Basil Lime Soap CPOP

Cold Process Soap

Difficulty: Beginners
Time: 1 hr
Yields: 1250 g soap

I’d like to introduce you to the CPOP (cold process oven process) technique. This is making your soap using the normal cold process method, but then we place the soap in the oven to induce the gel phase. There are many other ways to force gel in soap making, like insulating it with towels or putting it in a cardboard box, but since a good friend of mine, Skye from Shaddix & Co, introduced me to this technique, the quality of my soaps has improved so much. 

What is the gel phase?

The gel phase during soap making is when the chemical reaction reaches its optimum. The saponification process, turning the oils and lye into soap, is an exothermic reaction, which means it produces heat. The faster the reaction, the more heat it produces. If it produces too much heat, your soap can start to expand and literally ‘erupt’ out of the mould, the so called soap volcano.

Soap volcano in real time.

If the reaction is slow, your oil/lye mixture will still turn into soap, but it will go at a much slower pace and can can take up to 2-4 weeks to fully complete. The gel stage is when your the soap goes through the chemical reaction and, technically, the saponification is completed. Similar to hot process soap making. You can easily recognise the gel phase: the soap will go a dark translucent colour and appear gel-like before cooling down and turning solid and opaque again.

Why force gel?

Some soaps go through the gel phase, some don’t, and some only go through partial gel phase. Soaps that have gone through the gel phase appear brighter and more vibrant in colour, have this sheen to it, a bit like porcelain, and are usually firmer and harder and more resistant to going mushy in the soap tray. However, some soaps only manage the gel phase in the centre of the soap before they cool down too much and the reaction slows down. And some soaps never reach the gel phase at all. Those are usually the ones with swirling and colouring techniques, when I adjust the recipe to slow down the reaction to give me more time to work with the soap and swirl. Or castile soaps. Soaps with a high percentage of olive oil and little to no saturated fats (hard oils) struggle to reach the optimal reaction speed aka gel phase. But we can help it along by either insulating the soap or by placing the soap in a heated environment. Insulating the soap will contain the heat the soap produces from the reaction. This is more an uncontrolled method and can be a hit and miss. A far better method, and one in which you control the heat, is by placing the soap in the oven. By forcing gel, you ensure that all the soap goes through the gel phase and you are not left with this annoying ring of colour in your soap.

Please note, some ingredients, like sugar, milk, and even some fragrances, will add heat to the chemical reaction (like fuel). You don’t want to put those soaps in the oven, but rather you have to watch out that they don’t over-heat.

How to force gel in the oven

Place the soap in the oven at 50 degrees Celsius or 120 degrees Fahrenheit for 1-2 hours. Ideally you can keep an eye on it and check for the gel phase to begin. Once you see it starting, turn off the oven, but don’t open the door. Leave the soap in the oven overnight, or for at least 2-3 hours until the gel phase has completed.

Basil Lime Soap

My favourite kind of soaps are actually the very plain, simple soaps. This one is my current summer soap. I love the combination of the basil and lime essential oils: fresh and slightly herb – a radical change from the sweet and fruity soaps I’ve been using all winter. The recipe makes a nice hard bar of soap, and after force gelling, you end up with this gorgeous white bar of soap with a beautiful porcelain sheen to it.

If you have never made cold-process soap before, I strongly suggest you check out the basic cold process soap tutorial first.

Before starting, please read the safety and precautions post, especially since this tutorial requires the handling of caustic soda!

ONE: First, prepare your lye. Weigh out the caustic soda in a small container. Measure the water in a small pyrex or other heat proof glass jug or if you are using plastic, make sure it is polypropylene (PP) plastic. Then carefully add the caustic soda to the water and gently stir until all the caustic soda has dissolved. Stir in two teaspoon of sodium lactate. Sodium lactate is a natural liquid salt and will make your soap harder. Set the lye solution aside to cool down.

TWO: Weigh out the coconut oil in a microwaveable bowl or container, and heat it in microwave on high for 2-3 minutes or until it has completely melted. Alternatively, you can melt the coconut oil on the stove.

THREE: Weigh and add the olive oil, sunflower oil and castor oil to the now melted coconut oil.

FOUR: Add the essential oils to the oils and give it a good stir with a whisk. I’m using 25 ml of lime essential oil and 10 ml of basil essential oil here.

FIVE: Check if the lye has cooled down to approximately room temperature, by placing a hand on the outside of the jug, and if it feels cool or only slightly warm, it is time to add it to the oils.

SIX: Making sure you are still in protective gear (goggles and gloves), carefully pour the lye to the oils, avoiding any splashes. Using your stick blender, mix (on low-medium if it has settings) for about 5-10 seconds until the soap mixture starts to thicken. For those of you who work with trace, you want it at medium trace – thickened but still fluid enough to pour.

SEVEN: Pour the soap into the mould. Tap the mould gently on the bench a few times to get rid of any air bubbles and to even out the surface.

EIGHT: Now it’s time to force the gel phase. Place the soap in the oven and set the oven to 50 degrees Celsius or 120 degrees Fahrenheit.

NINE: After one hour, turn off the oven, but leave the soap in the oven and especially: DO NOT OPEN THE OVEN DOOR! You don’t want any of the heat to escape. Leave the soap in the oven overnight.

