Step 4: Purify the Fat
The first thing you have got to do with the rendered bacon fat is to purify it. When it comes out of the bacon pan or storage jug or rendering pan it will most likely have lots of little bacon bits in it. We don't want any of those to get into the soap because they would most likely turn rancid at some point and mess up the curing process. To purify the fat you need to first warm it up slowly and get it boiling with a bunch of added water in a pot. I put my containers of fat into a hot water bath to melt the fat first so I could get the lard out of the containers and into my pot.
Once the fat was soft enough to pour/scrape out of the containers I added them into a big pot and mixed my fat (about 4 cups worth) with 8 cups of water. The fat shouldn't be boiled alone. Use about two times as much water as fat you would like to purify.
With the liquid bacon fat and water in the pot I then slowly heated the mixture and brought it to a slow boil (do this process gradually so it doesn't splatter everywhere). I let it simmer for a little more than an hour - at which point it reduced its volume significantly and visually appeared cleaner - although it didn't smell any better. Take the mixture off of the heat and transfer it to the refrigerator until it solidifies again.
(Check out the refrigerator the bacon fat is cooling in - have you ever seen so many veggies? I hope the bacon fat didn't feel self conscious in there.)
Once the fat solidifies (2-8 hours depending on how much fat you have and your fridge temp) you have to get the fat cake out of the pot and into yet another big pot. At this point your trying to pour just the fat into the new pot - leave the watery substance and the bacon bits that have separated to the bottom of the fat glob behind - you want just the pure fat cake thats sitting on top of your water and nasty bacon bits. I used my hands to do this as just pouring it into the new pot will prevent you touching the fat glob, but won't yield quality results.
Take the solidified purified fat and return it to the stove. You want to heat it up just so it turns into a liquid. Try not to make it too hot, it will be important to have it at the right temperature in the next step.
Sold, liquid, solid, liquid - make up your mind fat!