I know this has been done before, but I thought I would document the latest batch of soap I've made. Hopefully this instructable should give you some practical hints as well as help you with the theory of making soap from leftover fat.

Soap makes a unique and personal gift, especially when it is home made. You can also sell your soap at a farmer's market or just use it yourself. And it's a great way to get rid of old grease and fat that would otherwise find it's way to sewers or landfills.

The process uses caustic chemicals, so it is slightly dangerous. But if you follow general safety guidelines and keep your wits you should be fine. You might make a mess, but it should be easy to clean up because most of the spills will be made of soap!

I got most of my info from this page and this lye table

I was inspired by this instructable:
https://www.instructables.com/id/How-to-Make-Bacon-Soap/?ALLSTEPS

Note: I would've gotten better pictures, but it's hard to use a camera with eye goggles and rubber gloves.

Step 1: Supplies

Ingredients you will need:
1. A lot of animal fat or vegetable oil.
2. I added some olive oil to my recipe to make it milder.
3. Lye (NaOH) crystals. You can get this at the hardware store next to drain openers.
4. Distilled water. You can use tap water, but I wanted to make sure there were no dissolved minerals that could ruin the soap.
5. Essential oils for a scent. This is optional, but I think it helps. I use tea tree oil and jasmine oil. You can find these at a natural foods or drug store.

Tools you will need:
1. Safety equipment: Rubber gloves, goggles, and an OPEN bottle of vinegar withing easy reach, in case you need to neutralize any spilled lye. Lye (NaOH) is very basic and caustic, and it WILL burn your skin, or any part of your body for that matter. If you've ever seen Fight Club than you know what I mean.
2. A large glass or ceramic bowl to mix everything in. Anything that touches lye must be glass, because lye can react with plastic, wood, and especially metal. I was lucky enough to find a big Pyrex bowl in my attic that an old roommate left behind.
3. An accurate scale. It doesn't matter whether you measure in grams or ounces, but you must be accurate. I have a digital scale that works in grams or ounces.
4. Something to stir with. You can always just use a big spoon, but it will take a long time and you will get tired quickly. I got a stick blender at Goodwill for a couple dollars that worked well until the motor burned out. So for my second batch I just pulled it apart and mounted the business end to a power drill. Make sure that whatever you use for mixing, you wash it very well before using it on food. I have dedicated equipment that I use for mixing soap and nothing else.
5. Some kind of mold to pour the finished soap into. Besides a casserole pan or plastic bin, you can also use lengths of PVC, or paper cups to mold the soap into round shapes (see the main photo).
6. Notepad and pen along with a calculator to make conversions.
7. You will also need a few more tools like bowls and scrapers etc.

Lay down some newspaper on a big table or counterspace. You will need plenty of room to make soap.

Step 2: Obtain Clean Rendered Fat

To make soap, you need some kind of fat or oil. Vegetable oil will make a softer soap than saturated fat like lard or shortening, so it's a matter of personal preference of whether you like a hard bar soap or a somewhat squishier soap. Blending several kinds of oil or fat together can give you a wide variety of characteristics.

Some commenters have pointed out that vegan (and therefore kosher and halal) soap can be made with only vegetable fats. Check out SoapyHollow's profile for vegetable oil recipes.

I got my fat mostly from bacon and hamburger drippings, with some olive oil too. It is important to make sure that you have each kind of fat separated and pure; i.e., don't mix beef tallow and lard together until after you have weighed them, or else it will throw off your calculations and your soap will not turn out right. Then you must render and clean the fat. Rendering is the process of melting the fat to separate it from water or bits of meat. In my case, I just fried my bacon and poured off the extra grease into a jar.

To clean the fat, you can either boil it, filter it, or both. I first filtered everything through an old sock and a coffee filter in a funnel that I made by punching a hole in a plastic cup. Then I boiled it in a big tall pot with half water and half fat. This gets rid of any water-soluble dirt or salt left in the fat. Boil it for about 20 minutes, then cover and let it cool (in a fridge or outside in the winter) until the fat floats to the top and solidifies. Then you can scoop out the purified fat and throw away the nasty water at the bottom. Scrape any dirt off the bottom of the fat.

Step 3: Make Measurements and Calculations

A big part of making soap is being precise. You need to weigh out ingredients precisely so you have them in the right proportions. Never measure by volume.

First weigh the containers you will put the fat into, then weigh out the fat. I measured 1626 grams for the bowl and the fat, then subtracted 906 g for the Pyrex bowl, leaving 720 grams of fat. I also measured out 102 grams of olive oil in another cup and added that to the rest of the fat for a total of 822 grams of fat.

In order to use the right amount of lye and water, you need a table of saponification values. You can find one set here: http://www.tlcsoaps.com/oilsapval.htm

You can also use this online lye calculator: http://www.tlcsoaps.com/tlcsoapcalc.htm

Since I am aiming for 5% residual fat at the end, I used values of 0.132 for lard, 0.129 for olive oil, and 0.38 for measuring the water.

720 g lard x 0.132 = 95 g NaOH
102 g olive oil x 0.129 = 14.2 g NaOH
Total: 108.2 g NaOH
Now is a good time to put on your gloves and goggles. Weigh out the lye crystals into a bowl. It is OK to use plastic at this point because the crystals aren't wet.

822 g of total fat x 0.38 = 312.4 g water.
On this last measurement you don't need to be too accurate. Just add a little extra water if you're not sure, since it will mostly evaporate in the end. Put the water in a smaller glass bowl so you can mix the lye in. I used a coffee pot because it is made of glass and has a pour spout.

Now CAREFULLY and SLOWLY add the lye to the water. NEVER add water to lye, because it can spit out of the container and burn you. Stir slowly as you add the lye, making sure to dissolve all the crystals. The water will heat up, but if you mix it slowly enough it won't heat up too much.

Step 4: Mix Lye and Fat to Make Soap!

Now comes the part where you actually make the soap. Keep your safety goggles and gloves on, because there is still a risk of chemicals splattering.

Make sure that both the fat and the lye/water mixture are warm, about 100F. This will keep the fat from solidifying before it actually turns into soap.

While stirring, slowly pour the lye/water mixture into the fat. The fat will start to turn milky and thick as you stir. If you're using a blender this step will go much quicker.

Once the soap is the thickness of pudding, test whether it is ready by trying to leave a "trace." Drip a trail from the spoon onto the surface of the soap. If it leaves a trail for a few seconds, that means it has "traced" and is ready to be molded. If the trail quickly drops back under the surface it isn't ready and you need to stir it more. Keep the mixture warm, and remember that you can't really stir it too much.

After it has traced, you can add whatever scent you want. You can also add ground spices or herbs. I added some nutmeg, ginger, and orange zest (mostly because that's all I had around). Just don't add anything really scratchy like coffee grounds, since you will end up making sand paper out of your soap. I learned that one the hard way! Also don't add anything with alcohol, acid, or any chemicals that might throw off the reaction.

You can now pour the soap into whatever mold you want. I used a simple casserole dish. After it solidifies (this takes a few hours or overnight) you can cut it into bars and wrap it. It's good to keep most of the air away from the soap while it cures to prevent carbonic acid from forming on the surface. This can happen due to a reaction with carbon dioxide in the air. Plastic wrap or wax paper works well for this. You can also imprint a brand or logo into the soap while it is still soft.

Now you have to wait. It takes about 3 weeks for soap to fully cure. During this time, any excess lye will reacting with any remaining fat. If you use the soap before this time, it could irritate your skin. Use this time to read a book or take up another hobby, like homebrewing (that's my next project).