Black walnut leaf contains tannins that cross-link proteins in mucous membranes and the skin. The tannins make the herb useful treating conditions in which the skin "leaks," such as excessive perspiration, eczema, and allergic rashes. There are many regional formulas for topical remedies that use walnut leaf as an ingredient. In France, walnut leaf is used to treat scalp itching, peeling and dandruff; as well as sunburns and superficial burns. In India, a walnut leaf decoction is used as a wash for malignant sores and pustules. The German Commission E notes their benefit for mild, superficial inflammations of the skin, and excessive perspiration of the hands and feet.
The bark and leaves have alterative, laxative, astringent and detergent properties, and are used in the treatment of skin troubles. They are of the highest value for curing scrofulous diseases, herpes, eczema, etc., and for healing indolent ulcers; an infusion of 1 oz of dried leaves to the pint of boiling water, allowed to stand for six hours, and strained off is taken in wineglassful doses, three times a day, the same infusion being also employed at the same time for outward application. Obstinate ulcers may also be cured with sugar, well saturated with a strong decoction of Walnut leaves.
No insects will touch the leaves of the Walnut, which yield a brown dye, which gypsies use to stain their skin. It is said to contain iodine.
The husks and leaves, macerated in warm water impart to it an intense bitterness, which will destroy all worms (if the liquid be poured on to lawns and grass walks) without injuring the grass itself.
English herbalist Nicholas Culpepper reported in the 17th century that walnut leaf, in combination with onion, salt, and honey could help to draw the venom from snake and spider bites. In America, there is archeological evidence that they were consumed and used by the Native Americans as far back as 2000 B.C.E.