Castilian soap: recipe, production and everything you need to know about it
TL;DR: Castile soap is made from 100% olive oil and sodium hydroxide. It is the gentlest for sensitive skin, but requires 6–12 months of curing. The lather is creamy and gentle, not thick.
Castile soap is one of the oldest and simplest forms of soap. It consists of only two ingredients: olive oil and sodium hydroxide (or potassium hydroxide for the liquid version). No palm oils, no synthetic additives, no preservatives. The result is a gentle soap suitable for the most sensitive skin - including that of children.
The name comes from the region of Castile in what is now Spain, where it was traditionally made from local olive oil. The original Castilian soap is aged in cool cellars for up to a year, producing a legendarily thick, creamy lather that is as different from modern commercial soaps as pro espresso is from instant coffee.
This page gives you everything: exact recipe, precise temperatures, reasons why castile soap needs such long curing, how to use it properly and a variant for liquid castile soap.
What exactly is castile soap?
Exact definition and properties. True castile soap contains 100% olive oil as the fat component. Nothing else. No coconut oil for hardness, no castor for lather, no palm for durability. Just olive oil.
Why is this interesting? Because olive oil is special. It contains mostly oleic acid - about 70-80%. This acid produces a specific type of soap foam: soft, silky, conditioning. Unlike coconut oil, which produces large bubbles and cleans strongly, olive oil gives small, dense bubbles and cleans gently.
Castile soap also naturally contains oleuropein and polyphenols from olive oil, which have antioxidant properties. Some of these survive saponification and contribute to the characteristic greenish-brown colour of fresh Castilian soap (it lightens after maturing).
Olive oil for soap - what quality? Here's the good news: you don't need 300 CZK/litre extra virgin olive oil. "Pomace“ olive oil (from olive pomace) is great for soap making - it has the same saponification number, the same properties, and you can get it for a fraction of the price. In the Czech Republic, look for it in Greek deli, big supermarkets or online (2-4 l packs for 200-350 CZK). For soap it is ideal.
Castile soap and sensitive skin: why does it work?
Why is Castilian soap ideal for sensitive skin? Skin has a natural pH of around 4.5-5.5. Most soaps have a pH of 9-10, Castile soap tends to be 9-9.5 after aging. This is still alkaline, but less aggressive than cheap commercial soaps.
The key factor is oleic acid. Unlike lauric acid (coconut, 48%) and myristic acid, which are strong cleansers and can be drying, oleic acid has a high compatibility with the skin's sebum. Human sebum contains approximately 25% oleic acid - which is why Castile soap is more natural to the skin.
Who is castile soap suitable for:
- Sensitive and dry skin
- Atopic dermatitis (with medical recommendation)
- Infants from 3 months of age (verify with pediatrician)
- Facial care — as a gentle cleanser
- Hair care — as a shampoo alternative (requires adjustment period)
Where castile soap doesn't work well: Hard water. In areas with high water hardness (Prague, Brno, Central Bohemia - see SOVAK water hardness map) olive soap does not lather well and leaves a greyish film. Solution: acidifying hair rinse with apple cider vinegar (1 tablespoon per 500 ml of water) or filtered water.
How to make bar castile soap (cold process)?
What are the ingredients and procedure? Recipe and detailed step-by-step guide.
Recipe overview
| Yield | ~900 g of finished soap |
|---|---|
| Difficulty | Medium (due to long trace time) |
| Preparation time | 2–3 hours |
| Curing | 6–12 months |
| Shelf life | 2–3 years |
Ingredients
Fat component:
- 700 g olive oil (100% of fat component)
Lye solution:
- 89.1 g NaOH (sodium hydroxide, 100% purity)
- 228 g distilled or boiled water
Super fat: 5% (standard for castile)
Calculation note: The saponification number of olive oil for NaOH is 0.134. Calculation: 700 × 0.134 = 93.8 g NaOH for 0% super fat. With 5% super fat: 93.8 × 0.95 = 89.1 g NaOH. Water = 33% of the weight of the oils = 231 g. (Results from the soapmaking calculator may vary slightly depending on the database used - see Soap calculator.)
Tools
- Digital scale (1 g accuracy)
- Two containers made of stainless steel or polypropylene (PP)
- Immersion stick blender
- Thermometer
- Silicone loaf mold or wooden mold lined with parchment paper
- Safety goggles and gloves
Procedure
1. Safety preparation (15 minutes) Put on goggles, gloves and an apron. Prepare your workspace — ideally near a ventilated window or under a range hood. Keep paper towels and water for rinsing at hand.
