Can You Make Your Own Castile Soap? 7 Mistakes to Avoid

Castile soap is loved for its eco-friendly properties, gentle cleansing, and versatility. It’s traditionally made from olive oil and other vegetable oils, and is naturally biodegradable and free from synthetic detergents, which is why dermatology sources often describe it as a mild, plant-based cleanser that suits people who prefer simpler ingredient lists. You can see a clear explanation in this Cleveland Clinic overview of Castile soap, and a more ingredient-focused breakdown in Dr. Bronner’s guide to what Castile soap is and how it’s used.

In this guide, we’ll walk you through the most common mistakes people make when attempting to make Castile soap at home, and how to avoid them for a successful, satisfying result.

Why Make Your Own Castile Soap?

Homemade Castile soap allows you to:

• Control ingredients – You can choose your own blend of oils, essential oils, and botanicals, and avoid additives you don’t want. If you’re trying to keep your routine low-toxin and planet-friendly, you might also like our guide to eco-friendly, chemical-conscious soaps in the UK, which shows how simple ingredients can still feel indulgent.

• Reduce waste – Making soap in bulk lets you minimise plastic packaging and reuse your own moulds and storage containers. This fits neatly alongside other low-waste bathroom swaps; our feature on eco-friendly bath products in the UK gives more ideas if you’re building a greener routine around your handmade bars.

• Save money over time – Once you have your basic tools, buying oils and lye in sensible quantities can be more economical than repeatedly purchasing small, finished bars.

• Support sustainability – You can prioritise organic, fair-trade or locally produced oils and essential oils, and avoid palm or other ingredients that don’t fit your values.

7 Mistakes to Avoid When Making Castile Soap

1. Using the Wrong Type of Oil

Traditional Castile soap is made primarily from olive oil, which provides gentle cleansing and moisturising properties. However, many people attempt to substitute olive oil with coconut oil or other oils without understanding their properties.

Why It’s a Problem:

• Coconut oil can create a soap that is too harsh and drying.

• Other oils may alter the consistency, lather, or effectiveness of the soap.

How to Avoid It:

Stick to high-quality extra virgin olive oil for the base. If you want to add other oils, research their effects and proportions carefully.

2. Not Measuring Ingredients Accurately

Soap-making is as much a science as it is an art. Precise measurements are crucial to achieve the right balance of oils and lye (sodium hydroxide).

Why It’s a Problem:

• Too much lye can make the soap harsh and unsafe for skin.

• Too little lye can result in a greasy, unusable product.

How to Avoid It:

Use a digital scale to measure all ingredients (oils, lye and water) by weight rather than volume, and follow a trusted recipe. Online lye calculators such as SoapCalc and similar tools recommended in round-ups of reliable soap calculators can help you confirm the correct lye-to-oil ratio and superfat level before you start.  This keeps your batch within a safe, skin-friendly range and reduces the risk of harsh or greasy soap.

3. Skipping the Curing Process

Curing is the process of letting your soap harden and dry for several weeks after it’s been poured into molds.

Why It’s a Problem:

• Skipping or shortening the curing time can leave your soap soft and prone to dissolving quickly.

• The soap may still contain active lye, which can irritate the skin.

How to Avoid It:

Allow your soap to cure for 4–6 weeks in a cool, dry, well-ventilated place, with bars spaced out so air can circulate. Makers and educators often explain that curing isn’t just about drying: it gives time for saponification to fully complete and for excess water to evaporate, which leads to a harder, longer-lasting, milder bar. A practical walkthrough like this guide to curing handmade soap shows why most cold-process soaps benefit from at least a month of curing before regular use.

4. Adding Essential Oils at the Wrong Time

Essential oils are often added for fragrance or therapeutic benefits, but timing is everything when it comes to incorporating them into your soap.

Why It’s a Problem:

• Adding essential oils too early can cause them to evaporate during the soap-making process.

• Adding them too late may result in uneven distribution.

How to Avoid It:

Add essential oils once your soap batter has cooled and reached trace – the stage where it thickens to a light custard or pudding consistency. At this point, the mixture is cool enough to reduce unnecessary evaporation, and thick enough to help the fragrance distribute evenly. Soap-making educators note that essential oils are volatile and can flash off if added when the batter is too hot, which is why most tutorials recommend stirring them in at or near trace, as explained in this guide on using essential oils in soap recipes.  If you’re interested in how essential oils work more broadly in skincare, you may also enjoy our piece on the benefits of essential oil soap for natural fragrance and skin comfort..

5. Ignoring Safety Precautions

Lye is a necessary ingredient in soap-making, but it’s also highly caustic and can cause burns if handled improperly.

