Note: If you're interested in the soap making methods below, The Nova Studio in Point Richmond, CA offers classes on most of them.
I've tried a lot of the different kinds of soap making out there, so I figured I'd write about the different types, for those interested. I'm making some at the moment, so it seems like an appropriate time.
A lot of soap you buy in the supermarket isn't really soap, but a detergent bar. Soap involves the reaction of lye and fats to make a solid. Those who buy or make handmade soap are probably most familiar with cold process or melt and pour soaps. I'll start with cold process.
Cold process isn't totally cold- when you add lye to your water, it heats up rather quickly, you also need to melt your oils and butters, and then when the soap is in it's mold, it's probably creating heat as well. I know that's probably obvious to a lot of people out there, but I still feel cold process is a bit of a misnomer. Safety is a bit of a concern, but with proper precautions (goggles, gloves, and covering yourself up, plus keeping your lye away from children & pets), it can be a safe and fun way to make soap. I'm admittedly not too good at it, though that's more to do with my experimental recipes thickening up to quickly. Impatience also doesn't help, since I might combine my lyes and oils too fast.
The advantages to cold process include freedom to choose your own oils & liquids and creating beautiful designs. You're also starting with raw ingredients, so it's totally handmade. I'm honestly in awe of what some people do with cold process soap- amazing swirls, fancy layers, and beautiful colors. There are a few disadvantages, which can be avoided to a degree. Start with good instructions and a good lye calculator, and you've avoided a lot of them. Using tested recipes from a trusted source also helps when you're beginning. One of the biggest issues is a bad batch, at which point you have to either toss a lot of soap or rebatch it. You're also a bit limited in scents you can use. With fragrance oils, buy them from a store that mentions any issues that can come up from using the oils, such as ricing, seizing, or accelerating trace. Some of these problems can arise from using essential oils as well. And while I've seen recipes calling for sandalwood and rose essential oils, this would be impractical for many people. This would cost a lot of money, though it would make for a lovely soap.
While not necessarily a problem, cold process needs several weeks to cure, during which time some water evaporates out (leaving you with a harder bar) and the lye fully saponifies. Which brings us to hot process soap. There are a few ways of doing hot process soap, but I make mine in a crock pot. Your soap essentially cooks, leaving you with fully saponified soap that's ready to use (though it can benefit from curing time as well). There are fewer fragrances you can use, since anything with a low flash point will not survive the high heat. You don't need as much as with cold process though, so it can save you a little money. Colorants are a bigger issue, since some of them don't work well in hot process. It's also much thicker than cold process soap, so when it comes time to pour, it's harder to make nice designs. I still like making it though, since there are generally no surprises once you pour it into the mold.
Melt and pour is a good beginner's soap- you can pick up supplies at a craft store, making it more accessible. If you really like it though, better supplies are available online, and you'll save money that way, too. It is what it says it is- you cut up and melt your soap base, add colorants/scent/additives and then pour it into a mold. While you don't get to choose what your soap is totally made of, since you're starting with a base, you can use all sorts of colorants and scents, and create fancy designs in pretty molds. It's also safe for kids to make, since you're not using caustic ingredients. It's a nice way to get into soap making, if you're not sure if you want to try making cold process soaps. One big disadvantage is that it can sweat if not packaged properly, leaving clear beads all over your soap. This isn't very attractive, though wrapping it in plastic wrap seems to help keep this at bay.
Want a real challenge? It's possible to make your own melt and pour from scratch. I have never done this, though I've learned how to in a class. Too me it seems like a lot of trouble for something that you can more easily buy, I guess that can be said of all soap making. It's more complicated than cold or hot process soap, and involves extra ingredients. The soap I saw made wasn't crystal clear, though the sample I did melt down and I was able to pour it. It could be worth it for someone who wants to make translucent soaps from scratch.
Another challenge is liquid soap- this is a longer, more drawn out process, and requires potassium hydroxide instead of sodium hydroxide. I make this every few months for personal use, starting on a Saturday night and ending on a Sunday afternoon. You start by making a soap paste, which you then add to distilled water to get your liquid soap. I've had mixed results, though with some experimentation and research, I can sometimes get a nice thick gel. Other times it's a bit runny, and every time it's a dark yellow color. Insoluble ingredients fall out, so if you don't want that to end up in your finished product, you need the soap to sit for several days before pouring into your bottles. I also use a recipe that starts off lye heavy to make sure all the fats are saponified, and then I need to adjust pH once it's dissolved in the water. Some scents don't mix well with the final product, so that requires testing. While I probably won't ever make it to sell, since I find the end results unreliable, it is nice to make a liquid soap for less money than what you buy at the store. And I feel great when I do make a nice batch.
As mentioned in cold process, one more way to make soap is rebatching or hand milling. This involves grating up your soap, melting it down (in a double boiler, in an oven bag in your oven, or in a heat proof bag in a crock pot of water), adding your scent/colors/additives and then pouring into your mold. Often the melted down soap is more of a paste, so it's usually more like glopping it into your mold. You can create your soap specifically with this in mind, or use it as a way to possibly salvage a botched batch of cold process soap. It gives you more freedom in terms of scents and additives. I am making rebatched soap today- one that I using soap specifically for this purpose, the other using a batch of funky soap. Should the first batch come out nicely enough, I will be selling it at some point. It's has chamomile powder and lemon essential oil. The color isn't the nicest, since the chamomile powder turned it a muddy yellow, but I'm hoping the benefits of the soap out way it's appearance. The funky batch is currently melting in a double boiler. Even if the final result is better than the initial soap, it won't be for sale, since I don't want to risk selling a bad bar.