There is no record of the addition of an adjuvant to increase fermentation until the 1300s, when yeasts from rye or barley were added and, later, those from potato juice. Hops were used in making beer during the 1300s or early 1400s, and in 1436 Vasily II in Russia gave his treasury the exclusive right to use hops when brewing with honey to produce 'honey beer', called pure.