The process time and energy consumption are important factors when yield and cost for a process isconsidered. One possibility for controlling the heat flux in a Crystalox DS-250 furnace is to use a shutter underneath the crucible which can be opened or closed in controllable ways. Originally this shutter is normally made of graphite. This work shows what happens when the shutter is replaced by one made a material with heat conductivity approximately 40% lower than standard graphite material. The use of this material gave a reduction of the process time of 2.4 hour. This was in good agreement with theoretical modelling of the furnace where the reduction in process time was calculated to be 2.5 hour. It is also shown that for a cast run with standard temperature profile, the energy consumption during the complete cycle is reduced significantly.