Optical fibre consisting of a pure silicon core in silica cladding combines the advantageous properties of silicon waveguides with the convenience of optical fibre. However, the optical quality of these fibres is highly dependent on the crystalline structure and the purity of the silicon. The fabrication of these fibres requires engineering of the thermal gradients during the drawing process to ensure optimal crystallisation of the silicon. Here, we investigated the effects of draw speed and analyse the induced stresses at multiple stages in the fabrication process. The thermal exposure of the silicon while in contact with a silica cladding was found to increase the optical losses. This was attributed to the diffusion of impurities from the silica cladding into the silicon core.