There are two major parties in this soap nation: the CP/ HP soap makers who make soap from oil and lye, and the MP soap crafters who combine soap base with myriad colors and scents. There is a friendly rivalry between the two parties, a little bit of mutual mistrust, and sometimes an unfortunate level of mutual misunderstanding. The CP/ HP soap makers will say that the MP folks are not actually soap makers, since the soap base they start with is already soap. The MP soap crafters will say that, unlike CP/HP soap, theirs does not contain lye and other harmful chemicals. While it seldom erupts into the violence we see in the political arena, we would all be better off if we understood one another a little better. This month, I would like to take on the role of uniter-in-chief by teaching both parties how to make a simple MP soap from scratch. 

Let us begin by recognizing that the word soap has multiple meanings. The definition used in both chemistry and commerce is that soap is the alkali salt of a fatty acid. Most often, the alkali is sodium for a solid soap, and potassium for a liquid soap. The list of ingredients on a grocery store soap might include, for example, sodium palmate, sodium cocoate, and sodium palmkernelate, the mixture of soap molecules that result from saponifying palm oil, coconut oil, and palm kernel oil, respectively. But CP/HP soap does not contain soap alone. It also contains some of the water used to make the lye solution, and, as we saw in last month’s column, glycerol, a by-product of saponification. The industrial process used to make commodity soap removes the glycerol, but the CP and HP processes leave it in.

CP/HP “soap” is more accurately understood to be a mixture of soap molecules and two solvents: water and glycerol. A CP/HP soap made with sodium hydroxide is generally a solid at room temperature, but if you make it hot enough, it will melt and become liquid. A CP/HP soap made from potassium hydroxide is generally a liquid at room temperature, but if you make it cold enough, it will fuse and become a solid. And the “melting points” of both sodium and potassium soaps depend on the relative amounts of soap, water, and glycerol. The higher the concentration of soap, the higher the melting point. The higher the concentration of solvent, the lower the melting point.

So, if any soap can melt, why not just put CP/HP soap in the microwave oven and press the “start” button? For that matter, why not do the same with grocery store soap? I put a grape-sized piece of CP soap (a commodity soap like Palmolive works the same) in a microwave-safe bowl, stuck it in the microwave oven, and turned it on. About 15 seconds was all it took.

What happened you might ask. The “melted” soap turned into a frothy mess. Why? Because the melting point of CP/HP soap is higher than the boiling point of water. When you heat most soap, the water boils away before the soap has a chance to melt. To make a soap that can be melted in a microwave oven, we need a solvent that does not boil before the soap melts. Two solvents are very common in MP soap bases. Propylene glycol has a boiling point of 188°C (371°F), and glycerol 290°C (554°F), both significantly higher than that of water. Adding one of these two solvents will produce a soap that can be melted in the microwave oven.

Propylene glycol is a wonderful substance, but you won’t nd it in grocery stores or drug stores. Glycerol, on the other hand, is sold in either of those places. So for my next experiment, I got my hands on some glycerol, often sold under the common name of glycerin. We’ll revisit propylene glycol next month.

Recall that CP/HP soap already contains between 7% and 9% glycerol as a byproduct of saponification. If you heat the frothy mess from the first experiment to a high enough temperature, it will eventually melt. But that temperature is inconveniently high. At that temperature, the soap molecules are likely to char, and if you spill some on yourself, you will get a nasty burn. To lower the melting point of the soap, we need to increase the concentration of glycerol.

My super-easy formula for made-from-scratch MP soap: (I do not recommend trying this at home as it is very easy to overheat and burn soap, and potentially burn yourself. Take my word for it ) 2 parts soap to 1 part glycerol. I suggest starting small. 2 ounces of soap to 1 ounce of glycerol. 50 grams of soap to 25 grams of glycerol. Grate the soap with a cheese grater, weigh it into a big microwave-safe container, because it’s going to get frothy as the water in the soap boils away. Weigh the glycerol into the same container and mix thoroughly. After 15 seconds in the microwave, it will look like Figure 1. 

Be very careful as you remove this hot, steamy mess from the microwave and stir the froth back into the glycerol. Continue the cycle of microwaving for 15 seconds and stirring until it looks like Figure 2 (view from above of a beaker sitting on a cooling rack). The result will be something that looks familiar to MP soap crafters: a hot, thick, transparent liquid, that is no longer frothy because all the water has boiled away. Being careful not to burn yourself, pour it into a mold and let it cool. I made a heart, the symbol of my love for both soap makers and soap crafters. This soap becomes opaque when it cools.