Soap Terminology

the language of soaping

© 2010 by KV5R — Rev. Nov. 24, 2010.

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Terms and Things to Know

Soapmaking, or “soaping,” has its own unique terminology, some technical and some traditional. Many soaping sites start right off talking about soaping and forget to define the terms, so I’ll put this part first. If you’re already an experienced soaper, feel free to skip ahead.

  • Lye Soap: Now a misnomer, soap was called “lye” soap back when the chemistry was not understood. People made lye of unknown concentration from wood ashes, then added more than enough to rendered animal fats to produce soap that was very alkaline. Many people still have a prejudice against lye soap because of this. I remember my father saying that his mother made lye soap that would “take the skin off your hands!” Nowadays, we understand the chemistry and can weigh ingredients precisely, making soaps in which no lye remains after the reaction. Thus, it is no longer called lye soap, but natural or homemade soap.
  • Soap measurements: Soap ingrediants are measured by weight, not volume. What’s the difference? 1 fluid ounce = 1 weight ounce at a specific gravity of 1.0 (water). But oils and fats are lighter than water, so a fluid ounce of oil might weigh 0.92 ounce. 1 fluid ounce of 30% caustic solution weighs about 1.3 ounces. Since we are trying to create a stoichiometric chemical reaction (not simply mixing a cake), we use weights, not volumes. It’s just that simple.
  • Cold Process (CP): Soap-making at about 110°F (43°C). A characteristic of CP soap is that everything that goes in, stays in, including the reaction byproducts like natural glycerin. Also, the fat-to-lye ratio must be exactly determined and measured (by weight), so that the finished soap has a mild pH (no caustic left over). Most homemade soap is CP. The reaction is started in the mix pot, but it finishes in the mold and during curing.
  • Hot Process (HP): The mixture is made similar to cold-process, but instead of pouring it at full trace, the soap is cooked for 1-2 hours, either in the oven or a double-boiler setup. The extra heat causes saponification to be completed in the pot, so the soap may be used right after cooling. Also, emollients and essential oils can be added after cooking, so they are not consumed in the reaction. The downside is that the cooked soap is much thicker (like jelly or thicker) and more difficult to mix and to mold than CP soaps. It’s also more dangerous to make, since the cooked soap is quite hot and may foam over or cause burns.
  • Lye = caustic soda = NaOH = sodium hydroxide: A strong alkali requiring careful storage and handling. Used for hard soaps. Byproduct of electrolytic chlorine manufacture. Though the terms are synonymous, it’s handy to refer to the dry granules as lye, and the wet solution as caustic. Some people fear soap-making because of the lye—they are chemophobic (brainwashed against chemicals)—yet they will handle a hot skillet or boiling pot every day…
  • Potash = KOH = potassium hydroxide: Ditto. Used for liquid soaps. Can be extracted from white wood ashes—that’s how pre-industrial people made soap (back to at least 2800 BC), and some people still do. Commercial KOH is made from an electrolytic process, and is preferable to home-made “potash” since the exact concentration is known.
  • Fat/Oil: Whether animal or vegetable, “fat” usually refers to fats that are nearly solid at room temperature (aka “hard oils”), while “oil” refers to fats that are liquid at room temperature (aka “soft oils”). The choice of what blends of fats/oils to use is up to the soaper, and determines various qualities of the soap. It’s very helpful to consult a fats/oils properties table when designing a new recipe.
  • Saponification: The chemical reaction between an alkali (i.e., caustic) and fats/oils (i.e., triglycerides). Soap is a “saponified” fat. The reaction is mildly exothermic and, in a proper mix, all the lye is consumed. Triglycerides (fats/oils) are cleaved into fatty acids and glycerol, then the lye reacts with the fatty acids, resulting in soap (a natural surfactant) and glycerin (a natural moisturizer). But don’t panic—you don’t need to know the chemistry to make soap!
  • Saponification Value (Sap.): The weight ratio of lye to fat/oil required to saponify the fat/oil. Every fat and oil has its own saponification value. The easiest way to calculate your lye weight is to individually weigh your oils and then use an online lye calculator. I like the Soapcalc.net calculator best because it supplies more information, like the various properties of your oils. It’s a great tool to design a new recipe. It’s very important to use exactly the right amount (weight) of lye so that it is all consumed in the reaction, leaving the soap mild but not oily.
