes of less than 100 nm, • wherein a weight percentage of the SiO 2 nanoparticles based on a solids content of the dispersion is in the range between 2 and 15% by wt., and • wherein the fine fraction consisting of SiO 2 nanoparticles with particle sizes of less than 100 nm accounts for a volume proportion of the casting skin of more than 70%, and (b) and wherein the SiO 2 -containing grain layer is thermally densified into the diffusion barrier layer through the heating up of the silicon in the crystal growing process. providing a crucible base body of transparent or opaque fused silica comprising an inner wall, providing a dispersion containing amorphous SiO 2 particles; applying a SiO 2 -containing slip layer with a layer thickness of at least 0.1 mm to at least a part of the inner wall by using the dispersion; drying the slip layer so as to form a SiO 2 -containing grain layer and thermally densifying the SiO 2 -containing grain layer so as to form a diffusion barrier layer, characterized in that (a) the dispersion contains a dispersion liquid and the amorphous SiO 2 particles that form a coarse fraction with particle sizes in the range between 1 μm and 50 μm and a fine fraction with SiO 2 nanoparticles with particle sizes of less than 100 nm, • wherein a weight percentage of the SiO 2 nanoparticles based on a solids content of the dispersion is in the range between 2 and 15% by wt., and • wherein the fine fraction consisting of SiO 2 nanoparticles with particle sizes of less than 100 nm accounts for a volume proportion of the casting skin of more than 70%, and (b) and wherein the SiO 2 -containing grain layer is thermally densified into the diffusion barrier layer through the heating up of the silicon in the crystal growing process. (a) the dispersion contains a dispersion liquid and the amorphous SiO 2 particles that form a coarse fraction with particle sizes in the range between 1 μm and 50 μm and a fine fraction with SiO 2 nanoparticles with particle sizes of less than 100 nm, • wherein a weight percentage of the SiO 2 nanoparticles based on a solids content of the dispersion is in the range between 2 and 15% by wt., and • wherein the fine fraction consisting of SiO 2 nanoparticles with particle sizes of less than 100 nm accounts for a volume proportion of the casting skin of more than 70%, and • wherein a weight percentage of the SiO 2 nanoparticles based on a solids content of the dispersion is in the range between 2 and 15% by wt., and • wherein the fine fraction consisting of SiO 2 nanoparticles with particle sizes of less than 100 nm accounts for a volume proportion of the casting skin of more than 70%, and (b) and wherein the SiO 2 -containing grain layer is thermally densified into the diffusion barrier layer through the heating up of the silicon in the crystal growing process.