Making soap from used cooking oil





I wrote the bulk of this post on January 5th, when I actually made the soap, but I wanted to wait until I knew whether or not the stuff was worth a toss before I went and gave out the recipe. I finally used a bar of it a few nights ago and it works just fine and dandy. No weird smell, no slime. Doesn’t lather as much as I’d hoped, but it got my hands nice and clean.  Next time I should use more lye. (For the record, those are Matt’s meaty mitts in the pictures, not mine!)

I have always wanted to make soap – but I have always been terrified of lye. A lifetime of reading books and articles on home soap making, with their dire warnings, and about a hundred viewings of Fight Club put a terror of chemical burns in my brain.

Well, consider that fear conquered. With enough vinegar you can kill anything, and apparently having two and a half gallons on hand will kill one’s fear of sodium hydroxide. As Matt has said many a time: “People have been making soap for millennia. It can’t be that hard.”

It’s not.

I found some very simple, straightforward, and well-researched instructions on a website called and followed the instructions, using my thrice-filtered used canola cooking oil, filtered tap water, and lye. There were no poison gas clouds, no burning flesh, no scorch marks on my kitchen, and I didn’t cry. (I will admit to doing the panic dance once or twice, very briefly.)

I recommend reading the “Homemade Soap” page on, but here’s the rundown of what I did:

  1. Weigh the fat you want to use. I found 2 pounds to be about the amount I felt comfortable sloshing around.
  2. Consult the handy Lye to Fat Ratio Table on to determine how much water and lye you will need to saponify (essentially, to “soapify”) your fat.
  3. 19e0e-soapmakingPour your lye slowly into your water in non-metallic and sturdy container. Not the other way around! I’ve heard some scary stories about what happens if you put water into lye instead of the other way around. It will rapidly heat up and give off stinky steam you don’t want to breathe. Stir (with a wooden or plastic spoon) until it’s clear and let it cool to about 85ºF.
  4. cdce3-soapmaking2Heat your oil to about the same temperature, in a non-reactive container. The only metal you can use is stainless steel.
  5. e1d9a-soapmaking252832529Slowly drizzle the water/lye mixture into the oil, while stirring.
  6. 4937c-soapmaking252842529Stir, stir, stir. Everyone recommends a stick blender for this part because it can shorten your stirring time from an hour to a few minutes depending on your ingredients. I used a stick blender and it took about half an hour for my ingredients to reach the mystical “trace” stage. (Fats and oils that are liquid at room temperature take longer to saponify than those that are hard at room temperature.) At this point your mixture is the consistency of soft-set pudding and when you pull the spoon out the liquid that sheets off of it leaves a trace on the surface before it is reabsorbed. Add whatever smelly stuff you want now.
  7. 7495d-soapmaking252852529Pour into non-reactive molds and put them out of the way for a few days.
  8. No one ever explains how to clean up the large and hazardous mess you’re left with at this point! No one! No one, that is, but me. Common sense told me to wipe everything down with vinegar, which neutralizes the lye – and I do mean everything, even stuff you’re putting it he garbage can, so that it doesn’t go nuclear and eat through the bottom of the can. That done, cleanup was no more thrilling than any other sinkful of dishes or splattered cooktop.
  9. It takes anywhere from one to five days for saponification to be truly complete. When your soap feels nice and hard it is fully saponified and you can unmold it and, if necessary, cut it into bars. Leave the soap alone for another two weeks minimum, to cure (that is, for all the water to evaporate so that the soap doesn’t turn back to mush when you try to use it). Many soapmakers recommend curing for 4-6 weeks before using your soap on your body.

Before adding the lye my oil smelled like oil. Nothing exciting, nothing rancid, no food odors. After saponification there was the faintest whiff of hashbrowns, so I added just a few drops of essential oil to take the edge off. After curing I couldn’t smell either hashbrowns or essential oil.

This solves a rag and bone dilemma for us: what to do with all that used cooking oil? It’s nowhere near enough to bother with making biodiesel but I can’t fathom tossing it. Now we can reuse it!

The pigs were slaughtered on Saturday so we should be getting the fresh (non-smoked) cuts back in a few days. Like last year, I asked to keep the lard. We didn’t much of a kick out of cooking with lard, but I have heard it makes some spanky soap!

— Amanda


3 thoughts on “Making soap from used cooking oil

  1. Pingback: Soap! | SterlingFink

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