How to make castile bar soap with rosemary and olive oil


What would you say if I told you that you can make 10 bars of natural rosemary and olive oil bar soap with just £6 and a couple hours to spare?

Soap-making is nowhere near as complicated as it is made out to be, and it is the cheapest way to avoid nasty chemicals in your skincare. So-called ‘natural’ soaps are made using the traditional method of combining an alkali with oils or fats in order to hydrolyse the oils into glycerol and fatty acids. The sodium/potassium in the alkali then bonds with the fatty acids to produce soap. This gives soap the special property that it is both water soluble and can dissolve grease or oils, making it a sort of bridge between water and oil. Industrially-produced soaps often contain harmful chemicals and usually cannot boast the same skin-care properties that natural ingredients can. Instead of paying a premium for naturally-made soaps, you can easily produce these in your kitchen for a lower cost. Making your own soap will also give you the option of deciding exactly which combination of vegetable oils and essential oils you would like to use.

All you need to make soap using this traditional method is: olive oil, water and sodium hydroxide (NaOH) also known as lye or caustic soda, which can be bought as ‘drain cleaner’ at hardware shops (if you’re over 21!).

I first made soap when I was 15 for a school project. I started making and selling soap at school events in order to raise funds for an team expedition to India. At the time I wasn’t particularly aware of what kinds of oils are good/bad to use, so I just picked the cheapest, which was a solid generic vegetable ‘fat’ from the supermarket. I made hundreds of bars of soap using this, and it worked really well. However, when I decided to revisit the idea of making my own soap, I had done some reading about palm oil and its effects on the rainforest in Borneo. I also wasn’t too keen on using synthetic dyes and fragrances due to the added chemicals and cost. So I decided to try a ‘castile soap’, which is made entirely from olive oil and has a wide variety of household uses.


After much searching on the web, I managed to find a soap formula that uses only olive oil, and I decided to combine this with a natural rosemary fragrance (because lets be honest here, there are few things that go better with olive oil than rosemary). Also my mum’s garden has a constant and free supply of rosemary. Olive oil is not the cheapest vegetable oil that can be used to make soap. However, I chose it because it is sourced from Europe, produces a nice hard bar even when used alone (unlike other vegetable oils), and it is great for your skin, producing a very mild lather that is great for sensitive skin (2). Rosemary is a very popular natural remedy, apparently preventing dandruff, acne and dermatitis, preventing cell-damage and moisturising skin. Rosemary is also supposed to relieve stress, boost the immune system and stimulate mental activity, “In a 2003 study, researchers found that of 144 test participants who inhaled rosemary oil during an exam displayed significantly higher cognitive function.” (1) (whaaaat?!) And of course, it smells wonderful!

If you have never made soap before, fear not. This is a beginner’s guide (I’m a beginner too). However, I also recommend that you do some background reading before you get started (see the bottom of this post for some recommended reading).


A beginner’s guide to making castile soap using rosemary and olive oil

Please note: I still consider myself a beginner soap-maker and the process outlined below uses dangerous chemicals so use my soap-making instructions at your own risk.

What you will need:

  • digital kitchen scale (absolutely necessary that its digital – measurements need to be really exact)
  • rubber gloves and some sort of eye protection (very important – sodium hydroxide can cause nasty burns)
  • meat thermometer (necessary – can be bought on amazon for about 4£)
  • large stainless steel pot, at least 2L
  • one smaller heatproof bowl or pot (not aluminium)
  • stick blender or large wooden spoon (stick blender highly recommended)
  • something to use as a soap mould that will hold at least 1.5L (loaf tin, tupperware container, brownie tin… anything that can withstand a bit of heat will do)
  • greaseproof paper to line the soap mould
  • cheap malt vinegar to neutralize the alkali when cleaning up (optional)
  • heatproof mat or a surface that will resist heat (you can use your hob or a wooden chopping board)


  • 900g cheap olive oil, the cheaper the better for soap-making (this is about 1L but please measure by weight)
  • 112g NaOH (aka sodium hydroxide, lye or caustic soda) – Note: you need to be 21 or older to buy this in the UK. Can be bought at hardware stores as ‘caustic soda’ drain cleaner)
  • 226g tap water
  • 5ml rosemary essential oil
  • handful fresh rosemary leaves

