minus temperature is not a prerequisite. you can do the same at a temperature of + 20C. but the higher the temperature in the room, the more 4-mmc will remain in the mother liquor. at -8C almost all 4-mmc will fall out of solution. at + 20C about 70% will fall out, about 30% will remain in the water. but what prevents you from driving off the water and getting the remains of 4-mmc, and then repeating the crystallization process again?