The evaporation of potassium from phlogopite was investigated by roasting phlogopite with different chemical reagents. The possible reactions between reactants and the sample at different temperatures were investigated by thermogravimetry. Gypsum, calcite, sodium chloride, activated carbon, calcium chloride and fluoride were used as chemical reactants. Similar tests were carried out by batch experiments using muffle furnace and tube oven and finally the sample mixtures were heated in a vacuum. The use of a vacuum enabled an almost complete extraction of potassium in solid state at the lowest temperature so far investigated. The structure of phlogopite was shown to be destroyed by raising the temperature. The extraction of potassium from feldspar, a sample from Lohja, Finland was also investigated.