Curing refers to the period after cutting during which the soap becomes milder and harder. Milder as saponification finishes and harder as water evaporates. You can extend the life of your soap through careful formulation and storage. Over time, exposure to heat and humidity can degrade the quality of your soaps. Soap that sits in water or is allowed to be in the stream of the shower will melt away rapidly so dry them between uses.
All solid handmade soaps will melt readily in water because the high level of naturally occurring glycerin keeps the bar softer than mass-produced commercial soap from which the glycerin has been removed. Your soap will probably become longer lasting as it is allowed to dry and cure, but it will never be as long-lasting as the commercial variety.
After cutting and while curing, cold-process soap needs to be kept at a relatively constant temperature and have air circulation. Depending on the amount of soap you make, you can dry your soap on a paper-covered cookie sheet, a small shelf, or create an entire curing and drying rack system. However you choose to cure it, be sure to turn it every few days during the first couple of weeks so that it will cure evenly.
The outcome of making cold-process soap is limited in a couple of ways. Lye is not kind to natural colorants. For example, the brilliant ruby-red color that you can infuse into oil with alkanet root will fade to a light pink. The same goes for the bright orange you can get from annatto: It will fade to a color ranging from buttery yellow to subtle orange.
You can, however, get beautiful colors from mineral pigments, food and cosmetic colorants, and micas. Be certain to use colorants that are safe for cosmetic use and stable in cold-process soap. Some micas, although safe in soap, will have their color destroyed by the process.
In order to enjoy the delights of essential oils, you must add a relatively large amount to the cold-process soap batter. The general usage rate is approximately ½ ounce of essential oils per 1 pound of base oils. This varies when using absolutes, concretes, and resins.
Fragrance oils that are designed for use in cold-process soap hold up beautifully through the soapmaking process. It is important that you use “soap safe” fragrance oils. These oils have been tested and selected by suppliers.
As a starting point, for each pound of oils used in a batch, you can start with 1 teaspoon of fragrance oil or 1 tablespoon of essential oil per pound of oils. Once you've made a number of batches, you will find the level of fragrance you like. For spice oils such as clove, start with ⅛ teaspoon per pound of oils.
When you create blends using both essential oils and fragrance oils, you need to be sure you use the proper measurements for each. Fragrance oil usage is usually about one-third the rate of essential oils. Not all fragrance oils are the same, so be sure you get the manufacturer's or distributor's rate of use for each oil you use.
In cold-process soapmaking, you need a recipe that creates a hard, releasable soap. In the recipe section, you will find recipes with additives such as beeswax to help a bar release from a single mold. Some complicated designs need a very hard bar to release at all.
In cold-process soapmaking, the soap goes through a “gel” phase during its insulation time. In a block mold, the heat generated by the soap mass inside the towel-wrapped, insulated mold will be enough for the soap to get through that phase.
When you pour the soap into a series of three 4-ounce single bars, it is more difficult for the soap to generate enough heat to gel. Help the soap get hot enough by stacking the multicavity molds in a large plastic storage container. Stack them so the soaps above don't sit directly on the open molds below. Put on the lid and wrap the container with towels. If the weather is warm, you can put an insulated box of molds outside in the sun to help it get up to temperature. In some cases — milk soaps, formulas containing honey, some floral fragrance oils or spice essential oils, for example — you may want to avoid gel to prevent overheating by not insulating.
A Word of Caution
The following materials should never be used in soapmaking: aluminum, cast iron, nonstick finishes, weak plastic, and thin glass. The soap batter will eat your aluminum pot, contaminating the soap and turning it black. Similarly, your cast iron pot will become pitted and oxidized, contaminating the soap with rust. Nonstick pans begin to smoke and burn. Essential oils and extreme heat will melt less resilient plastics, and thin or delicate glass may be easily broken during the bustle of soapmaking activity. The resulting caustic mess must be disposed of properly.