Cold Process Castile Soap Recipe: Formula, Method, Cure and Compliance

Castile soap is where handmade soap begins, a simple, ancient bar made entirely from olive oil. It is also a cosmetic product, so the moment you sell it in Great Britain it must comply with the UK Cosmetic Products Enforcement Regulations 2013 and the retained Regulation (EC) No 1223/2009, overseen by the Office for Product Safety and Standards (OPSS). In this complete guide you will get the full olive oil formulation, the method, the cure, and a clear path to selling your soap legally.

This recipe is part of our maker series. If you want a wider library of tested formulas across skincare, hair care and more, our 100 Beauty Formulas cosmetic formulation guide collects one hundred professional formulations in one place.

What is Castile soap?

Classic Castile soap is soap made entirely from olive oil, with no coconut oil, palm oil or added butters. Named after the Castile region of Spain, it is prized for being mild, gentle and deeply conditioning. In cosmetic terms it is a rinse off cleansing product, and on an ingredient label its single oil appears as Olea Europaea Fruit Oil.

Because olive oil is soft and slow to react, a Castile bar takes longer than most soaps to reach its best. A well cured bar is creamy, hard and gentle, which is why Castile suits sensitive skin, mature skin and, once fully cured, even baby skin.

Makers choose Castile soap because it is:

• Gentle. Ideal for sensitive and delicate skin.
• Simple. One oil means fewer variables and less that can go wrong.
• Economical and vegan. Olive oil is affordable, and the bar is palm free and plant based, a strong selling point for conscious buyers.

What is saponification?

Saponification is the chemical reaction at the heart of every real soap. When olive oil (a triglyceride) is mixed with sodium hydroxide dissolved in water, the two react to form soap and glycerin. The important safety point is that the sodium hydroxide is used up in the reaction. A correctly made, fully cured bar contains no free lye, which is why finished soap is safe to use on skin.

A safety word before you start

Cold process soap is made with sodium hydroxide, also known as lye or caustic soda. It is a corrosive alkali that can cause serious chemical burns and permanent eye damage before it is saponified. Treat it as a hazardous substance and follow the Health and Safety Executive guidance on the control of substances hazardous to health (COSHH), along with the safety data sheet supplied with your lye. The essential rules are:

• Wear personal protective equipment. Safety goggles, chemical resistant gloves and long sleeves at all times.
• Work in a well ventilated space, away from children and pets.
• Always add lye to water, never water to lye. Adding water to lye can erupt. Makers say snow floats on water, so you sprinkle the lye onto the water.
• Keep white vinegar nearby for surface spills, and rinse any skin contact with plenty of cool running water.
• Use dedicated soaping equipment that never touches food again.

This care is only needed at the raw lye stage. Once your soap has saponified and cured, it is simply soap.

Equipment you will need

• A digital scale accurate to one gram
• A stick blender (immersion blender)
• Two heatproof jugs, one for the lye solution and one for the oils
• A stainless steel or heavy plastic mixing bowl (never aluminium, which reacts with lye)
• A silicone spatula and a thermometer
• A silicone loaf mould, easiest for beginners
• Goggles, gloves and an apron

The Castile soap formulation

This makes roughly 1.3 kilograms of soap, enough for a standard loaf mould. Everything is given by weight, because soap making is chemistry and volume measurements are not accurate enough.

• Superfat. Calculated at a five percent superfat, so five percent of the oil stays unsaponified to condition the skin and give a safety margin against excess lye.

• Verify before you make it. Saponification (SAP) values vary between suppliers, so always run your exact recipe through a reputable lye calculator before mixing. This single step protects you from a lye heavy, unsafe bar.

• Water and lye concentration. The water above gives a lye concentration of about thirty three percent, a forgiving level for beginners. A steeper water discount makes a harder bar and a faster cure.

• Sodium lactate is a liquid salt that hardens the finished bar and eases unmoulding. Stir it into the cooled lye solution.

• Fragrance and allergens. Soap is a rinse off product, but fragrance and essential oils still carry restricted allergens that must be declared on the label above set thresholds. If you scent your soap, an allergen compliance check confirms what has to appear on your ingredient list.

How to make Castile soap, step by step

Allow about ninety minutes of active work, then a day or two before unmoulding.

1. Prepare and weigh

Put on goggles and gloves, clear your surface, keep children and pets away, and weigh every ingredient before you begin.

2. Make the lye solution

Weigh the distilled water and the sodium hydroxide separately. Near ventilation, slowly sprinkle the lye into the water while stirring. It heats to around eighty degrees Celsius and gives off fumes briefly, so do not lean over it. Stir until clear, add the sodium lactate, then set it aside to cool.

3. Warm the oils

Weigh the olive oil into your bowl. It needs little or no heat. If the room is cold, warm it gently to around forty degrees Celsius.

4. Combine at the right temperature

Bring the lye solution and the oils to a similar temperature, ideally thirty five to forty five degrees Celsius, then pour the lye solution slowly into the oils with the stick blender resting in the pot to avoid splashing.

