How to Make Soap

If you haven’t noticed in my writing, I’m very much against being wasteful.  At home, I hate making food from a recipe where you only use a part of something and throw the rest out.  When I explained how to make biodiesel I briefly noted that you could use the leftover glycerin to make all sorts of things.  One of those things is soap.  Don’t get me wrong, soap is absolutely a luxury item.  But ideally, for every liter of biodiesel you make, you’ll end up with 200mL of glycerin.  That adds up, so why not make some soap.  You’ll be clean and you can use it to barter with other people.

Bar Soap or Liquid Soap?

Interestingly enough, the procedure for making bar or liquid soap is essentially identical.  The only difference is what type of lye you use.  Something about sodium hydroxide causes the glycerin to crystallize during the saponification process that doesn’t happen with potassium hydroxide (chemists or chemical engineers, feel free to let me know the specifics).  So if you want bar soap use NaOH, otherwise use KOH.

It should also be noted that if you used a different type of lye in the biodiesel process than you are using to make soap, your soap may not turn out exactly how you planned.  If you are making bar soap, you can add more NaOH to help it solidify, but if you do, you need to let it sit for longer afterwards to ensure all the lye has reacted.


  1. Glycerin, and lots of it.  This is the ingredient that all others are keyed off of, so it doesn’t matter how much you use.
  2. Water. You need 200mL of water for every liter of glycerin.
  3. Lye.  If you are using sodium hydroxide (NaOH) you will need 50g per liter of glycerin.  If you are using potassium hydroxide (KOH) you will need 75g per liter of glycerin.
  4. Smelly Stuff.  You don’t really need this, but if you want your soap to smell pretty you’ll need something to make it do so.

The Procedure

  1. First things first, you need to clean your glycerin.  This ensures that you have no particulate matter (dirt, old food, twigs, leaves, zombies, etc.) in your glycerin.  You typically don’t want these things in your final product, so get them out now before your process becomes more complicated.
  2. Remove any alcohol remnants.  Any alcohol left in your soap can be bad for your health, especially methyl alcohol.  You need to bring your glycerin to a temperature of 65C for methyl alcohol or 80C for ethyl alcohol.  FOR THE LOVE OF PETE, be careful when you do this.  What you are doing is boiling of the alcohol and methyl alcohol will sink.  So make sure that you do this in a well ventilated area do everything you can to avoid exposing your flame to the vaporized alcohol.  If your flame turns an odd color or starts to crackle, then disperse your flame and rethink your setup.
  3. Once you are sufficiently sure that there is no alcohol left in your glycerin, begin heating it to 60C.  While you wait you can combine your water and lye.
  4. Add your water and lye to the glycerin.  Stir continuously and heat to boil.
  5. Once at a boil, reduce heat and let simmer.
  6. If you’re making smelly soap, add your smelly stuff at this point.
  7. Once a skin starts to form on the surface of your concoction, check it by ladling out a spoonful and pour back in, if a film is left behind, it is done.
  8. If you are making bar soap, pour into molds.  If you are making liquid soap, pour into the containers you intend to store it in.
  9. Let the soap sit for around 3 weeks.  During this time, the glycerin and lye are reacting and you need to let this reaction complete.  If you try to use the soap and it burns or tingles, it isn’t ready yet and you just gave yourself a lye burn.

As you can see, making soap is really easy and a good use for the glycerin byproduct of biodiesel.

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