Castile soap

Synopsis

Castile Soap

Identity and Nomenclature

Castile soap is an olive oil-based soap made in a style similar to that originating in the Castile region of Spain. In contemporary usage, the term has broadened: in its simplest definition, castile soap is soap made from vegetable oil; it originated from the Castile region of Spain and was originally made from pure olive oil. Some formulations now include a blend of plant-based oils, though purists maintain that the designation properly applies only to soaps in which olive oil is the primary or sole fat source.

The principal botanical source of traditional castile soap is Olea europaea L. (the common olive), family Oleaceae. The ingredient Olea europaea Fruit Oil, listed on cosmetic labels, is the fixed oil obtained from the ripe fruit of the Olive — in other words, olive oil. The alkali component in bar soaps is sodium hydroxide (NaOH), while liquid castile soaps are produced with potassium hydroxide (KOH). Castile soap is created through a chemical reaction called saponification, where fats — in castile soap's case, vegetable oils — are mixed with an alkali (commonly lye, or sodium hydroxide for solid soap and potassium hydroxide for liquid soap).

Common forms and preparations include:

Bar soap: Made by saponifying olive oil (or olive oil blends) with sodium hydroxide; the hard bar hardens further on curing.
Liquid soap: Produced using potassium hydroxide in place of sodium hydroxide, yielding a softer, pourable consistency. True liquid castile soap requires potassium hydroxide (KOH) rather than sodium hydroxide used in bar soaps.
Concentrated liquid: A highly viscous, dilutable form sold in various scented and unscented variants for body, hair, and household use.
Modern multi-oil blends (sometimes called "Bastile" soaps): Traditional castile soap is 100% olive oil, though modern versions sometimes include a small percentage of coconut or palm oil to improve lather and hardness. Some formulations use no olive oil at all.

Historical and Traditional Use

Origins in the Levant

The start of castile soap goes back to the Levant, where Aleppo soapmakers have made hard soaps based on olive and laurel oil for millennia. It is commonly believed that the Crusaders brought Aleppo soap back to Europe in the 11th century, based on the claim that the earliest soap made in Europe was just after the Crusades. However, this is an oversimplification: soapmaking was present in Europe prior to this. It was known to the ancient Phoenicians, whose empire stretched from the Levant across all of northern Africa and who engaged in trade with the southern Iberian Peninsula, and also to the Romans who controlled the area for nearly 600 years from the 2nd century BC to the 4th century AD.

Ancient Romans and Egyptians utilized olive oil in numerous ways, including cooking, as cosmetics, as salves for wound treatment, and for anointing. Ancient Greeks used olive oil in their baths.

Development in Spain

Following the Crusades, production of this soap extended to the whole Mediterranean area. Early soapmakers in the Mediterranean area did not have easy access to laurel oil and therefore dropped it from their formulations, thereby creating an olive oil-based soap now known as castile soap.

A key distinguishing feature of the Castilian product was its alkali. Castile soaps were special not only because they contained olive oil, but also because their lye component was barilla, an alkaline powder made from plants found only in the Castile region. John Hunt maintains that barilla (an impure form of sodium carbonate obtained from halophyte plant ashes that were high in sodium) was boiled with locally available olive oil, rather than tallow. Adding brine to the boiled liquor made the soap float to the surface, where the soap-boiler could skim it off, leaving the excess lye and impurities to settle out. While Aleppo soap tends to be green, this produced what was probably the first white hard soap, which hardened further as it aged, without losing its whiteness, forming jabón de Castilla.

The Arabic influence on Iberian soapmaking is traceable linguistically: traces of soapmaking's Arabic origins are still woven through the Spanish language today, evidenced in part by the "al-" prefix. Almona is the Spanish word for a soapmaking factory, stemming from the Arabic almuna with the same meaning. The word álcali, a cognate of the English "alkali", stems from the Arabic word meaning "to roast", indicative that early alkalis used in soapmaking were derived from the burning of certain plants.

