Oil and water don't mix. More precisely, oil and water don't dissolve in one another. Non-chemists frequently use the terms mixing and dissolving interchangeably, but there is an important distinction that is particularly relevant to soapmaking. When you dissolve sugar or alcohol in water, the resulting solution is transparent because the sugar or alcohol is present as individual molecules floating in and amongst the water molecules. These molecules are too small to deflect light, and so light travels straight through the solution without changing directions. The result is that the solution appears transparent. But when you mix oil and water, the resulting mixture is a cloudy emulsion. The oil is present as suspended droplets, not as individual molecules, and while they are not visible to the naked eye, these droplets are large enough to deflect light. A beam of light entering an emulsion from one direction exits in another. The result is that the emulsion appears cloudy. You are now prepared to distinguish solutions like honey and wine from emulsions like milk and blood.

When you mix oil and water, the resulting emulsion is unstable. The oil is initially dispersed into droplets, but these droplets gradually coalesce into larger and larger droplets, which float to the surface. Eventually, you are left with a layer of oil floating over a layer of water. The oil/water emulsion can be stabilized, however, by the addition of a surfactant, a molecule with a hydrophilic head (attracted to water) and a hydrophobic tail (attracted to oil). The surfactant sits on the surface of an oil droplet and prevents the droplets from coalescing. The stabilized droplets are called micelles, and the stabilized emulsion is called a colloid.

Now, when a soapmaker adds a solution of sodium or potassium hydroxide (lye) to an oil and agitates the mixture by shaking, stirring or stick blending, the oil is dispersed into droplets and a creamy, non-transparent emulsion forms, as it would with oil and water alone. But the lye at the surface of each droplet reacts with the oil, forming soap (a surfactant), which gradually stabilizes the emulsion and converts it into a colloid. As more and more soap forms, the colloid gets more and more viscous, until it is for all intents and purposes a solid. When crafting soap into layers and swirls, we rely on it being thick enough to hold the patterns we wish to form, but thin enough to flow into and fill the mold. That “thick but not too thick” condition is called trace.

Experiments detailed in Chapters 22 and 23 of Scientific Soapmaking (Clavicula Press, 2010) indicate that at trace only about 10% of the oil has been converted into soap. Soap at trace is best imagined as a colloid whose micelles are approximately 90% oil and 10% soap. Beyond trace, the soap at the surface of each micelle gets thicker and the oil at the core diminishes until eventually only soap remains. The key feature of cold process soap that makes crafting possible is that there is an extended period of gradual thickening during which layers and swirls may be crafted.

Soap seizes when this period of gradual thickening is shortened to the point that the soapmaker has insufficient time for crafting. Normally, the surfactant (soap) forms slowly as the oil reacts with the lye. But when a surfactant is present from the beginning, the emulsion is converted prematurely into a colloid. This surfactant may be present in additives like scents and colors, or it may be present in the oil itself. How does oil get contaminated with surfactant? One form of oil rancidity comes from the spontaneous decomposition of oil into fatty acid and glycerol. Whereas oil reacts with lye slowly, fatty acid reacts almost instantaneously with lye to form soap. Thus the “free” fatty acid in rancid oil converts quickly to soap, cutting short the early pre-trace mixing period and hastening the formation of the thick colloid. A simple test can determine the acid value of a batch of oil, a measure of the amount of acidity (presumably fatty acid) it contains.

To perform this test, you need phenolphthalein indicator (usually supplied as a liquid solution in alcohol), some dilute lye (1% sodium hydroxide, 1.0 g NaOH in 99.0 g water), two medicine droppers, some denatured alcohol (sold alongside paint thinners), a Pyrex measuring cup, a stainless steel spoon and the oil to be tested. If you are testing a solid fat, melt it before testing. Pour 250 mL (about 1 cup) of denatured alcohol into the measuring cup and add a few drops of phenolphthalein. Then add 1% sodium hydroxide one drop at a time (stirring with the spoon after each drop) until the alcohol turns faint pink. Do not add any more hydroxide solution than it takes to achieve this color. Now add 30 drops of the oil to be tested and stir with the spoon. If the pink color disappears, your oil contains acid. Add 1% sodium hydroxide one drop at a time, stirring between drops and counting the drops, until the pink color returns. The more sodium hydroxide needed to restore the pink color, the higher the acid value of the oil and the higher its concentration of free fatty acid. If fewer than 30 drops of 1% NaOH are required, the acid value is below 1% and the oil is not rancid enough to cause seizing. Scientific Soapmaking Chapter 17 gives a more quantitative method for measuring acid value. Next month, we'll look at additives that may contribute to seizing.