2. Lye solution preparation (30 minutes) Weigh the water into a larger stainless steel or plastic container. In a dry plastic container, weigh the NaOH. Slowly pour the NaOH into the water (never the other way around), stirring constantly. The solution will immediately warm to 70-80°C and may appear hazy. This is normal. Set aside where it will not interfere and allow to cool to 35-40 °C.
3. Oil preparation (10 minutes) Weigh out the olive oil. Castile soap has an advantage: olive oil is liquid even at room temperature, so you don't need to melt it. The temperature of the oil should be 35-40 °C. In winter, warm the oil slightly with a water bath.
4. Mixing — patience is key (30–90 minutes) Pour the lye solution into the oils (never the reverse). Start mixing with an immersion stick blender — pulses of 5–10 seconds, then pause, then again. Castile soap is known for the fact that trace comes slowly — due to the high content of oleic acid. Expect 15–45 minutes of mixing, sometimes even longer. Don't panic — this is normal.
How to recognize light trace: when you drag a spoon across the surface, it leaves a trail that holds its shape for 3–4 seconds. Consistency should be like thin pudding.
5. Adding optional additives (5 minutes) At light trace you can add:
- Essential oils: 14–21 g (2–3% of 700 g oils). Lavender and ylang ylang are traditional for castile.
- Natural colorants: chlorophyll for green, spirulina for olive, turmeric for golden.
Caution: essential oil can cause rapid trace in castile soap — add slowly and mix by hand, not with a stick blender.
6. Pouring into mold and gel phase Pour into the mold, tap out air bubbles by tapping the mold on the table. Cover with plastic wrap directly on the soap surface (prevents soda ash). Wrap the mold in towels or a blanket.
Castile soap may not always go through full gel phase — it depends on temperature and mold. Gel phase is not mandatory, but helps achieve more vibrant colors.
7. Unmolding and cutting (after 48–72 hours) Castile soap is still soft after 48 hours. Be patient - this is normal with pure olive oil. If the soap is still too soft to cut after 72 hours, place the mould in the freezer for 1 hour - it will firm up.
8. Curing: the hardest step Stand the cut soaps on their side on wire rims or wooden planks in a well-ventilated area. Ideal curing temperature: 15-20 °C, humidity below 60 %. Turn the soap once every 2 weeks to dry evenly.
Minimum curing: 6 months. I recommend 9–12 months for full lather. Castile soap that cures for a year produces a legendary thick lather and is significantly gentler than a 2-month-old bar.
Why does castile soap need such a long curing time?
Why is curing so important? This is the most common question when making castile soap, and the answer lies in the chemistry of oleic acid.
Oleic acid (C18:1) is a monounsaturated fatty acid with one double bond. Unlike saturated fatty acids (lauric, palmitic, stearic), soap molecules derived from oleic acid arrange themselves more slowly into a crystalline structure. Fresh Castilian soap has an amorphous, 'sticky' texture - hence the typical gel-like consistency and weak foam.
During 6–12 months of curing, several things happen:
- Water evaporation — castile soap loses 15–20% of its original weight as water after 6 months. The resulting hardness and density are significantly higher.
- Crystallization of soap molecules — sodium oleate (soap salt of oleic acid) gradually transitions to a more stable crystalline form, which creates a finer, denser lather.
- Surface oxidation — fresh castile soap tends to be green-yellow due to chlorophyll and carotenoids in olive oil. With curing, it lightens to beige or white. This coloring does not affect quality.
In practice: If your castile soap barely lathers and is sticky after 2–3 months — this is normal. Don't throw it away. Let it continue curing.
How to use castile soap?
What are the best practical applications?
As body and face soap
Use with soft water or rinse your face with filtered water. For maximum effect, let the lather sit for a moment — 30–60 seconds before rinsing.
As shampoo (no-poo method)
Castile soap as shampoo requires an adjustment period of 2–4 weeks, during which hair may be oily or waxy. After adjustment, the hair cycle normalizes. Key: always use an acidic rinse after washing (1 tablespoon of apple cider vinegar per 500 ml of water), which neutralizes the alkaline film and closes hair cuticles.
As liquid hand soap
Solid Castilian soap can be grated and dissolved in hot water for homemade liquid soap. Ratio: 1 tablespoon of grated soap per 200 ml of water. The result will be thinner than a commercial dispenser - add xanthan gum for a thicker consistency.
For household cleaning
Liquid castile soap is the base of many DIY cleaning products — for bathrooms, floors, dishwashing. Do not use in combination with acidic ingredients (vinegar, lemon juice) — they react and neutralize each other.
