Glycerin soap is often transparent, with ribbons of bright color and a nice slippery feel. Cold process soap is always opaque, usually with more natural color and a creamy, often rich, lather. Beneath physical appearances, what really is the difference?
Cold Process Soap
Cold process soap is also sometimes called Lye Soap. It is created by mixing lye & water with fats & oils, then mixed briskly. The lye reacts chemically with the oil mixture in a process called saponification, meaning to turn into soap. The batch is said to have reached light trace when it reaches a pudding-like state in which the spoon leaves a brief trail. When trace is reached, scents, colorants and exfoliants can be added. The mixture is stirred more and then poured into molds. The batch continues to heat and complete saponifying in the molds. In 24-48 hours, the soap can be turned out of the molds and sliced into bars. The bars then must cure for about a month before they can be used. This allows the bars to harden, making them last longer, and ensures that the bar has completed the chemical process.
If the ratio of lye to oils is too high, then the soap will be what is called lye heavy. Lye heavy soap is generally crumbly and cannot be used on skin because it will cause a chemical burn. If the ratio of lye to oils is too low, the soap is said to be superfatted. Some of the oil stays in the soap, unmodified by lye. While it is generally a good practice to superfat soap (usually 3-7%) to ensure that no lye is ever left in the soap; too much and the soap will be greasy and have a drastically shortened shelf life.
In truth, all homemade and handcrafted soaps are glycerin soap, because the naturally forming glycerin from the saponification process is not removed. However, the term glycerin soap has come to be understood as transparent soap.
Glycerin soap is usually translucent, but the addition of ingredients can make it opaque. It is often very decorative with ribbons or chunks of bright colors and shapes. Crafters may even embed objects such as toys in the soap. Glycerin soap made from cold process soap or by coloring and scenting melt-and-pour bases and does not require curing; it can be used straight from the mold.
The melt-and-pour bases are purchased from a craft store or soap supplier, melted in either a microwave or double boiler, colored, scented and poured into molds. These are not homemade soaps, the crafter did not create this soap from scratch and has no control of what ingredients go into the soap (other than what she adds to it). That said, melt-and-pour can create very unique and beautiful handmade bars of soap, without the concern of working with caustic lye.
Transparent glycerin soap can be homemade. Start with an unscented, uncolored batch of cold process soap that has little or no superfatting. The soap is shaved into small pieces and melted in either a crockpot or double boiler. By adding water, sugar, glycerin and either high proof ethanol (such as vodka) or rubbing alcohol, transparent glycerin soap can be created. Homemade transparent soap can be colored and scented like the melt-and-pour base, but is a less forgiving of repeated meltings. This method takes some practice as the alcohol tends to evaporate quickly, and is more likely to be done by experienced soapers. This method allows full control of all ingredients in the soap from start to finish.
Which should I Use?
This really is a personal decision. Both cold process and transparent allow for lovely soaps with a great variety of scents. While the transparent soap is not as good for your skin as cold process soap because of the added alcohol, it will likely be better for your skin than commercialized soaps purchased in a department store.
If you just want a pretty bar to match your bathroom decor, then try the transparent glycerin soaps. If skin care is your primary concern or if your skin tends to be very sensitive, then go with cold process soaps.