The chemical reaction between lye and fats is called saponification. This reaction is exothermic - it releases energy in the form of heat. (Some reactions are endothermic, or require energy to occur, and take it out of the environment in the form of heat. Paradoxically, even though heat is being "added" to the reaction, the reactants and products become cooler. This happens when you make ice cream, and add salt to the ice around the ice cream maker. If you checked the temperature of the ice, and then the ice and salt mixture, you would see that the temperature decreases. That's how you get temperatures cold enough to actually freeze the ice cream. Same thing when you put salt on your sidewalk to melt the ice. The temperature of the salt water would be colder than just the ice itself.) But I digress. Depending upon the oils you use, the temperatures you soap at, fragrance oils (FO) you use, and environmental temperatures, the saponification reaction can generate more or less heat. If the reaction gets hot enough, the soap goes through a gel phase. You can see this happening - it starts in the center of the mixture, because that's where it will get hottest first - the forming soap becomes darker and somewhat translucent in appearance. The gel phase will spread outward, sometimes stopping before it reaches the edge, sometimes completely gelling the entire batch. Large batches, and using log molds where the surface area ratio is low compared to the volume of the batch, tend to trap the heat being created by the reaction, encouraging gel to occur. Soap that goes through saponification without getting hot enough to go through a gel phase will be lighter in color and opaque in appearance compared to gelled soap.
Many new soapers ask which is better, gelled or non-gelled soap? The answer is neither, or rather, it depends upon your aesthetic preference. Gelled and non-gelled soap last the same amount of time, have the same lather, and the same properties. The only real difference is in appearance. Gelled soap is darker and has a tiny bit of translucence. Non-gelled soap is significantly lighter and opaque. I made a batch of soap, and put most of it in a log mold, and put the left overs in individual molds. The individual molds did not gel, while the log mold did gel. Here is a picture:
As you can see, the soap on the left is much lighter than the soap on the right, even though it came from the same batch.
The one nemesis with gelling is the dreaded "partial gel". This is where the center of the soap gels, but the edges do not. This results in a soap that is lighter and opaque on the outside, but darker on the inside. Imagine putting a half moon of the soap on the right in the middle of the soap on the left. Not attractive. Because you want to avoid partial gel, most soapers either encourage a full gel, or try to discourage it from happening at all. You can encourage gel in several ways: soap warm and keep the mold in a warm area, turn the oven on to 170 degrees F (76 - 77 degrees C) then turn it off and put your soap mold inside for several hours, or wrap the mold up in towels. I emphasized turn off the oven because if you don't, you could end up with the soap overheating, and a "volcano" eruption. I use a space blanket, which is cheap, easy, and doesn't use any gas or electricity like the oven. It is essentially a large sheet of silver mylar, and you can buy them in the camping sections of places like Target and Wal Mart, and even online at Amazon.com. I lay down a towel, then lay the space blanket on top (it is large, so I double it up). Put my mold on top, fill it, and wrap the blanket and towel over the mold. The soap will often rise in the middle, so I put some wood at the ends of my log mold to keep the space blanket off the top of the soap. I leave everything like that until the next day. Works like a charm. Here are pictures of my set up:
Avoiding gel can be much harder. If you live in a hot and humid place, it can be nearly impossible. Recipes high in some oils, like coconut, can be very difficult to keep from gelling. Suggestions: soap in small batches, use tray molds that spread the batter out, put it in the freezer or refrigerator, and soap cool. I just made a small, 1 lb test batch, put it in the freezer, and it gelled in the freezer! Preventing gel is trickier than ensuring a full gel. But if you absolutely love the creamy look of ungelled soap, you will eventually find what works for you.
Colloidal Oatmeal: I needed some colloidal oatmeal for a facial soap recipe I was testing, but didn't want to pay the high price of ordering it online. I did some Internet research, and my problem was solved.
The reason to use colloidal oatmeal is the particles are small enough that they evenly disperse throughout the soap mixture, so you get the benefit of the soothing quality of the oatmeal in the lather, and you don't get any rough pieces that might scratch your skin. To make it, just take old-fashioned oats (not instant, and I don't use quick-cook either), and put some in a food processor. Grind it up as small as possible. At this stage it is probably okay to use, but colloidal oatmeal is an extremely fine powder, and you probably won't get that with your food processor. Small coffee grinder to the rescue. Put some of the ground oatmeal in the coffee grinder (cleaned, of course), a few pulses, and voila! real colloidal oatmeal. WAY cheaper than buying online.
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