A method for identification of plagioclase feldspars involves formation of a glass and determination of its refractive index. An investigation was made to test the reliability of the technique when samples are fused by an acetylene-air flame and in a high-temperature electric furnace. Specimens of albite from Quebec and labradorite from Newfoundland were used. No significant loss in Na or K occurred. Glasses formed by the fusion of the plagioclase powders under these conditions probably suffer no change in composition.