27 CFR 19.302: Treatment during production.

During production, the proprietor may purify or refine the spirits by using any material that will not remain in the finished product. Juniper berries and other natural aromatics or their extracted oils may be used in the distillation of gin. Spirits may be percolated through or treated with oak chips that have not been treated with any chemical. The proprietor must destroy or so treat any materials used in treatment of spirits, and which do not remain in the spirits, so as to preclude the extraction of potable spirits.