Beginner's Guide to Cold Process Soap Making

What Is Cold Process Soap?

Cold process (CP) soap is made by combining oils or fats with lye (sodium hydroxide) at room temperature or slightly warmed — without applying external heat throughout the process. The heat comes from the saponification reaction itself, which converts the oils and lye into soap and glycerin.

This is different from hot process soap, where you apply external heat (usually a slow cooker) to speed up saponification, and melt and pour, which uses a pre-made soap base you melt and customize. Cold process gives you the most control over design and ingredients, but requires a cure time of 4–6 weeks.

Equipment You'll Need

• Digital scale — soap making requires precise measurements by weight, not volume
• Two heat-safe pitchers or containers — one for lye solution, one for oils
• Stick blender (immersion blender) — dramatically speeds up trace
• Thermometer — infrared or probe; you want oils and lye solution at similar temperatures (~90–110°F / 32–43°C)
• Soap mold — silicone loaf molds are beginner-friendly; wooden molds work great too
• Rubber or silicone spatulas
• Safety gear — rubber gloves, safety glasses, long sleeves. Non-negotiable. See our Lye Safety Guide.

A Simple Beginner Recipe

This basic recipe makes a hard, cleansing bar with good lather — a great starting point:

• 70% Olive Oil — conditioning, long-lasting bar
• 20% Coconut Oil — hard bar, bubbly lather
• 10% Castor Oil — boosts lather and creaminess

Use our Lye Calculator to calculate the exact NaOH and water amounts for your batch size. A 500g oil batch (350g olive, 100g coconut, 50g castor) is a good starting size — not too much to waste if something goes wrong, but enough to make a few bars.

Set superfat to 5% for a balanced bar. Water at 38% of oil weight is a safe default for beginners.

The Basic Process

1. Weigh your oils and melt any solid oils (coconut, palm, shea) gently. Set aside to cool.
2. Weigh your water in a heat-safe pitcher. In a separate container, carefully weigh your lye.
3. Add lye to water (never water to lye — it can erupt). Stir until dissolved. The solution will heat to ~200°F / 93°C and release fumes — do this near ventilation or outdoors. Let cool.
4. When both oils and lye solution are between 90–110°F, slowly pour the lye solution into the oils while stick blending.
5. Blend to trace — the mixture thickens to a pudding-like consistency. Light trace looks like thin custard; heavy trace is thicker. For beginners, light to medium trace is ideal.
6. Add fragrance, colorants, or additives at trace, stir in, then pour into your mold.
7. Insulate the mold with a towel or blanket for 24–48 hours to help it go through gel phase.
8. Unmold and cut after 24–48 hours. The soap will be firm but still caustic — wear gloves.
9. Cure for 4–6 weeks in a well-ventilated spot, turning bars occasionally. Curing finishes saponification and evaporates excess water, making a harder, milder bar.

Common Beginner Mistakes

• Adding water to lye instead of lye to water — this can cause a dangerous volcanic reaction
• Using volumetric measurements — soap making must be by weight for accurate lye calculations
• Skipping safety gear — even "finished" soap batter is caustic before cure
• Cutting too early — soft soap crumbles; wait until it's firm
• Skipping the cure — fresh soap is harsh; the full cure matters

Ready to calculate your recipe? Use our free lye calculator to get your exact NaOH and water amounts.