What Is Cold Process Soap Making?

Cold process (CP) soap making is one of the most popular methods for crafting handmade soap from scratch. Unlike melt-and-pour, cold process gives you complete control over every ingredient — from the oils you choose to the fragrances and colorants you add. The result is a truly custom bar of soap with a natural, moisturizing lather.

The "cold" in cold process refers to the fact that you don't apply external heat during the mixing stage. Instead, the heat is generated naturally through the chemical reaction between lye (sodium hydroxide) and water — a process called saponification.

What You'll Need to Get Started

Essential Equipment

  • Digital kitchen scale — soap making is measured by weight, not volume
  • Stainless steel or high-density polyethylene (HDPE) containers — for mixing lye solution
  • Stick blender (immersion blender) — speeds up trace significantly
  • Soap mold — silicone loaf molds are ideal for beginners
  • Thermometer — to check oil and lye solution temperatures
  • Safety goggles and gloves — non-negotiable when working with lye
  • Dedicated mixing bowls and spoons — never use these for food after soap making

Core Ingredients

  • Lye (sodium hydroxide / NaOH) — the essential catalyst for saponification
  • Distilled water — for dissolving lye safely
  • Base oils — such as olive oil, coconut oil, and palm oil or sustainable alternatives
  • Optional additives — essential oils, clays, botanicals, natural colorants

Understanding Lye Safety

Lye is a caustic substance that requires careful handling. When mixed with water, it generates significant heat and releases fumes. Always follow these safety rules:

  1. Work in a well-ventilated area or outdoors.
  2. Always add lye to water — never pour water into lye.
  3. Wear safety goggles and chemical-resistant gloves at all times.
  4. Keep children and pets out of the workspace.
  5. Have a bottle of white vinegar nearby to neutralize any spills.
  6. Store lye in a cool, dry, airtight container away from moisture.

Once fully cured, there is no lye remaining in finished cold process soap — it has been completely converted into soap and glycerin through saponification.

The Basic Cold Process Method

  1. Calculate your recipe — Use a soap calculator (like SoapCalc or Brambleberry's lye calculator) to determine the exact lye and water amounts for your chosen oils. Never skip this step.
  2. Prepare your workspace — Cover surfaces, put on your safety gear, and set out all equipment.
  3. Weigh and melt your oils — Combine your base oils and gently heat until all solid fats are melted. Allow to cool to around 43–49°C (110–120°F).
  4. Mix your lye solution — Carefully add weighed lye to weighed distilled water. Stir until dissolved. Allow to cool to a similar temperature as your oils.
  5. Combine oils and lye solution — Slowly pour the lye solution into your oils while stick blending. Alternate between blending and stirring.
  6. Reach trace — Blend until the mixture reaches "trace" — a pudding-like consistency where a drizzle on the surface leaves a brief impression.
  7. Add fragrance and extras — Stir in essential oils, colorants, or botanicals at light trace.
  8. Pour into mold — Pour the batter into your prepared mold and tap to release air bubbles.
  9. Insulate and wait — Cover with a towel or cardboard lid and leave undisturbed for 24–48 hours.
  10. Unmold and cut — Unmold when firm and slice into bars.

Curing: Why It Matters

After cutting, cold process soap requires a curing period of 4–6 weeks. During this time, excess water evaporates, making the bar harder and longer-lasting. The bars also become milder as saponification fully completes. Place bars on a rack with good airflow, out of direct sunlight, and turn them occasionally.

Common Beginner Mistakes to Avoid

  • Skipping a lye calculator and eyeballing measurements
  • Using aluminum equipment (lye reacts dangerously with aluminum)
  • Adding fragrance oils at too-high a temperature, which can cause ricing or seizing
  • Not letting soap cure long enough before use
  • Forgetting to account for water discount in your recipe

Ready to Start?

Cold process soap making has a learning curve, but the rewards are well worth it. Start with a simple three-oil recipe — olive, coconut, and castor oil — and master the basics before experimenting with more complex formulations. Each batch teaches you something new, and soon you'll be crafting beautiful, luxurious bars tailored exactly to your skin's needs.