Applying healing herbs to the skin can stop bleeding, protect against infection and ease pain. The skin also absorbs some chemicals from the plant and moves them into the bloodstream. This is how herbs applied to the skin can stimulate internal organs or circulation, which encourages healing.
Poultices are made by adding hot water to dried, powdered herbs or fresh, coarsely ground herbs and placing the herb directly on the affected area. The herbs may be covered with gauze and taped in place. Poultices can be kept warm with a hot-water bottle or heating pad and preferably are left on overnight or for a few hours.
Plasters are oily or waxy mixtures blended with herbs and applied to the chest area or abdomen to stimulate the internal organs. Plasters may include such ingredients as olive oil or beeswax. In past times, the mixture was spread onto a cloth and rolled tightly for storage, then unrolled and applied when needed.
Source: Hobbs, Christopher. Handmade Medicines. Loveland, Colorado: Botanica, 1998.