How to make biodiesel lye

  1. Start a rain barrel to catch soft water. This is a key step. Depending upon how much lye you want to leach, make sure that you have 2 or 3 gallons of soft water before you proceed.
  2. Find a local brewer’s supply house and pick up a wooden barrel and a cork about 3″ long. You can use a cask-sized or waist-high barrel.
  3. Take the barrel home and drill a hole in it approx. 2″ above the bottom. Make sure that the cork will fit snugly into the hole.
  4. Find a place that the barrel will be undisturbed. Lye is caustic. Take the necessary precautions. Put some bricks down and place the barrel on top of them. The brick base must be stable. It raises the barrel up so that you can easily drain off the lye into a container when it is ready. Give yourself room to work.
  5. Cover the bottom of the barrel with some palm-sized clean rocks (e.g. river rock). Cover the rocks with approximately 6″ of straw (this can be hay or grass). This will filter the ashes and help your lye drain cleanly.
  6. Gather branches and/or logs of oak, ash, or fruitwoods. Remember that the best lye is made from hardwoods. Avoid pine, fir, and other evergreens.
  7. Burn it outside in a pile, or better yet, use it in your fireplace or woodstove.
  8. Scoop the ashes out and put them in the prepped barrel. (Make sure that the ash is completely cold, or you’ll set your barrel and anything around it on fire.) You can fill the barrel with ash but it is not necessary you can make smaller amounts with less ash.
  9. Put a pan under the hole and remove the cork. Pour the soft water in until you see it start to drain into the pan, then put the cork back in tight. The water level should be about 6″ from the top. After a day, the first ash should settle and you can add more ash.
  10. Let it sit for at least 3 days. You can add ash all week and drain it regularly on a specific day of the week.
  11. Check to see if your lye is ready. For what purpose are you leaching this lye? Body soap or heavy cleaning? Lye concentration gets stronger with each leaching. For average soap making, you can use these measures: Drop a fist-sized potato or a raw egg into the barrel. If it floats enough for a quarter-size piece to rise above the water, it is ready. If it doesn’t, you need to add more ashes or drain all the water and re-leach it (pour it back into the cask and let it set one more cycle).
  12. Make sure that you have a wooden crock or glass container to catch your lye when it’s ready. Put it under the tap, gently pull the cork, and fill your containers. Leave enough head room that they will be safe and easy to pour. Make sure that you have tight fitting lids.
  13. Store your lye in a cool dark place until use. (The sooner the better.)


  • Do not start this project until you have collected 2-3 gallons of rain water and have purchased or scavenged all of your supplies.
  • Make sure that your lye barrel has a stable foundation and is in a secure place where it cannot be knocked over by, for example, roving children.
  • If you run a dehumidifier its collected water is an alternative to rainwater.
  • To dispose of old leached ashes, dig a hole away from everything and pour the muck into it. Don’t cover it until the ashes dry thoroughly.


  • Lye is a base, also known as an alkali. Both acids and bases are caustic; they “burn” anything that they touch. Please use common sense and follow the tips provided.
  • Wear rubber gloves and eye protection when draining off or handling your lye, it can burn your skin and blind you!
  • For all backyard chemists, chemical-resistant gloves (the yellow kitchen ones will do), safety glasses and arm and body covering are mandatory.
  • Educate yourself on poison treatment before you begin making soap or biodiesel. visit for appropriate actions to take if lyewater or lye crystals spill on you, are accidentally swallowed, or get in your eye.
  • In the instance that lye should come in contact with your skin, brush off any solids and do not run it under water. Neutralize the burn with vinegar. Water will only make it hurt worse. The strong base can cause severe burns, and you may not feel the effects right away due to nerve damage. So always (see next dot)
  • In any emergency, call 911 or your local poison control center’s emergency number.

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