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How to Make Lye

Lye, also known as NaOH, sodium hydroxide, or caustic soda, is used in making soap and in biodiesel fuel production. Caustic potash, or potassium hydroxide, is also called lye. Like sodium hydroxide, it can also be used in the production of biodiesel, although the recipe will have to be adjusted somewhat; unlike, NaOH, however, it can more easily be made at home. This recipe is for KOH, potassium hydroxide. However, KOH does not generally produce hard or bar soap.


  1. Image titled Make Lye Step 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 (7.6 or 11.4 L) of soft water before you proceed.
    • Water from a dehumidifier works as well.
    • You can also use electrically distilled water. The purer the water, the more potassium that can be leached from the ashes. Do not use bottled spring water or water from the tap! (You can use bottled distilled water that was processed using steam distillation.)
  2. Image titled Make Lye Step 2
    Get a wooden barrel and a cork about 3in (7.6cm) long. A cask-sized or waist-high barrel will work. You can find these at a local brewer's supply house.
  3. Image titled Make Lye Step 3
    Drill a hole in the barrel approximately 2in (5cm) above the bottom. Make sure that the cork will fit snugly into the hole.
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    Put the barrel on a brick base someplace where it 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. Image titled Make Lye Step 5
    Cover the bottom of the barrel with some palm-sized clean rocks (e.g. river rock). Cover the rocks with approximately 6in (15cm) of straw (this can be hay or grass). This will filter the ashes and help your lye drain cleanly.
  6. Image titled Make Lye Step 6
    Gather branches and/or logs of oak, ash, or bushes that grow fruit. Remember that the best lye is made from hardwoods, so avoid pine, fir, and other evergreens. Palm leaves work well if they are completely dried and brown.
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    Burn the branches to ash. You can do this outside in a pile or, better yet, in a freshly-emptied fireplace or wood stove where the ashes won’t become mixed with anything else.
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    Scoop the ashes out and put them in the prepped barrel. (Make sure that the ash is completely cold, or you could 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. Image titled Make Lye Step 9
    Soak the ashes. 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 tightly. After a day, the first ash should settle and you can add more ash.
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    Let the ash soak for at least three days. If you want to use more ash, you can add it all week and drain it regularly (ex. on a specific day of the week).
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    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, measure the concentration by dropping a fist-sized potato or a raw egg into the barrel (making sure to throw either of these away afterwards). If it floats enough for a quarter-sized area 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 for one more cycle).
  12. Image titled Make Lye Step 12
    When it's ready, catch your lye with a wooden crock or glass container. Put it under the tap, gently pull the cork, and fill your containers. Leave enough head room so that they will be safe and easy to pour. Make sure that you have tight, fitting lids.
  13. Image titled Make Lye Step 13
    Store your lye in a cool dark place until use. The sooner you use it, the better.


  • 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.
  • 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.
  • Do not start this project until you have collected 2–3 gallons (7.6–11.4 L) of rainwater and have purchased or scavenged all of your supplies.
  • Remember: lye is a base (alkaline), opposite of an acid.


  • Educate yourself on poison treatment before you begin making soap or biodiesel. Visit for appropriate actions to take if lye water or lye crystals spill on you, are accidentally swallowed, or get in your eye.
  • Keep lye away from kids, flammable materials, and metal containers; lye can eat through some metals.
  • For all backyard chemists, chemical-resistant gloves (the yellow kitchen ones will do), safety glasses, and arm and body covering are mandatory.
  • In any emergency, Call the Emergency Services or your local poison control center's emergency number.
  • Lye is a base, also known as an alkali. Bases are caustic; they "burn" anything that they touch. Please use common sense and follow the tips provided. Failure to follow correct procedures may lead to injury or even death.
  • Run burns under water. Do not try to treat a burn with vinegar. The strong base can cause severe burns, and you may not feel the effects right away due to nerve damage.

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