Summary of steps:

1. Line the soap moulds with greaseproof paper
2. Heat the olive oil in the large pot
3. Pour the water into the heatproof bowl and place it on the heatproof mat
4. Add the NaOH to the water. Always pour the NaOH into the water. I am told that bad things will happen if you do it the other way around.
5. Wait until both the olive oil and NaOH-water mixture are between 40-45 °C. Switch off the hob.
6. Add the NaOH-water mixture to the oil, and mix with the blender periodically for 20 minutes until ‘trace’ is achieved
7. Add the rosemary leaves and fragrance
8. Pour your soap into the mould

Now in more detail:

The first thing you need to do is gather all your materials and ingredients and prepare for all the steps. Open all the windows and turn on the extractor fan before you begin, as mixing NaOH with water will produce nasty fumes. Make sure you have a basic idea of the steps above before you begin. Remember that when handling sodium hydroxide you should wear rubber gloves and protective eyewear and always pour the chemical into the water, not the other way around. I am told that sodium hydroxide reacts with aluminium, so don’t use any aluminium kitchen equipment.

1Line your soap moulds with greaseproof paper.





2Weigh 900g olive oil and pour into the large pot. Put this on the hob on medium heat. You will need the oil to reach between 40-45°C, but while that is happening you can get your NaOH mixture ready.




3Weigh 226g tap water and pour into the heatproof bowl/pot. Place this on the heatproof mat, as it will get really really hot (~60°C) when it reacts with the NaOH.





4Put on your rubber gloves and protective eye wear. Make sure the windows are open and the extractor fan is on.





5Weigh 112g NaOH in a small bowl. Then pour this into the heatproof bowl with the water in it. The sodium hydroxide and water will react vigorously and heat up quickly, so step back to avoid breathing in the fumes. After a minute or two, you can take a plastic or steel spoon to lightly stir the mixture so it keeps reacting. Check the temperature with the thermometer. You need to wait until it has finished reacting (no crystals left in the bottom) and cooled off to 40-45°C. This may take between 5-10 minutes.


6.2While you are waiting for that to happen, you can check the temperature of the oil. Rinse your thermometer under the tap first and pour a bit of vinegar on it to neutralise it (I’m not sure if this works, but I do it anyway. If you’re studying chemistry your wisdom would be much appreciated). You also want the oil to be between 40-45°C, so use your judgment to turn the heat up/down.




When the NaOH-water mixture and oil are both between 40-45°C, switch off the hob and pour all of the NaOH mixture into the oil.





8Use your stick blender to stir and blend the mixture periodically for about 20 minutes. I used a 600W stick blender on the medium setting and blended in circular motions for a couple minutes, then off for a couple minutes. It will start to look more opaque and thicken after a while (5-10 minutes). This process is called saponification. Soon you will notice the soap mixture coating the blender and sides of the pot. This is the beginning of the ‘trace’ stage. You can add the rosemary essential oil at this stage.


9After about 20 minutes, when the mixture gets so thick that blender starts leaving traces in the soap and you notice that soap dripping off the blender leaves marks in the mixture, then you know that your soap is ready! Add the rosemary leaves and stir (don’t blend!). If your soap reaches this stage before 20 minutes or takes longer, don’t worry – this is just because different qualities of olive oil take different amounts of time. Always go by the consistency of your mixture over the time it takes to get it there.


10.2Pour the soap into the moulds. It needs to stay there for about 24-48h until solid enough to be cut.





After a day or two, take the soap out of the moulds, cut it into bars and leave these bars to ‘age’ for 4-8 weeks before using. You want to make sure that each bar has access to air while it ages (don’t stack them).


– Kasia


Good introductions to soap-making:

If you want to change the proportions of the recipe, use this lye calculator to help you figure out how much lye and water you need:
The Sage Lye Calculator


One thought on “How to make castile bar soap with rosemary and olive oil

  1. Pingback: Lavender soap and a very good cause | The Conscious Student

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