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Cold-Process Soap from Animal Fat and Wood Ash Lye

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Overview

Cold-Process Soap

Soap-making-kettle.jpg

Subtitle: Soap from animal fat and wood ash
Domain: Practical Skills
Difficulty: Moderate (requires attention to lye strength)
Time Required: 24–48 hours active; 4–6 weeks cure
Materials: Animal fat, wood ash, water
Tools: Pot, wooden spoon, cheesecloth
Yield/Output: ~2.5 lbs per 1 lb fat
Skill Level: Intermediate
Safety Level: See Safety section
Variants: Multiple
Related Articles:

Part of The Practical Alchemist: A Technical Guide for the Resourceful Engineer

Cold-process soap is one of the oldest and most reliable methods for making soap from basic materials: animal fat (tallow or lard) and plant-derived lye (potassium hydroxide) extracted from wood ash. This method requires no heating beyond an optional warm water bath, making it accessible to communities with limited fuel or equipment.

The process relies on saponification—a chemical reaction in which fat molecules are broken down by lye, producing soap and glycerin. Unlike industrial soap-making, cold-process soap retains natural glycerin, making it gentler on skin.

This article assumes you have access to:

  • Animal fat (beef tallow or pork lard preferred)
  • Hardwood ash (from oak, hickory, or similar—not softwoods like pine)
  • Water
  • Basic containers and stirring equipment

Time required: 24–48 hours for initial setting; full cure 4–6 weeks.

Yield: ~2.5 lbs of soap per 1 lb of fat (approximate; varies with water content and lye strength).

Materials

Essential

  • Animal fat: 1 lb (450 g) beef tallow or pork lard, rendered (see Rendering Animal Fat for details)
  • Wood ash: 2–3 lbs (900–1,350 g) from a wood fire, hardwood preferred
  • Water: ~1 gallon (3.8 L) clean, preferably rainwater or well water
  • Salt (optional): 1–2 tablespoons (15–30 g) for "salting out" (see Troubleshooting)

Additives (Optional)

  • Essential oils or plant extracts: fragrance (added at trace, before pouring)
  • Natural colorants: charcoal, clay, plant powders
  • Herbs: dried lavender, calendula, or other skin-safe botanicals

Tools

Critical

  • Large pot or bucket: non-aluminum, 2–3 gallon capacity (cast iron, stainless steel, ceramic, or food-grade plastic)
  • Wooden spoon: for stirring (never use aluminum or reactive metals)
  • Thermometer (optional but helpful): to monitor temperature
  • Cheesecloth or fine cloth: for straining lye solution

Helpful

  • Glass jars: for testing lye strength (see below)
  • Mold for soap: wooden box, cardboard box lined with parchment, or silicone mold (~2–3 qt capacity)
  • Rubber gloves: lye is caustic; protect skin
  • Safety goggles: to protect eyes from splashes

Alternatives

  • A wooden or ceramic crock can replace a pot
  • Linen cloth or burlap can replace cheesecloth
  • A bare board or tray can serve as a mold if lined with cloth

Procedure

Part 1: Extracting Lye from Wood Ash

Step 1: Prepare the ash.

  • Collect hardwood ash from your fire (oak, hickory, maple, ash wood preferred; avoid softwoods like pine, which produce weak lye)
  • Sift the ash through cheesecloth or a fine cloth to remove charcoal, twigs, and debris
  • You need approximately 2–3 lbs of clean ash for 1 lb of fat

Step 2: Make a lye solution (leaching).

  • Place the clean ash in your large pot or bucket
  • Pour ~1 gallon of water slowly over the ash, stirring gently
  • Cover loosely (allow air circulation but prevent dust from entering)
  • Let sit for 24–48 hours at room temperature, stirring occasionally
  • The water will begin to look tan or brown as potassium hydroxide (lye) dissolves

Step 3: Strain the lye solution.

  • Carefully pour the liquid through cheesecloth into a clean container, leaving the settled ash behind
  • The resulting liquid is your lye solution (a dilute potassium hydroxide mixture)
  • Keep this away from skin and eyes—it is caustic.

