Figure 140-150F. You can also process "hot" at that temp for making higher than 20PPM Colloidal Silver, gel-capped, and it works just fine. I do it all the time and I recall others here saying the same - that they use around 150F also. Hotter means more evaporation of the water in the cell and that could be problematic. If you walk away and enough water evaporates it could drop the level below the cathode submerged depth, the cell will stop and you'll have NO idea what PPM you actually made.