Chemists often melt the reactants together and then later anneal the solidified melt. If volatile reactants are involved, the reactants are often put in an ampoule and then all air is removed. Often, the chemists keep the reactant mixture cold (for example, by keeping the bottom of the ampoule in liquid nitrogen) and then seal the ampoule. The sealed ampoule is then put in an oven and given a specified heat treatment.