Why It’s a Problem:

• Failing to follow safety measures can lead to accidents and injuries.

How to Avoid It:

• Always wear gloves, eye protection (goggles or a face shield), and long sleeves when handling lye or fresh soap batter.

• Work in a well-ventilated area and keep children and pets away from your workspace.

• Use heat-resistant, lye-safe containers (stainless steel or suitable plastics) and tools.

• Slowly add lye to water – never water to lye – to prevent dangerous splashing or eruptions.

Comprehensive safety guides such as this lye safety guide from Bramble Berry and broader round-ups of soap-making safety precautions emphasise that sodium hydroxide is safe to work with when you respect its hazards, suit up appropriately, and label everything clearly.

6. Using Tap Water Instead of Distilled Water

Water is an essential ingredient in soap-making, but the type of water you use can affect the final product.

Why It’s a Problem:

Tap water often contains minerals (like calcium and magnesium) and other impurities that can interact with lye and fats, potentially causing soap scum, shorter shelf life, or unpredictable results. Soap-making resources frequently recommend using soft, impurity-free water so your recipe behaves consistently; for example, this explainer on distilled vs tap water in soap making points out that minerals in tap water can contribute to rancidity and aesthetic issues over time.

How to Avoid It:

Use distilled or deionised water for dissolving your lye and any water-based additives. It’s a small change that helps you avoid unwanted reactions and gives your bars a cleaner, more predictable finish.

7. Not Testing the pH Level

The pH level of your soap helps you gauge whether it’s likely to feel gentle or harsh in use.

Why It’s a Problem:

• Very high pH can feel harsh and drying, particularly on sensitive or compromised skin.

• Soap that hasn’t fully saponified can contain active lye, which can irritate or even burn.

Research measuring the pH of commercial soaps and shampoos has shown that many solid soaps sit around pH 9–10, while healthy skin tends to prefer a slightly acidic environment (approximately pH 4.5–5.5).  That difference doesn’t automatically make well-formulated soap unsafe, but it does explain why testing and good formulation matter. Educational pieces on the pH of natural soap emphasise that most traditional soaps fall between pH 8–10 and that other factors (like superfatting and ingredients) also influence how they feel on the skin.

How to Avoid It:

Once your soap has cured, test the pH using pH strips or a reliable pH meter if you have one. Aim for a pH in the region of 8–10, which is typical for traditional soap but still within a range considered acceptable for rinse-off products. If your readings are unusually high or your skin reacts badly, review your recipe, double-check your lye calculations with a tool like SoapCalc, and adjust future batches accordingly.

Best Practices for Making Castile Soap

1. Follow a proven recipe

   
   

   Use a tested Castile soap recipe from a trusted book, teacher, or soap-making site, and run it through a lye calculator such as SoapCalc to confirm the lye amount and water ratio before you begin. Step-by-step tutorials like this lye calculation walk-through using a saponification chart can help you understand what’s happening behind the numbers.

2. Invest in quality tools

   
   

   Use stainless steel, heat-resistant plastic or silicone equipment that won’t react with lye. Keep your “soap gear” separate from your everyday cooking equipment so there’s no cross-contamination.

3. Be patient and keep notes

   
   

   Soap-making rewards patience and observation. Record your oil percentages, temperatures, trace time, and curing conditions so you can repeat good batches and troubleshoot any problems. Over time, these notes become your own personal recipe book.

If you prefer to focus on enjoying beautiful, gentle Castile soap without the science and safety steps, you can always balance your DIY experiments with ready-made options from our handcrafted Castile and olive-oil soap collection.

Final Thoughts

Making your own Castile soap can be an enjoyable and sustainable way to customise your skincare products. However, it’s essential to avoid common mistakes that could compromise the quality or safety of your soap. By following best practices and using high-quality ingredients, you can create a gentle, eco-friendly soap that benefits both your skin and the planet.

If DIY isn’t your thing, or you’d simply like a reliable bar to use while your own batches are curing, you can explore The Humble Shepherd’s collection of handcrafted Castile and olive-oil soaps. Each bar is made with carefully chosen plant oils, gentle formulations and eco-conscious packaging, so you still get all the simplicity and sustainability you’re aiming for, without having to handle lye yourself.

If you’d like to be the first to hear when new small-batch soaps are released, you can join our Shop Now waitlist, or browse more of our soap-making and skincare guides on the blog for further inspiration before your next project.

Discover the joys of Castile soap — whether you make it, buy it, or mix the two approaches, you’re still taking a thoughtful step towards skincare that’s kinder to your skin and the environment.