The device for producing a block of crystalline material from a bath of molten material comprises a crucible having a bottom and heat extraction means arranged under the crucible. It also comprises means for modulating the thermal conductivity fitted between the bottom of the crucible and the heat extraction means. The means for modulating the thermal conductivity comprise a plurality of plates made from thermally conducting material of low emissivity, parallel to the bottom of the crucible, and means for moving said plates closer to and away from one another.