Castile soap is so called because it was produced on a large scale in the territories of the Crown of Castile, from where it was exported to numerous places in Europe, mainly during the Modern Age. Importations of "Castile soap" through Antwerp appear in the London port books of 1567–1568. Though the Oxford English Dictionary has no references to "Castile soap" earlier than 1631, it is also mentioned in the English poem Libelle of Englyshe Polycye dating from 1436 or 1437.

Religious and Political Controversies

Spanish nobility prized castile soap for its gentleness and effectiveness, setting a trend that would eventually reach throughout Europe despite religious and economic controversies in Protestant England, where it was derogatorily referred to as "Popish soap" due to its association with Catholic Spain. England had been a large market for castile soap, but savvy marketers for English soapmaking brands began referring to the castile soap as "Catholic soap" stemming from Catholic Spain. This was unpalatable to the Protestant English. The implication was that use of the soap implied religious allegiance. So even though English soap was of lesser quality than the Spanish, it was a point of national pride to use it.

Furthermore, castile soap did not have the protection of royal patronage that Marseilles soap did in neighboring France. King Louis XVI instituted regulations that outlived him dictating that Marseilles soap must be made in Marseille and contain at least 72% olive oil. Because of this lack of regulation over where or how castile soap was made, the name was vulnerable to cheap, poor quality imitations flooding the market.

Near Eastern Traditions

Not far away in Palestine began the production of Nablus soap, more simply made with only olive oil and lye. In both Syria and Palestine, there remain still a few soapmakers in operation today, making soap in the same manner as their ancestors.

Key Constituents and Chemical Composition

The Saponification Process

The saponification process is the chemical reaction that occurs when fats or oils are combined with an alkali, such as lye (sodium hydroxide), to produce soap. In the case of castile soap, the recipe uses plant oils as fat sources. When the oils are mixed with the lye solution, a chemical reaction breaks down the fats into glycerol and fatty acids. The lye reacts with the fatty acids to form soap molecules, while the glycerol remains in the mixture, giving the soap its moisturizing properties.

Through the traditional process of saponification, natural oils are blended with water and lye, transforming them into a gentle, effective, and long-lasting soap. No lye remains in the finished product — only pure, moisturizing soap enriched with natural glycerin. This point is chemically significant: the alkali is fully consumed during reaction and is not present in the finished soap bar or liquid.

Fatty Acid Profile of Olive Oil

The functional character of traditional castile soap is determined primarily by the fatty acid composition of olive oil. The fatty acid composition of olive oil varies widely depending on the cultivar, maturity of the fruit, altitude, climate, and several other factors. Oleic Acid (C18:1), a monounsaturated omega-9 fatty acid, makes up 55 to 83% of olive oil. Linoleic Acid (C18:2), a polyunsaturated omega-6 fatty acid, makes up about 3.5 to 21% of olive oil. Palmitic Acid (C16:0), a saturated fatty acid, makes up 7.5 to 20% of olive oil. Stearic Acid (C18:0), a saturated fatty acid, makes up 0.5 to 5% of olive oil. Linolenic Acid (C18:3), a polyunsaturated omega-3 fatty acid, makes up 0 to 1.5% of olive oil.

Published peer-reviewed analysis of olive oil composition confirms these ranges: virgin olive oil is the sole edible olive oil extracted from the fruits of Olea europaea L. without refining processes. Olive oil has established hypolipidemic and antioxidant properties, and its regular integration into the diet results in major health benefits, including fewer cardiovascular diseases and lower rates of certain cancers. Researchers have argued that these benefits stem from its high monounsaturated fatty acid (MUFA) content, where the main component is oleic acid, and from minor molecules such as phytosterols, carotenoids, tocopherols, and polyphenols.

With respect to skin-relevant properties specifically, the most abundant fatty acids responsible for the therapeutic effect of vegetable oils are oleic, linoleic, and linolenic acids. The non-saponifiable lipids are usually responsible for the anti-inflammatory and antioxidant effects of vegetable oils. Fatty acids are necessary for the maintenance of epidermal integrity and the water barrier of the skin. They are metabolic precursors of arachidonic acid and prostaglandins in the epidermis and important for the regulation of cell division and epidermis differentiation.

