Now we know how to make a very basic soap, from a chemical reaction perspective. If you missed anything so far, go back and check out Part 1, Part 2, and Part 3 for more information.
The actual chemical process of making soap is a fairly simple one. However, in Part 3, we looked at only using one oil to make our soap. This is a perfectly valid way to make soap, but most soap makers prefer to use a combination of different oils and butters to create their own special recipe. This is where things begin to get interesting!
Each oil or butter will have it's own set of chemical bonds and structures. Because of this, the way each one interacts with the lye solution will be slightly different. Some oils have higher levels of unsaponifiables than others. This is not necessarily a bad thing! Depending on what exactly makes up an oil or butter, we may actually want some of these things to remain in our finished soaps. I highly recommend taking a look at Susan's blog, SwiftCraftyMonkey, for more information. She has plenty of great information, and she explains it in an easy to understand way. If you scroll down on the right side, she even has PDF files of various comparison charts. These are some invaluable resources for the beginning soap maker, and even for those who have been making soap for many years.
The variety of oils available is why there are so many different options for soap recipes. Every soap maker has his or her own "perfect" recipe, and they are likely all different. I encourage everyone to research on their own to find out as much information as possible, and then to experiment to see what happens.