Castile Olive Oil Cold Process Soap Recipe

Create an authentic, traditional castile soap made with 100% pure olive oil. This historic soap recipe dates back centuries to the Castile region of Spain and remains one of the gentlest, most versatile soaps available. Perfect for sensitive, reactive, or allergic skin, true castile soap contains no coconut oil or other additives—just olive oil, water, and sodium hydroxide. This soap produces a mild, creamy lather and is incredibly nourishing, making it suitable for the whole family, including babies and those with eczema.

Ingredients

Oils & Butters

• 54 oz (1531g) pure olive oil (extra virgin or pure)

Lye Solution

• 7.9 oz (224g) sodium hydroxide (NaOH)
• 17.0 oz (482g) distilled water

Additives (Optional)

• 0.6 oz (17g) fragrance oil (vanilla, rose, or unscented—keep it minimal)
• Mica or oxide colorant (0.25 tsp maximum—optional)
• Essential oils (0.3-0.5 oz if desired)

Equipment Needed

• Digital scale accurate to 0.1 oz
• Two stainless steel pots or large bowls
• Stainless steel spoon for stirring
• Stick blender (immersion blender)
• Thermometer
• Soap mold (preferably deep loaf mold for tall bars)
• Safety goggles and gloves
• Soap cutter or serrated knife
• Vinegar (for cleanup)
• Large work surface (castile is thick and requires space)

Instructions

Step 1: Safety Preparation

Put on safety goggles and chemical-resistant gloves. Set up your workspace with all equipment ready. Ensure excellent ventilation, as lye fumes can be irritating. Keep vinegar nearby for accidental spills. This recipe makes a lot of soap, so ensure you have adequate space.

Step 2: Mix the Lye Solution

Pour 17 oz of distilled water into a heat-safe stainless steel pot. Slowly add 7.9 oz of sodium hydroxide while stirring constantly with a stainless steel spoon. The solution will heat rapidly to 160-180°F as the lye dissolves. This is a larger lye batch, so be cautious and work carefully. Set aside to cool to 100-110°F.

Step 3: Prepare Pure Olive Oil

Pour all 54 oz of pure or extra virgin olive oil into a large pot. If using extra virgin olive oil, the soap will be a deeper green or golden color. Pure olive oil will create a lighter-colored bar. Gently heat the oil to 100-110°F. Take your time—olive oil takes longer to heat than blended oils.

Step 4: Check Temperatures

Ensure both the lye solution and olive oil have reached 100-110°F. Temperature matching is crucial for proper saponification. Use your thermometer to verify both are ready.

Step 5: Combine Oils and Lye Solution

Once both reach 100-110°F, very slowly pour the cooled lye solution into the olive oil while stirring steadily and carefully. The large volume means you'll need to stir for a full 2-3 minutes to ensure complete incorporation. Continue stirring until the mixture is uniform.

Step 6: Begin Stick Blending

Using your stick blender, pulse for 30 seconds, then stir by hand for 2-3 minutes. Continue this alternating pattern for 15-20 minutes. True castile soap reaches trace much more slowly than blended-oil soaps. Be patient and persistent.

Step 7: Monitor Trace Carefully

With castile soap, trace can be subtle—the mixture may become slightly thicker but still flow readily. You're looking for a very light trace where the batter slightly holds an impression from the stick blender. This may take considerably longer than blended-oil recipes.

Step 8: Add Fragrance and Color (Optional)

If desired, add up to 0.6 oz of fragrance oil or 0.3-0.5 oz of essential oils at trace. Castile soap is traditionally unscented, but gentle scents can be added. Castile soap accepts color slowly; if coloring, add just a touch of colorant mixed in oil.

Step 9: Pour Into Mold

Pour the thick castile soap batter into your prepared mold. Work carefully, as castile is thick and dense. Use a spatula to smooth the top. The soap will be noticeably heavier than other soap recipes due to the large volume of oil.

Step 10: Insulate Heavily

Wrap the mold extensively with blankets, towels, and newspapers. Castile soap benefits greatly from insulation and can go into gel phase fully. Some soapmakers use a cooler or place the mold in a barely-warm oven (90-100°F) to encourage gel phase.

Step 11: Wait Longer for Saponification

Leave castile soap undisturbed for 24-48 hours, but castile often benefits from a longer initial cure period. Some soapmakers wait up to 72 hours before unmolding to ensure complete saponification.

Step 12: Unmold the Soap

After 24-48 hours (or up to 72 hours), carefully remove the soap from the mold. Castile soap will be quite dense and heavy. If the soap is still soft, wait another 12-24 hours. Castile takes longer to firm up than other soaps.

Step 13: Cut Into Bars

Using a soap cutter or sharp serrated knife, cut the loaf into 24 equal bars. Castile soap is denser and may require more pressure to cut cleanly. Work slowly and steadily. Some soapmakers prefer to wait 1-2 weeks before cutting castile to allow it to firm up more.

Step 14: Air Cure

Place bars on a well-ventilated rack with space between each one. Cure in a cool, dry location (60-75°F) for 4-6 weeks. Castile soap benefits from a longer cure time—many soapmakers cure for 8-12 weeks to achieve optimal hardness and mildness.

Step 15: Test for Readiness

Before using, perform a simple pH test: wet your finger, apply a tiny amount of soap, and rub on your arm. If the soap feels slippery (indicates excess lye), continue curing. A properly cured castile should feel gentle and creamy without the slippery sensation of lye.

Tips for Success

• Castile soap reaches trace very slowly—this is normal, don't overhend or use excessive blending
• Extra virgin olive oil creates a greenish color; pure olive oil is paler
• Castile is traditionally unscented; fragrance is optional and should be minimal
• The longer castile cures, the milder and harder it becomes—patience is rewarded
• Castile produces a gentle, creamy lather—not abundant bubbles, but luxurious nonetheless
• This soap is excellent for babies, sensitive skin, and those with skin conditions
• Castile bars last longer than blended-oil soaps due to their density
• Some traditional castile makers cure for 6-12 months before use for ultimate mildness

Variations

• Scented castile: Add up to 0.5 oz of rose, lavender, or vanilla fragrance for gentle scent
• Green castile with herbs: Add dried herbs like chamomile or lavender buds at trace
• Castile with honey: Add 2-3 tbsp of honey at trace for extra conditioning
• Unscented pure: Skip all additives for the purest traditional castile soap

Cost Breakdown

Estimated Cost per Bar: $0.42

Related Recipes

• Bastille Soap - 80% olive oil for a harder bar
• Basic Cold Process Soap - Blended oils for faster results
• Eco-Friendly Palm-Free Soap - Sustainable oil blend alternative

Historical Note: True castile soap originated in the Castile region of Spain and was historically made with pure olive oil from that region. The process is slower and the soap is milder than modern blended soaps, but it's considered superior for sensitive skin and gentle cleansing. Many traditional soapmakers continue this historic tradition, creating some of the finest soaps available.

Usage Note: True castile soap is so mild that it can be used for the entire body, face, hair, and even as a gentle laundry soap. Some users dilute castile soap to create a liquid soap for various household uses. A single bar lasts significantly longer than other soaps due to its density and mild nature.