Glycerin (Glycerol)

A saponification byproduct of notable biological relevance retained in artisanally made castile soaps is glycerin (glycerol). The diverse actions of the polyol glycerol on the epidermis include improvement of stratum corneum hydration, skin barrier function and skin mechanical properties, inhibition of the stratum corneum lipid phase transition, protection against irritating stimuli, enhancement of desmosomal degradation, and acceleration of wound-healing processes. Additionally, an antimicrobial effect has been demonstrated. Topical application of glycerol-containing products improves skin properties in diseases characterized by xerosis and impaired epidermal barrier function, such as atopic dermatitis.

Minor Bioactive Compounds

Olive oil also contains phytochemically active minor fractions. O. europaea extracts are primarily composed of phenolic compounds, represented by phenolic alcohols hydroxytyrosol, tyrosol, and secoiridoids, which include oleocanthal, oleacein, oleuropein, and ligstroside. However, it is important to note that the saponification process chemically transforms the starting oils; not all minor compounds in the starting olive oil necessarily survive unchanged into the finished soap product. The literature on which specific polyphenols remain bioavailable in the saponified soap form is limited.

pH

Saponified olive oil soap is alkaline. Pure olive oil soap (often called castile soap) typically has a pH between 10 and 11.5, which is higher than the skin's natural pH of around 4.5 to 5.5. Soaps made with a higher percentage of olive oil tend to land at the upper end of that range. Commercial liquid castile soap formulations, particularly those that include coconut and palm oil, tend to measure somewhat lower; one major manufacturer reports a pH of 8.7–9.9 for its liquid products.

Mechanisms of Action

Surfactant and Mechanical Cleansing

Traditional castile soap is not antibacterial in the clinical sense. It cleans through surfactant action, physically lifting and washing away bacteria rather than chemically killing them. Soap molecules, being amphiphilic (having both hydrophilic and lipophilic ends), surround dirt, oil, and microorganisms and allow them to be rinsed away with water. It does not kill germs with antibacterial chemicals; instead, it helps remove dirt, oil, and germs from skin and surfaces when you scrub and rinse with water. Consistent with this, according to the US Food and Drug Administration, there isn't enough scientific evidence to suggest that over-the-counter antibacterial soaps are more effective in preventing illness than washing hands with plain soap and water.

Moisturization and Skin Barrier Support

Plant oils used in castile soap — particularly olive oil — are rich in oleic acid, which closely mimics the skin's natural sebum. Unlike detergent-based body washes that strip the skin's lipid barrier to create lather, castile soap cleans without depleting moisture. Skin feels soft and hydrated after washing rather than tight or dry.

Oleic acid, comprising 55–83% of olive oil, provides gentle cleansing properties while supporting the skin's barrier function. Unlike synthetic surfactants that can penetrate and disrupt cellular membranes, oleic acid works primarily at the surface level, removing dirt and excess oils without compromising the skin's protective structures. Linoleic acid, present in smaller quantities, provides additional moisturizing benefits and helps regulate sebum production, making olive oil soap suitable for both dry and oily skin types.

The glycerin retained from saponification plays a humectant role: the saponification process that turns plant oils into soap creates natural glycerin as a byproduct — a humectant that attracts moisture to the skin while the soap carries away dirt, bacteria, and dead skin cells.

Scientific Evidence by Area of Use

Skin Cleansing (General)

Castile soap has been widely used for general body and facial cleansing. The scientific evidence at the level of direct controlled trials of castile soap itself as a named product is sparse; most clinical and dermatological research has studied topically applied olive oil rather than saponified olive oil (castile soap) per se. The two are not chemically identical, and findings from olive oil studies cannot be directly extrapolated to castile soap without qualification. Nevertheless, Cleveland Clinic-affiliated dermatologist Dr. Alok Vij has noted that if you have acne-prone skin, atopic dermatitis, hidradenitis suppurativa, psoriasis, or rosacea, castile soap may be an added benefit alongside other first-line forms of treatment. "It's unlikely to worsen any condition because it's relatively mild and it has common ingredients that are tolerated universally."

