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

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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.

Making soap using drip lye can be challenging because the purity, density, and consistency of homemade lye is uneven. For home soap-makers, I recommend preparing hot-process soap (which I describe below) rather than cold-process because an exact amount or specific purity of lye isn’t required for successful saponification. In hot-process soaps, saponification — the chemical reaction between lye and fat — is controlled by added heat, not by the pH.

During my research, I uncovered a historic trick for checking the density of drip-ash lye using a fresh egg. Because an egg has about the same density as lye that’s the correct strength, the egg will float. Many colonial recipes for drip-ash lye recommend using the homemade lye if it can float an egg with 1⁄4 of its shell showing above the liquid. This lye will produce “Black Soap,” a strong laundry soap that historical re-enactors complain is too harsh. On the other hand, I uncovered a 16th century shampoo recipe that recommends using lye dense enough to suspend an egg in the middle of the liquid. Suspended-egg lye makes near-neutral soap, perfect for personal use because it doesn’t “bite.” This same historic recipe also confirms the 3-to-1 ratio of lye to fat that consistently works for me: “thre pottels of lye to one pot of oyl.” It’s nice to find confirmation that’s five centuries old!

Based on experience and historical research, I’ve developed these instructions for making a soft, creamy, hot-process soap from scratch — including homemade lye.

Soft Soap in 8 Steps

1. To make a leaching barrel, drill a small hole at the bottom of a 5-gallon bucket and stuff it with a piece of dishcloth as a filter. Fill the bucket with sieved ashes, tamping down intermittently. Level the top, leaving about 2 inches of headroom. Slowly pour in about an inch of rainwater. When the water has absorbed, add about an inch more. Continue to slowly add water until liquid starts to drip out the hole in the bottom of the bucket — it should take about a day. Prop the ash bucket on top of a second bucket to collect this drip lye. You’re ready to test the strength when you’ve collected about 1 gallon.

2. If you’ve used regular ashes from a woodstove or fireplace in Step 1, the drip lye will be dark brown and probably won’t suspend an egg. Slowly heat the drip lye and allow it to evaporate until it has the desired strength, which is likely about a quarter of its original volume. Use stainless steel vessels for heating lye, never aluminum (which creates noxious gases in combination with lye) or enamel (which lye will etch). Cool down the lye before soap making. Contaminants will settle to the bottom. The next day, pour the purer lye solution off the top (that is, decant it).

If you’ve used white ashes from a high-efficiency stove in Step 1, the lye may be light yellow and will float an egg from the get-go. Use this lye as is to make a strong, sharp laundry soap. To make neutral hand soap, slowly add water in small amounts until the egg is suspended in the solution.

3. Measure out (by liquid volume) 3 parts lye to 1 part oil. Fats that are solid at room temperature, such as tallow and lard, should be heated until they liquefy, and then cooled. If you’re a soap-making beginner, use olive oil because the process will be easier. Add the lye to the oil and mix very well — a stick blender works great — and let sit overnight.

4. After 12 hours rest, the solution will have separated. Mix it very well. Heat the solution for an hour or two in a slow cooker on the high setting with the lid in place. Stir only occasionally, as the soap should not be allowed to cool down. When the soap starts to rise, you’ll see foam forming under the lid. Remove the lid, and stir the soap well to settle the foam. Replace the lid, but prop a toothpick under the lid to create a little gap for hot air to escape.

5. As the soap cooks in the slow cooker (still on the high setting), you’ll see bubbles form at the edge of the ceramic insert. This is the soap “turning itself.” It should only bubble at the edges — never boil in the center. You’ll observe finished soap starting to form on top. Stir occasionally to ensure that no areas dry out at the edges of the insert. Be sure to stir gently, because the mixture can foam up suddenly at this stage.

6. The soap will get thicker and thicker until it incorporates, or finishes. Remove the slow cooker’s lid so excess moisture can evaporate. The soap will look like custard (soap-makers call this “trace”), leave droplet marks (“trace marks”) on the surface when scooped, and drip off the spoon in globs. You’re nearly finished!

7. Cook until it starts to have a glazed, sleek look, like petroleum jelly, and leaves little wavy points when stirred. If you part the soap at the bottom of the cooker, it should not come back together. Stop cooking at this point for a thin, soft soap. Or, keep evaporating the moisture until the soap is your desired density — it will never get hard. I prefer a whipped cream consistency.

8. Your finished product will vary by  color and consistency. I’ve made many successful batches of soap using “old-fashioned” techniques and each one takes about six to seven hours. If you decide to try this process over open heat, be aware that a more erratic heat source will make the soap behave erratically as well, and your mixture may not come to trace.

Making soap from homemade drip lye is a fun and rewarding project, and one that doesn’t require specialized or expensive equipment. I hope you’ll give it a try!