How to make liquid castile soap (KOH, hot process)?
What is KOH used for? Liquid castile soap is made with potassium hydroxide (KOH) using the hot process method — cooking until the paste is completely processed.
Ingredients
- 500 g olive oil (100%)
- 76 g KOH (potassium hydroxide, 90% purity — note: purity must be factored into calculation)
- 190 g distilled water for cooking the paste
- 500–1,000 g distilled water for diluting paste into final liquid soap
KOH Calculation: Saponification number of olive oil for KOH = 0.188. For 500 g: 500 × 0.188 = 94 g KOH for 0% SF. With 3% SF: 94 × 0.97 = 91.2 g of pure KOH. If you have 90% purity: 91.2 ÷ 0.90 = 101.3 g of real KOH.
Procedure (hot process in a slow cooker)
- Dissolve KOH in water (same as NaOH — pour KOH into water).
- Heat olive oil to 70°C in a slow cooker (LOW setting).
- Pour in KOH solution while stirring constantly.
- Cook on LOW for 2–4 hours, stir every 30 minutes.
- The paste is done when: it has the consistency of thick vaseline, is transparent or golden, and phenolphthalein drops on a small sample do not turn red (= no free hydroxide).
- Let the paste cool, then add it gradually to warm distilled water while stirring until you reach the desired consistency.
- Let it rest for 24 hours — the liquid clears and settles.
Store the resulting liquid soap in a sealed container. Shelf life: 1–2 years.
Most common problems when making castile soap
Soap is still too soft to cut after 48 hours. Normal for castile. Leave it for 72 hours, then place it in the freezer for 1 hour. Olive oil has a high content of unsaturated fatty acids — it hardens more slowly than coconut or palm.
Soap barely lathers after 2 months. Normal! Castile soap achieves good lather after 6 months. Let it continue curing.
There is white powder on the soap surface (soda ash). Soda ash on castile soap is very common. Shave it off with a scraper or sharp knife — the soap underneath is fine. Next time, cover the surface with plastic wrap immediately after pouring.
The soap has „zeylonite“ or transparent foci in the core when removed from the mould. Gel phase occurred only partially (partial gel). This is an aesthetic issue — the soap is fine. Next time ensure even heat throughout the mold (wrap in a blanket).
Where to buy olive oil for soap making in the Czech Republic
- Pomace olive oil (economical choice): Greek and Italian deli shops, wholesalers, Makro. 2–5 l packages priced at 180–350 CZK.
- Regular olive oil light/extra light: large supermarkets, Kaufland, Tesco — 1 l for 120–160 CZK.
- Extra virgin olive oil: fully functional for soap, but unnecessarily expensive — properties are identical to pomace for soap making.
Frequently asked questions
How long should castile soap cure? At least 6 months, but ideally 9–12 months. After a year of curing, it achieves the best lather and gentleness. Soap won't spoil even after longer periods — it remains stable for 2–3 years.
Can I use castile soap earlier (e.g. after 3 months)? Yes, but the lather will be weaker and the soap softer. It will be functional — just not the full castile soap experience.
How to tell when castile soap is ready to use? You should feel it's hard when you press it (not plastically soft). Lather should be thick and fine. If you're not sure, let it cure longer.
Can I remove soda ash from castile soap? Yes — with a scraper, sharp knife or light sanding. Soda ash is purely aesthetic — the soap underneath is fine.
Is castile soap suitable for all body parts? Yes — it is the gentlest soap that can be recommended. Suitable for babies (from 3 months with pediatric approval), face, intimate areas and as shampoo.
Why does castile soap lather poorly in hard water? Hard water (high calcium and magnesium content) binds to soap molecules and forms a non-lathering film. Solution: filtered water or an acidic rinse (apple cider vinegar).
Do you have your first Castilian soap ready? Don't forget to label it correctly - the date of manufacture and the date you plan to use it from. Castile soap is the best investment in patience a soapmaker can make.
See also:
- Recipes for beginners — simpler recipes for your first batch
- Soap calculator — recalculate the recipe for your mold
- Oil encyclopedia — properties of olive and other oils
- Most common problems — what to do when something goes wrong
⚠️ Recipe disclaimer
This recipe was created or revised with the help of artificial intelligence tools and has undergone NaOH gram recalculation. Nevertheless, we recommend verifying lye amounts in an independent calculator (e.g. SoapCalc or Brambleberry). Working with sodium hydroxide requires protective equipment — see Lye safety. Information is for educational purposes; the manufacturer is not liable for damages resulting from their use.
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