Atopic Dermatitis (Eczema) and Sensitive Skin

The bulk of the directly relevant clinical evidence concerns olive oil derivatives and skin disease, reviewed through a peer-reviewed systematic lens. A systematic review published in SKIN The Journal of Cutaneous Medicine, which used EMBASE and PubMed databases adhering to PRISMA guidelines and identified 44 articles, found that olive oil demonstrated effectiveness in reducing erythema, scaling, and pain associated with radiation and contact dermatitis, and in managing symptoms of atopic dermatitis and psoriasis through modulation of inflammatory pathways. It also promoted wound healing, benefiting patients with pressure ulcers, chronic wounds, and burns.

A separate systematic review published in Our Dermatology Online (2025) identified 21 studies meeting inclusion criteria from a PubMed search. The evidence across these studies spanned multiple skin conditions and was generally positive for olive oil-containing preparations, though study heterogeneity is a limitation. Atopic dermatitis, psoriasis, eczema, seborrhea, contact dermatitis, different inflammations, pruritus, and burns have all been reported to respond well to olive oil treatment.

A 2025 within-subject controlled trial conducted at the Dermatology Department of Virgen de las Nieves University Hospital in Granada (Spain), published in Journal of Clinical Medicine (PMC12251406), included 54 individuals (50% female), with a mean age of 28.57 years. The study's aim was to investigate the effects of topically applied extra virgin olive oil (EVOO) and petrolatum on the skin barrier function (SBF) and microtopography in healthy adult volunteers, assessing key indicators such as transepidermal water loss (TEWL), hydration, and surface texture, to elucidate the dermatological potential of EVOO and support its evidence-based use in skin care and therapy. While the study did not use castile soap as such, it provides mechanistic context for the role of olive oil in skin barrier maintenance.

Evidence strength: For olive oil applied topically (not in soap form), multiple small RCTs and systematic reviews support anti-inflammatory and skin-barrier benefits. For castile soap itself as a named formulation, direct clinical RCT evidence is very limited. The existing clinical guidance and expert commentary is generally supportive of castile soap as a mild option, but this rests largely on the properties of its constituent oils rather than trials of the finished saponified product.

Pressure Ulcer Prevention

A 2022 systematic review and meta-analysis of randomized controlled trials (PMC9690722) in International Journal of Environmental Research and Public Health examined the efficacy and safety of topical application of olive oil for preventing pressure ulcers. This review focused on unadulterated olive oil preparations rather than castile soap; the meta-analysis of RCT data provided supportive, though heterogeneous, evidence for olive oil in ulcer-risk reduction. It underscores the wound-healing context for olive oil derivatives that underpins the rationale for castile soap in wound-adjacent cleansing, but direct extrapolation to castile soap requires caution.

Wound Healing

Oleuropein extract has been shown to accelerate skin wound healing in aged male mice via increased collagen fiber deposition and reduced cell infiltration in wound sites. This is animal-only evidence and relates to a specific olive polyphenol rather than to saponified soap. Plant oils have been utilized for a variety of purposes throughout history, with their integration into foods, cosmetics, and pharmaceutical products. They are now being increasingly recognized for their effects on both skin diseases and the restoration of cutaneous homeostasis. Review data from plant oils including olive oil address their therapeutic benefits according to their anti-inflammatory and antioxidant effects on the skin, promotion of wound healing, and repair of skin barrier. Again, these findings relate to olive oil itself and the extrapolation to the soap form should be treated cautiously.

Hair and Scalp

Castile soap is used as a shampoo alternative, particularly by those seeking surfactant-free or low-chemical approaches to hair cleansing. Applying a dime-sized amount to the hands and lathering it into the scalp may be enough to rinse the hair of dirt and oil. Castile soap is not right for all hair types; for example, some may find that it is too drying for their scalp, whereas others might find that it does not strip enough oil from the hair.

If you're into the "no poo" method, castile soap is a great option for a squeaky-clean scalp — try pre-mixing a tablespoon of liquid castile with a cup of water. However, the soap can also leave hair tangled and matted owing to the interaction between its alkaline pH and hair's cuticle. Like baking soda, castile soap can be harsh on dyed or highlighted hair, stripping follicles of color. To protect colored hair, it's probably best to stick with a color-safe shampoo.

