Handcrafted soaps made from scratch require three things to become soap: oil, water and lye. It is the chemical reaction between these ingredients that turns them into soap.
Most soap also has other ingredients added to provide benefits to the soap, or to color or scent it.
The soapmaking process will work with any animal or vegetable fat or oil, but not with petroleum- based oils.
Traditional soaps were typically made with the most available oils/fats - those from animals. Lard and tallow make excellent soaps. With improvement in global trade over the last century, vegetable oils sourced both domestically and internationally have replaced a large portion of the tallow and lard previously used in soap making, particularly for commercial soaps.
The chemical makeup of each different oil has an effect on the finished bar of soap. For example, olive oil makes a very hard bar of soap, but the bubbles are small and somewhat "slimy." Coconut oil, on the other hand, makes big, fluffy bubbles and a hard bar of soap, but the it can be drying to the skin.
Handcrafted soapmakers have the advantage of being able to formulate soaps using all types of basic and specialty oils in order to make their unique "perfect bar." While you are very likely to find coconut or palm kernel, olive, so or palm oils in many soaps, you will also see oils such as castor, apricot, avocado, almond, jojoba, hemp or other nut or seed oils, or butters such as cocoa, mango or shea butter.
Many handcrafted soapmakers are also becoming more globally aware and are choosing their ingredients and oils not only for the quality they bring to the soap, but for their sustainability and fair trade.
Yes, lye is necessary in all handcrafted soaps made from scratch. If there isn't any lye, there isn't any soap. It is the reaction between the lye and the oils that produces soap. Once that reaction (called saponification) is complete, all of the lye is converted into soap; there is no lye remaining in the finished soap bar.
There are two types of lye used by soapmakers - sodium hydroxide and potassium hydroxide. Sodium hydroxide is used to make solid soap; potassium hydroxide is used to make liquid soaps. A combination of the two is used to make cream soaps.
Water is used to create the lye solution that is mixed into the oils. The amount of water is dependent on the specific soap recipe, but it must be enough to allow the lye and oil molecules to get together and make soap, but not so much as to result in a soft bar of soap. The majority of the water evaporates out of the soap as it cures and ages.
Although some handcrafted soaps left unscented, most are scented using either plant-based essential oils or fragrance oils, depending on the preferences of the soapmaker and consumer.
Plant-based Essential Oils
As the name implies, essential oils come from plants and are generally considered "natural". There are several methods for extracting the essential oils, but even so the range of possible scents is limited. Perfumers, and soapmakers who have experience blending essential oils, can produce some amazing scents with just essential oils. Some essential oils are extremely expensive making it unrealistic to use them in true soaps. Real rose essential oil, for example, takes 5,000 pounds of rose petals to produce just 16 oz, which may cost over $3,000.
Fragrance oils are synthesized from aromatic chemical compounds which are then blended to produce the scents we know and recognize. Some fragrance oils blends may include essential oils or "nature identical compounds" (compounds which are produced in a laboratory but have the same molecular structure as those found in nature).
Most food-like fragrances (i.e. butter, coffee, chocolate) or fruity scents (i.e. apple, blackberry, cucumber, mango) are synthesized fragrance oils. Soap scented as real florals, such as jasmine, lilac, or rose are usually made with fragrance oils as essential oils from these flowers are either impossible or extremely expensive to produce.
Dyes, which must be approved by the FDA before being used in soaps or cosmetics, pigments and mica are often used to change the color of soap. In fact, any ingredient used in a soap or cosmetic for the purpose of changing the color must be on the list of FDA approved colorants, and must be approved for the specific use. For example, some colorants are not approved for use on lips, others are not approved for use in eye products.
In addition to color additives, some specialty ingredients may cause the color of the soap to change. For example, adding french green clay to a soap will cause it to have a green color, cinnamon will turn the soap brown and paprika will turn it orange. These ingredients are not used specifically to change the color of the soap, but for other properties they bring to the finished product (although the color change is something that a soapmaker must keep in mind when formulating a specific type of soap).
True soaps, made from oil, lye and water, don't generally require preservatives. You will rarely find preservatives added to handcrafted soap. Some liquid soaps, which have a high proportion of water may require preservatives. Ready-made soap bases may contain or require preservatives.
Ready-Made Soap Bases
Ready-made soap bases may have additional ingredients necessary to make the soap able to be melted down and poured into molds or as a preservative. They can be made as "true soap" or be based partially or completely on synthetic detergents.
A note about Ingredient Lists
The FDA requires that the label for any cosmetic includes a complete declaration of ingredients. In the United States, if a soap is a "true soap" (made primarily with lye, water and oil) and makes no other claims than it is soap and cleans, it not considered a cosmetic and therefore does not require the listing of ingredients (although many soapmakers still provide an ingredient list). However, if the soap is not a "true soap" (mostly in the case of ready-made soap bases that are detergent-based) or if a cosmetic claim is made for the soap, then the complete ingredient declaration is required.