The best lye soap is one that has the correct pH balance, is superfatted, and contains a fragrance and additives that appeal to you. Lye soap can be handmade, purchased from a soapmaker, or commercially produced. Most bars of soap sold in big-box stores and grocery stores are actually synthetic detergent bars, not real soap at all. Real lye soap can be found at health-food stores, farm fairs, online, and at bath and beauty specialty shops.
Lye soap that is out of balance can be too alkaline and have a drying effect on skin. Most soapmakers check their soap for pH balance by testing for what they call a zap before the item is put out for sale. Buying soap directly from someone who makes cold-processed or hot-processed soap is a good way to get a quality bar. If there is any doubt about the balance of the soap, do your own zap test before using.
To test for pH balance, wet your finger and draw it across a bar of lye soap. Touch your finger to your tongue. Properly balanced bars will simply taste like soap, while bars with poor pH balance will actually zap the tongue with an almost electric punch. If a bar zaps, it may simply not have cured long enough. Storing the soap in a linen closet or drawer for a month or so may eliminate the zap.
The best lye soaps are more than just pH balanced; they are superfatted. When soap is made, fats and an alkaline mixture of lye and water are combined, and a chemical reaction called saponification takes place. The best soaps use more fat than lye, so when the bar has finished processing, some fat is left over to moisturize and nourish skin. Look for bars labeled "superfatted" or that contain fats like shea butter, cocoa butter, and avocado oil in the mixture. These are just a few of the exotic oils used for superfatting.
Since personal preferences can vary, the best scent or fragrance may differ from one individual to another. If you prefer a natural soap bar, look for lye soap scented with essential oils; these are fragrances that are naturally derived from plants. If you prefer more whimsical, perfume-like scents, then consider a bar that contains a synthetic fragrance oil.
Additives can also affect the way a lye soap bar performs. Items like poppy seeds, lavender buds, and clay can add value to a lye soap for some, while others prefer additive-free bars. Sample a new soap brand or formulation by trying a travel or guest-sized bar first. You'll be able to see if a bar works for you without a large investment.