Evidence strength: No controlled clinical trials specific to castile soap for hair or scalp use were identified in the peer-reviewed literature. Existing guidance is based on general soap chemistry and empirical consumer experience.

Antimicrobial / Hand Hygiene

Castile soap performs the same mechanical germ-removal function as any plain soap in hand hygiene contexts. Castile soap is not antibacterial in the way an antimicrobial wash or disinfectant is. It does not kill germs with antibacterial chemicals. Instead, it helps remove dirt, oil, and germs from skin and surfaces when you scrub and rinse with water. The FDA's position, as referenced in the peer-reviewed hygiene literature, is that plain soap and water, when used with proper technique, are as effective as antibacterial-formulated soaps for preventing illness in everyday settings.

Household, Agricultural, and Pest Control Uses

A person can use castile soap as a body wash, household cleaner, and insect repellent. Soaps such as castile soap may have use as a pest deterrent for ants and other bugs around some plants. A spoonful of liquid castile soap can be added to a quart-sized sprayer bottle and sprayed on beds of affected plants or areas where bugs get in. Care should be taken not to use too much soap, and it should not be sprayed directly on the plant. Even when diluted, soaps may strip the natural oils and waxes from plants.

Castile soap can be a powerful insecticide, but it's important not to overdo it on plants. Because it can remove a plant's natural protective, waxy coating, spraying too much castile soap directly on plants could leave them more susceptible to pathogens or even burn them.

Evidence strength: The use of soap-based sprays as horticultural insecticides is a recognized practice backed by extension agriculture literature, though most specific evidence uses general insecticidal soaps rather than castile soap specifically.

Body Systems and Health Areas of Association

Integumentary system (skin and hair): The primary area of use; general cleansing, moisturization, sensitive skin management, and adjunct care in inflammatory dermatoses such as atopic dermatitis, psoriasis, and rosacea.
Wound care (topical): Olive oil — the major constituent of castile soap — has documented wound-healing properties in multiple controlled studies. Castile soap is used in some settings to clean wounds prior to dressing; however, direct evidence for saponified castile soap in wound management is limited.
Scalp and hair: Used as a shampoo alternative for dandruff, seborrheic dermatitis, and general cleansing, though evidence is empirical.
General hygiene and microbial surface reduction: Through mechanical surfactant action.

Dosage Forms and Use Patterns Reported in Sources

Castile soap is not regulated as a drug or dietary supplement and therefore has no formally established therapeutic dosage. The following represent dilutions and quantities described in referenced sources for various uses:

Body wash / facial cleansing: Castile soap can be used on the face, but should be used with care due to its high pH. For facial cleansing, dilute the soap with water (about 1 part soap to 3 parts water), gently massage onto wet skin, and rinse.
Scalp / shampoo: Pre-mixing one tablespoon of liquid castile with a cup of water is one approach described for scalp cleansing.
Pest deterrent spray (garden): One tablespoon of castile soap per quart of water is the concentration cited for spraying on insects.
All-purpose household cleaner: Dilution with water in a spray bottle is described for surface cleaning, with ratios varying by application.
Produce wash: Adding castile soap to a full sink to wash vegetables can help reduce dirt, bugs, and germs on the food, and may help lift off waxes or stickers. The soap should be rinsed off thoroughly before consuming the food.

The key is dilution — a small amount goes a long way, and adjusting the ratio to water allows the user to fine-tune the strength for different skin needs or household uses.

Safety Considerations

pH and Skin Acid Mantle

The most evidence-supported safety concern for castile soap as a topical cleanser relates to its alkaline pH. The skin's natural pH typically hovers around 4.7 to 5.75, which is slightly acidic. This acidity helps to form a protective barrier known as the acid mantle, which guards the skin against harmful microorganisms and environmental damage. Castile soap has a higher pH, usually between 8.9 to 9.1, making it more alkaline than the skin's natural pH. This alkaline property can affect the skin in multiple ways: frequent use of high-pH products like castile soap may disturb the skin's acid mantle, potentially leading to dryness, irritation, and increased vulnerability to bacteria and viruses.

Each application of castile soap temporarily destabilizes the acid mantle. Research shows that washing with alkaline soap causes a measurable rise in skin pH that persists for hours. That "squeaky clean" sensation — often misinterpreted as a sign of effective cleansing — actually signals barrier disruption.