Part 2: Testing Lye Strength (Critical)

This step is essential to prevent failed batches or dangerous caustic soap.

Step 4: Test with an egg or potato.

  • Place a fresh egg (in shell) or a small potato into the lye solution
  • If the egg floats with about a quarter of it above the liquid surface, your lye is the correct strength
  • If it sinks, the lye is too weak; return it to a pot and simmer gently (outdoors, well-ventilated) for several hours to concentrate it, then test again
  • If it floats too high, dilute with a small amount of water and retest

Alternatively, you can use a simple float test:

  • Whittle a small wooden peg or dowel
  • Mark it at its balance point
  • Place it in the lye solution; if it floats at your mark, the lye is correct strength

Step 5: Document your lye strength.

  • Note the volume of lye solution you have (e.g., "approximately 1 gallon")
  • This will help you scale batches in the future

Part 3: Cold-Process Saponification

Step 6: Prepare your fat.

  • Render your animal fat if you haven't already (see Rendering Animal Fat)
  • Weigh or estimate: you need 1 lb of clean, rendered fat per batch
  • Allow the fat to cool to room temperature (it should be solid or semi-solid, but still pliable)

Step 7: Mix fat and lye (the critical step).

  • Pour the lye solution into a large pot or bucket
  • Slowly add small chunks or dollops of rendered fat to the lye, stirring constantly with a wooden spoon
  • Stir gently but thoroughly, working for 15–30 minutes
  • The mixture will initially look separated (oil floating on lye), but as you stir, it will begin to emulsify and thicken
  • Continue stirring until the mixture reaches trace—a consistency where, when you lift the spoon, the dripping mixture leaves a faint trail (like thin pudding)

Note: This is called "trace" because the soap has begun to solidify and will hold the trace of the spoon.

Step 8: Pour into mold.

  • Once trace is reached, pour the soap mixture into your prepared mold (lined with parchment or cloth)
  • Work quickly; the mixture will continue to thicken
  • Smooth the surface gently with a damp spoon if desired

Step 9: Insulate and cure (24–48 hours).

  • Wrap the mold in blankets, towels, or burlap to insulate it
  • This keeps the soap warm and allows the saponification reaction to continue (sometimes called the "gel phase")
  • Leave undisturbed for 24–48 hours
  • Do not move or jostle the mold during this time

Step 10: Unmold and cut.

  • After 24–48 hours, carefully unmold the soap block
  • It should be firm but not rock-hard
  • If it's still very soft, wait another 12–24 hours before cutting
  • Cut into bars using a sharp knife or wire cutter (approximately 1–2 inches per bar)
  • Arrange bars on a shelf or rack in a dry, well-ventilated space

Step 11: Cure (4–6 weeks).

  • Allow the bars to cure for 4–6 weeks in a cool, dry location away from direct sun
  • During this time, residual moisture evaporates and the soap hardens
  • After 4 weeks, test: the soap should be hard, white or tan, and should not feel greasy when you rub your hands together
  • Once cured, soap is ready to use and will keep for months or years if stored dry

Troubleshooting

Problem: Soap didn't reach trace (mixture stayed oily)

  • Cause: Lye was too weak, or you didn't stir long enough, or fat was too cold
  • Fix: Re-heat the mixture gently (warm water bath, not direct flame), continue stirring for another 30 minutes
  • Prevention: Test lye strength carefully (Step 4); ensure fat is at room temperature; stir vigorously for at least 20 minutes

Problem: Soap is grainy or chalky (after curing)

  • Cause: Lye was too strong, creating excess lye in the finished soap (called "false trace" or "lye pockets")
  • Fix: This batch may be harsh; test on a small patch of skin first before general use; save for laundry soap
  • Prevention: Always test lye strength with the egg or wooden float test; dilute if needed

Problem: Soap is slimy or oily (doesn't firm up)