  • Exothermic: Chemical reactions that produce heat. Can be fast or slow: explosion=fast, fire=medium, soaping=slow. Reaction is faster at higher temperature, and since it produces its own heat, it can accelerate the reaction on its own. Cold-process saponification is usually so slow that it is necessary to insulate the mold to retain some of the generated heat (see Gel Phase below). Hot-process saponification actually adds more heat to accelerate the reaction.
  • Superfat, superfatting (SF): Increasing the fat, or reducing (“discounting”) the lye a little bit (typically 5-8% by weight) to ensure that all the lye is consumed in the reaction, and just a little oil is left over. Too much SF and your soap is greasy; too little and it’s caustic. There is no caustic left in properly made bath soap. Caustic soap may irritate your skin, but is fine for laundry, and helps the cleaning process.
  • Trace: During mixing, when lye/oil mixture starts to react and thicken. Hand stirring=slow; stick blender=fast. “Trace” is simply an observed, subjective measure of viscosity that, with experience, is a good indicator of the onset of saponification. A more accurate (less subjective) measure is temperature rise: in small batches, a 2-3°F rise indicates readiness to pour—e.g., the exothermic reaction is now underway, and will now ramp itself up until all the happy caustic ions marry all the happy triglycerides and make lots of happy little soap babies—with a few nice hydrophilic glycerin in-laws and emollient godmothers left over for pampering…
  • Fragrance Oils and Essential Oils (FO and EO). Usually added at “light trace” (the first visible sign of thickening), they add the scent and/or some extra property to soap. Study carefully before using, as some of them will accelerate saponification; others may cause problems like glycerin weeping. Most will change the chemical reaction to some degree. They are usually added at the rate of about 1.5 to 3 percent of the total weight of the fats/oils. Most are quite expensive, like $10-$20 per ounce, so it’s usually best to either use less expensive oils like peppermint or orange, or not use them at all.
  • Exfoliants: Abrasives that make the soap slightly scratchy—things like clay, oatmeal, cornmeal, ground nut shells, and pumice. Some people don’t like their soap so slick, or want their soap to “exfoliate” dead skin cells, or cut heavy grease. (Exfoliosaponificators: soapers who don’t feel clean unless they’ve had a good scratch! 😉 I prefer to just use a scratchy sponge, and leave the grit out of the soap.
  • Seize: During mixing, the reaction can accelerate so fast that it sets up in the pot before you can pour it. Adding certain ingredients, particularly synthetic fragrance oils, may cause this to happen unexpectedly. The mix may suddenly act like mashed potatos, cottage cheeze, or taffy; or even set up into a waxy brick. Should never happen if you follow instructions.
  • Volcano: This is when the pot foams over and makes a huge mess. Caused by the oil being too hot, and/or certain additives that react and release gases into the goop. Should never happen if you follow instructions.
  • Gel Phase: After pouring the mold and insulating, the reaction continues to produce heat ’til it’s done (several hours). During this time, the self-heated soap will look and feel about like axle grease. It should be solidified, and opaque, by the next day. Some soapers prefer to prevent the gel phase by putting the poured mold in the freezer. Full saponification will take much longer, but they claim it makes a better soap. But most CP soapers encourage the gel phase by insulating the mold with towels or blankets. To get all the molecules married even faster, see Hot Process above.
  • Molding and Cutting: Pouring one large rectangular loaf mold, then cutting it into bars, is best done with a loaf mold and matching miter box. Using plastic or silicone molds with several individual cavities will make prettier bars, but unless you precisely measure your batch, may result in a short bar or some waste. The advantages of the loaf-and-cut method are that you don’t have to be so precise with the batch volume, and that you can cut bars to desired sizes.
  • Curing: After the soap cools and solidifies, it needs to sit for 2-6 weeks to allow it to lose some moisture and firm up. Bars are usually placed on racks for drying. Cold-process soaps will slowly finish up the last little bit of saponification during curing, as the water leaves and forces the last bit of lye into contact with the last bit of fatty acids. Some soapers put their soap in the oven with only the light turned on the speed things up a bit. Hot process soap does not require curing, because all the water is “cooked” out of the soap.
  • pH: A base-10 logarithmic measure of acidity and alkalinity, where 0=100% acid, 7=water, and 14=100% alkali. During curing, pH may be tested to determine the amount of lye still remaining. Good soaps should have a pH well below 10. Most soapers don’t test pH, they just make it right, and then try it. Some even taste it! If tasting it bites the tongue, the pH is still too high.

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