How to Make Hot-Process Soft Soap

This step-by-step article explains the process, tools, and benefits of making your own natural, homemade soap using wood ash lye and a slow cooker.

  • Add the lye to the oil, mix well with a hand mixer or stick blender, and let sit overnight.
    Photo by Susan Verberg
  • After letting it rest for 12 hours, heat the solution for an hour or two in a slow cooker on the high setting with the lid in place.
    Photo by Susan Verberg
  • The soap will get thicker until it incorporates and looks like custard.
    Photo by Susan Verberg
  • As the soap cooks in the slow cooker, you’ll see bubbles form at the edge and finished soap starting to form on top.
    Photo by Susan Verberg
  • Your finished soap will vary by color and consistency, but in the end you'll have a unique and useful product.
    Photo by Susan Verberg
  • If you’ve used regular ashes from a woodstove or fireplace, the drip lye will be dark brown and probably won’t suspend an egg.
    Photo by Susan Verberg
  • If you’ve used white ashes from a high-efficiency stove, the lye may be light yellow and will float an egg from the get-go.
    Photo by Susan Verberg
  • Cook until the soap starts to have a glazed, sleek look like petroleum jelly, and leaves little wavy points when stirred.
    Photo by Susan Verberg
  • You can easily make your own leaching barrel for turning wood ash into lye.
    Photo by MOTHER EARTH NEWS Staff/Amanda Nagengast
  • Making soap using “old-fashioned” techniques is a fun and rewarding project that doesn’t require specialized or expensive equipment.
    Photo by Matthew T. Stallbaumer

Soap-makers love to tell the story of how ancient Romans first “discovered” soap by burning animal sacrifices on Mount Sapo, and how the creeks at the bottom of that mythological mountain were the best places to do laundry. They’ll tell you that the water, ash, and animal fat on those sites accidentally created the soap that filled the creeks. The reality is that the Romans didn’t actually make soap. They traded for it with the Celts, who dominated the market because of their access to abundant limestone and seashells, from which they produced slacked lime to make a caustic soda lye (sodium hydroxide).

After years of professionally making all-natural goat’s milk soaps to sell at our local farmers market, I decided to develop a self-sufficient soap-making process based on ancient techniques. My goals were to make my own lye and to turn kitchen-waste fats into soap. I finally took the plunge — and what an interesting adventure it’s been! I dug through old articles and manuscripts, learned to decipher medieval English, and filled my kitchen with weird, bubbling concoctions. And I wondered how something that seemed so simple could be so challenging.

Don’t let me discourage you. If you’re an outdoor enthusiast, you may have made soap already. Scrubbing a greasy frying pan with campfire ashes doesn’t just scour the dirt away: When rinsed with a little water, the hydroxide salts in the ashes combine with the cooking grease to form a primitive cleanser.

Understanding Soap-Making Basics

To undertake the process of making soap, known as “saponification” (from sapo, the Latin word for soap), let’s first review what soap is and why it works the way it does. Because soap is made from water-soluble bases known as alkalis, it neutralizes acids while retaining its ability to be dissolved in water. More specifically, soap is a surfactant with the unusual ability to diffuse fats and oils into water, which is why it can rinse away oily stains.

Soap is made by mixing dissolved hydroxide salts, generally called “lye,” with fatty acids. To make your own lye that you can use to produce a soft soap, you leach (or drip) water through ashes to dissolve the hydroxide salts. Ashes are highly concentrated minerals of hydroxides, nitrates, carbonates, sulfites, and more. The quality of the lye produced depends on how well the plant material was burned. I’ve found that the more complete the burn (all organic material combusted), the more hydroxides will be dissolved, and the more basic (that is, higher pH) the resulting lye will be. In the case of incomplete burns, such as you’d find in fire pits and fireplaces, you can add lime to the ash to help change carbonates (charcoal) into hydroxides.

My drip lye made from ashes has a pH of about 11, while commercial lye has a pH of 14 — making it 1,000 times more basic than ash lye. This is a big reason why making drip-lye soap is so different from conventional soap making. Because of the lower pH, drip lye is a lot less dangerous to handle than modern commercial lye. As a precaution, though, you should always keep some vinegar handy during soap making because its acid will help neutralize the lye’s base.

9/5/2018 10:59:54 AM

Very well written article. Explanations are simple and easy to understand. Thanks for word definitions. It has been many years since I have made soap. I messed up the last batch, and in trying to save it, I took a hand mixer to it and ended up with a soap that floated, like Ivory soap. Everyone enjoyed it. It kept my skin looking beautiful. I am certainly looking forward to trying your plan. I would also like to know when to add dried herbs and essential oils. I'm starting anew. Thank you...

NJ Knopp
8/22/2018 12:31:23 PM

Thank you for the interesting - and thorough! - article. I can hardly wait to try it :) I do have a few questions before I do: - can coconut oil be used in its liquid state? - do different oil sources behave or turn out differently? (lard vs tallow vs olive oil, etc.) - does this soap perform well in very hard water? - at what point can you add essential oil(s) for scent? I'm definitely going to give this a try, and thanks again! NJ Knopp Johnson City TX

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