  • Cause: Lye was too weak, or insufficient lye was added for the amount of fat
  • Fix: After unmolding, you can re-batch: grate the soap, add more lye solution, and re-stir to trace
  • Prevention: Test lye strength and measure fat carefully

Problem: Soap turned rancid or smells bad (during cure)

  • Cause: Fat was rancid before you started, or soap was stored in a humid environment
  • Fix: Discard batch; start with fresh, good-quality fat
  • Prevention: Render fat fresh from recent meat; store rendered fat in a cool, dry place; cure finished soap in dry conditions, not humid basements

Problem: Soap is too hard (like a rock)

  • Cause: Lye was too strong, creating a very hard bar (not harmful, just difficult to use)
  • Fix: Use as laundry soap, or grate and dissolve in warm water for liquid soap
  • Prevention: Test lye strength; aim for the egg-float or wooden-peg equilibrium

Problem: Didn't reach trace after 1 hour of stirring

  • Cause: Likely lye is too weak
  • Fix: Continue stirring slowly for another 30 minutes (trace can take a long time with weak lye); if it never thickens, add a small amount of fresh lye solution (tested first) and stir more
  • Prevention: Start with strong lye; don't dilute unnecessarily

Safety

  • Lye is caustic: It will burn skin and eyes. Wear gloves and eye protection when handling the lye solution.
  • Proper ventilation: Work outdoors or in a well-ventilated space. Do not inhale fumes from hot lye.
  • Never add water to lye: Add lye to water, or lye to fat, never the reverse (adding water to lye causes violent exothermic reaction).
  • Washing: If lye contacts skin, rinse immediately with large amounts of water for at least 15 minutes. If in eyes, seek medical attention.
  • Storage: Keep lye solution in labeled containers, away from children and pets.
  • Finished soap: Once cured, soap is safe to use. However, test first on a small patch of skin (behind ear or inside wrist) to ensure you are not sensitive.
  • Laundry soap vs. body soap: If your lye was too strong, the soap may be harsh for skin but perfectly safe for laundry or general cleaning.

Variations

Scented Soap

  • Add essential oils (5–10 drops per pound of fat) at trace, just before pouring
  • Stir in thoroughly but quickly
  • Good choices: lavender, cedarwood, peppermint, lemon
  • Avoid citrus oils in large amounts (can inhibit cure)

Colored Soap

  • Add natural colorants at trace:
 - Charcoal powder (gray/black)
 - Clay (tan, red, or green depending on type)
 - Madder root powder (reddish)
 - Indigo (blue; requires careful stirring to avoid streaking)
  • Use sparingly (1–2 tablespoons per batch); excessive colorant can interfere with cure

Herbal-Infused Soap

  • Steep dried herbs (lavender, calendula, rose petals) in the lye solution before use
  • Strain well before adding fat
  • The infused solution will carry herbal properties into the finished soap

Softer Soap (Castile-style)

  • Use mostly or entirely plant oils (if available) instead of animal fat
  • Olive oil, coconut oil, or rendered nut oils will produce a softer, more luxurious bar
  • The ratio of lye to oil changes; start with the same egg-float test to establish lye strength

Liquid Soap

  • If you prefer liquid soap, you can grate finished cold-process soap and dissolve it in warm water
  • Add 1 part grated soap to 2–3 parts water, stir until dissolved, and let cool
  • Use as hand soap or laundry liquid

Large Batch (Scaling Up)

  • The process scales linearly: if you use 5 lbs of fat, extract and test lye for 5 lbs, follow the same steps
  • Larger batches may take longer to reach trace (up to 1–2 hours of stirring)
  • Keep notes on your yields and cure times for future reference

See Also

References

  • Caveman Chemistry (Kevin M. Dunn) – Chapter on soap-making https://www.cavemanchemistry.com/
  • Henley's Twentieth Century Recipes, Formulas, and Processes – Sections on soap and cleanliness
  • "Saponification," Encyclopaedia Britannica Online
  • USDA Farmers' Bulletin 1383: "Soap and Candle Making" (1928)

External Links

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