The significance of this disruption is, however, debated. Regardless of whether the product is acidic or alkaline, all soaps and cleansers use the power of micelle formation to remove the acid mantle. However, the acid mantle is constantly regenerating, meaning it will return to its natural state in 30–90 minutes. Although pure castile liquid soaps are quite alkaline (pH 8.7–9.9), no cleanser can alter skin's pH in a way that causes permanent damage to the acidic mantle.

A controlled study comparing alkaline soap users with pH-balanced cleanser users documented that the alkaline soap group showed increased skin pH, proliferation of problematic bacteria, and elevated skin irritation. Persons with pre-existing skin conditions may be more vulnerable: studies have found significantly higher pH in both eczematous and uninvolved skin of atopic dermatitis patients compared to healthy individuals; this higher pH correlates with increased itching and dryness.

Contact with Eyes and Mucous Membranes

Castile soap's alkaline pH can cause eye irritation if it contacts the eyes. Dilution is generally recommended prior to use on the face or in any area near mucous membranes. If, after trying castile soap, the skin becomes red, dry, scaly, itchy, or a burning feeling is experienced, use should be stopped and a mild moisturizer applied for a few days until symptoms resolve. If symptoms persist or worsen after a couple of days, a healthcare provider or dermatologist should be seen. "Although castile soap is generally mild and tolerated by most people, everyone's skin is different."

Interactions with Hard Water

Castile soap reacts with the calcium and magnesium ions found in hard water, producing an insoluble soap scum that can leave a white film on skin, hair, and surfaces. This is an inherent chemical property of true soap (as opposed to synthetic detergent) and not a safety concern per se, but it affects cleansing efficacy and can contribute to a feeling of dryness or residue on skin and hair.

Hair and Colored Hair

Castile soap can be harsh on dyed or highlighted hair, stripping follicles of color. To protect colored hair, it's probably best to stick with a color-safe shampoo.

Plant Phytotoxicity

When used as a plant spray, castile soap can be a powerful insecticide, but because it can remove a plant's natural protective, waxy coating, spraying too much castile soap directly on plants could leave them more susceptible to pathogens or even burn them. Rinsing plants with clean water after application is recommended.

Use on Pets

It may be best to use unscented, natural castile soap on animals. Even natural ingredients such as essential oils can be irritating to a pet's skin. Additionally, pets tend to groom themselves, and ingesting other ingredients from the cleaner may be harmful.

Environmental Profile

Castile soap is one of the few personal care products that is truly 100% biodegradable. Its plant-derived ingredients break down completely in the environment within weeks to months, making it safe for use in off-grid settings like camping or van living where greywater may reach natural waterways. By contrast, synthetic surfactants in commercial soaps can persist in water systems and harm aquatic life.

Absence of Certain Synthetic Agents

Traditional castile soap is free from sodium lauryl sulfate (SLS), sodium laureth sulfate (SLES), synthetic fragrances, parabens, and triclosan — all agents that carry documented risks of skin irritation, allergic sensitization, or endocrine concern. This absence is, from a safety standpoint, one of castile soap's most well-characterized advantages for sensitive skin users, though it does not constitute therapeutic evidence for the soap itself.

Evidence Characterization Summary

The evidence base for castile soap as a named formulation is primarily composed of: (1) well-established soap chemistry and saponification science; (2) clinical and systematic-review evidence for topical olive oil, from which mechanisms are inferred but not directly transferred; and (3) dermatological expert opinion and observational consumer data. There are no large, double-blind, randomized controlled trials that use "castile soap" as the named intervention in a therapeutic context. Evidence from systematic reviews on topical olive oil (in non-saponified form) is generally positive for inflammatory skin conditions and wound healing, but these studies evaluate unadulterated oil rather than finished soap. The saponification process chemically transforms olive oil; minor bioactive constituents (polyphenols, vitamin E fractions) may not survive the reaction at pharmacologically relevant levels in the finished product. Any benefit attributed to olive oil polyphenols specifically should therefore not be assumed to apply to the soap without direct evidence.