5. Blend to trace

Pulse the stick blender in short bursts, alternating with stirring, until you reach trace, when a drizzle across the surface leaves a faint trail. Pure olive oil soap traces slowly, so be patient, and do not be fooled by a false trace caused by cooling rather than real saponification.

6. Add extras, mould and insulate

At light trace, blend in the kaolin clay slurry and fragrance, then pour into the mould, tap out air bubbles, cover, and wrap in a towel overnight for a smooth gel phase.

7. Unmould and cut

Leave the soap one to three days. Castile is soft at first, so do not rush. When firm, unmould and cut into bars while wearing gloves, as the soap is still alkaline at this stage.

How long does Castile soap take to cure?

Curing is where a good Castile soap is truly made. Saponification finishes, so the bar becomes completely mild, and water evaporates, so the bar hardens. Most soaps cure in four to six weeks, but Castile is the exception. A fresh bar is soft and slimy, yet given time it becomes hard, silky and gentle. Cure your Castile for a minimum of eight weeks, and ideally three to six months, on a rack in a cool, dry, airy place, turning the bars occasionally.

Testing and troubleshooting

• The zap test. Touch a cured bar briefly to your tongue. Soap tastes of soap. A sharp, battery like zap means unreacted lye, so the bar needs more cure or has a recipe error.
• pH. Handmade soap is naturally alkaline at around pH nine to ten, which is normal. A reading well above this suggests a lye heavy batch.
• Slow trace. Normal for Castile. Keep blending in bursts and check your temperature.
• Orange spots (DOS). The dreaded orange spots are rancidity. Use fresh olive oil, store bars dry, and consider a little rosemary oleoresin extract as an antioxidant.
• White soda ash. Harmless and cosmetic. Prevent it by covering the soap during cure.

Variations to build a range

• Bastile soap. Replace fifteen to twenty five percent of the olive oil with coconut and a little castor oil for more lather and a harder bar sooner.
• Clay and botanical bars. Add French green or rose clay, or top with dried lavender.
• Milk Castile. Replace part of the water with goat or oat milk for extra creaminess.
• Gentle scented bars. Lavender, chamomile and light citrus suit the mild character of Castile.

From kitchen to shelf: selling your Castile soap legally

Handmade soap is a cosmetic, so the moment you sell it, it must meet UK and EU cosmetic law before it goes on the market. For a soap specifically, our companion guide, the CPSR for whipped soap, walks through the same duties in detail. Here is what compliance involves.

• A Cosmetic Product Safety Report. Every cosmetic sold in the UK or EU needs a Cosmetic Product Safety Report (CPSR), a formal safety assessment signed by a qualified Safety Assessor under Regulation (EC) No 1223/2009. If you make several scent variants, they can be assessed together as a range through our multiple variant CPSR service.

• The full document set. Alongside the CPSR you must hold a Product Information File, and your product must be notified. Our all in one compliance package covers the CPSR, PIF, SDS and notification together.

• Notification. Before sale, your soap is notified in Great Britain through the Submit cosmetic product notifications service (SCPN), and in the EU through the CPNP, which we handle in our CPNP product notification service. It is declared under the correct frame formulation, a toilet soap, so your exact recipe stays private.

• Responsible Person and labelling. Every product needs a Responsible Person who is legally accountable to the authorities, and a compliant label. Phoenix can act as your UK and EU Responsible Person and carry out a label and packaging review so your labels are right first time.

• Selling in the United States? Under the Modernization of Cosmetics Regulation Act, the US Food and Drug Administration now requires facility registration and product listing for many cosmetics, which we manage through our FDA MoCRA listing service.

None of this needs to slow you down. When you are ready to take your soap to market, contact Phoenix Safety Consultants and we will guide you through every step.

Frequently asked questions

Is Castile soap good for sensitive skin?

Yes. Made entirely from olive oil with a gentle superfat, a well cured Castile bar is one of the mildest soaps available, which is why it suits sensitive, mature and baby skin.

How long does Castile soap take to cure?

Longer than most soap. Cure it for at least eight weeks, and ideally three to six months. The longer it cures, the harder, milder and more luxurious it becomes.

Can I make Castile soap without lye?

No. All real soap is made by saponifying oils with sodium hydroxide, and the lye is fully consumed. To avoid handling lye, use a ready made melt and pour base, although that is not a true cold process Castile.

Do I need a CPSR to sell Castile soap in the UK?

Yes. Any cosmetic sold in the UK or EU, including handmade soap, needs a Cosmetic Product Safety Report, a Product Information File, a Responsible Person and a compliant label before it goes on the market.

What INCI name does Castile soap use for olive oil?

Olive oil is listed as Olea Europaea Fruit Oil. If you add sodium hydroxide, water and any fragrance or clay, these are declared too, following the labelling rules under Regulation (EC) No 1223/2009.

Related reading

• CPSR for Whipped Soap (UK and EU guide)
• CPSR for Lip Scrub (UK and EU guide)
• CPSR for Body Mist (UK and EU guide)

About the author

This article is educational. Always run your own recipe through a lye calculator, follow proper safety practices, and have any product professionally assessed before selling it.