A method for purifying crystals in a glass or metal container. A hydrocarbon is introduced into feed material containing tetrahydrocannabinol (THC). The feed material and hydrocarbon is placed in a glass or metal container. The hydrocarbon is then removed within a few minutes after introduction, leaving at least some hydrocarbon in the feed material. Pressure is allowed to build within the container in an oven or in a jacketed vessel for 2-3 weeks. During this time, THC acid crystals precipitate out and fall to the bottom of the container. The contents of the container are poured into a Buchner funnel and a vacuum is applied thereto in order to pull terpenes into a beaker. The terpenes are placed into an oven in order to purge off any remaining solvent. The funnel is then scraped to acquire THC acid crystals.