Castille soap is made of a naturally derived vegetable oil (no animal fat or synthetic materials), cold water, and lye crystals. Olive oil is traditionally used, but others like to experiment with their own mixtures of natural oils.
Castile soap is made with very simple and time honored ingredients (it's creation dating back to the mid 1500's in Europe). It's a contemporary green alternative to many of the harmful and expensive brand soaps available in supermarkets today, which are often made from fats derived from animals and loaded with synthetic chemicals. Aside from olive oil, mixtures of jojoba, coconut, palm oil, and soybean oil can also be used. Essential oils are occasionally added to the recipes to add scent to the soap, including Ylang Ylang, Sweet Orange, Lavender, Tea Tree, Eucalyptus, Citronella, and Peppermint. Fresh herbs can also be added to the soap, like crushed rosemary, pulverized chamomile heads, calendula petals, rose petals, and pulverized orange peel. The basic recipe is very simple, but many variations can be made using different types of vegetable and essential oils. Home soap makers often have their own special recipes, trying many different variations of oil and scents before settling on a concrete ingredient list.