TEN: The next morning, cut your soap into bars and cure the bars for another 6-8 weeks. (Yes, you still need to cure the soap – this is to remove the extra moisture in the soap, and to create a better and harder bar of soap!)

Basil Lime Soap

  • Difficulty: easy
  • Print
Before starting, make sure you wear protective goggles and gloves and work in a well-ventilated area, free from any distractions!


  • 350 g coconut oil (35%)
  • 450 g olive oil (45%)
  • 150 g rice bran oil (15%)
  • 50 g castor oil (5%)
  • 138 g caustic soda (8% SF)
  • 280 g water (2:1 water to lye ratio)
  • 2 teaspoon sodium lactate
  • 25 ml lime essential oil
  • 10 ml basil essential oil


  1. Weigh out your water in a lye-safe jug and weigh out your caustic soda (NaOH) in a separate small container or paper cup.
  2. Carefully add the caustic soda to the water, avoiding any splashes, and stir gently until all the caustic soda has dissolved.
  3. Add the sodium lactate. Set aside to cool.
  4. Weigh out the coconut oil in a pyrex jug and heat in the microwave on high for 2 minutes or until melted. Alternatively, melt on the stove.
  5. Add the other oils to the now melted coconut oil.
  6. Add the essential oils and give the oils a good stir with the whisk.
  7. When the lye has cooled down to room temperature, add the lye to the oils and, using your stick blender, mix for about 5-10 seconds until you have a medium trace (the soap has started to thicken, but you can still pour it).
  8. Pour the soap into the mould and tap the mould on the bench a few times to disperse of any bubbles and to even out the surface.
  9. Place the mould in the oven and set it to approximately 50 degrees Celsius or 120 degrees Fahrenheit.
  10. Let it warm up it for 1 hour and then turn off the oven, but leave the soap in the oven and keep the doors of the oven closed.
  11. Leave the soap in the oven overnight.
  12. The following day, unmould and cut the soap into bars.
  13. The bars will need to cure for another 6-8 weeks before they are ready to be used.


    • Hi Deb! Unfortunately many soap makers think that hot-processing or oven gelling reduces the curing time. The only thing it does is speed up the saponification, meaning that the chemical reaction has finished and the soap won’t be caustic anymore. Technically yes, you can use your soap immediately after hot-processing and even oven gelling if done correctly. However, curing is not about the chemical reaction or about caustic soap. Curing is about removing the excess water contained within the soap. When we make soap, we add extra water to allow us to work with the soap. The chemical reaction (combining oils and lye to make soap) only requires water in equal amount to the caustic soda in your formulation. However, the reaction would be really fast, which in soap makers terms is called acceleration or fast moving. It is too fast to work with. So to slow down the reaction, we add extra water. It gives us time to make our soap, but it also means that we need to cure it to remove the excess water afterwards. Why? Soap that contains water within will dissolve easier leading to mushiness that we want to prevent. I like comparing it to clay soils. When soils that are really dry, completely dried out, and it rains, the rain will just flood the area, because the water cannot penetrate into the soil. When clay soils are already wetted and it rains, it gets really muddy like the stuck in the mud kind of situation. The same goes for soap, except we want the really dried out soap, so that when we use the soap, we only remove some of the outer layer, and none of the water penetrates into the soap. The more water a soap contains, the easier it is for water to penetrate into the soap, turning more and more of the outer layer of the soap into mush. I hope that explains the curing process a bit and why you cannot shorten it. The ideal way of curing is to weigh your soap regularly, and once it has stopped reducing in weight and the weight remains constant, you know that all the excess water has evaporated out of the soap. (There is one more reason for curing which as to do with the growth of the soap crystals, and why soaps such as marseille soaps and aleppo soaps are cured for a year or more, but I won’t go into the detail for that. The bottom line though is that the longer you cure the soap, the better quality soap you will have)

  1. Hi Jackie

    Looking forward to making this, but wanted to check first if the oven method is suitable for individual soap moulds, or just the loaf stye? I don’t want to end up with soap volcanoes in my oven lol

    • Yes, you can use the oven gel method for individual moulds too, as long as the moulds themselves are suitable up to a heat of about 70 deg C, which is less than boiling temperature. You can always test this by pouring boiling water in the mould and see if it warps. If it doesn’t, you’re safe!

  2. This looks like a lovely recipe and I really want to give it a go. I think there’s an error somewhere though – the instructions state: “THREE: Weigh and add the olive oil, sunflower oil and castor oil to the now melted coconut oil.”, but in the ingredients there’s no sunflower oil listed. It seems to have RBO listed instead. Is this recipe formulated to be made with sunflower oil or rice bran oil? That is my only confusion.

    • Oops, sorry! Thank you so much for pointing out the mistake. I’ve fixed it. It’s definitely rice bran oil and no sunflower oil in this one. Thanks again for spotting this.

    • You can process it like the other recipes, but I’ve learned in the meantime that if you oven process your soaps, you do get better quality soaps that are harder and last longer.

  3. Hi there, i recently discovered that basil oil is regarded as carcinogenic and is use in cosmetics and soaps tightly regulated in the EU. the allowed usage rate is very very low – around 0.03%. This recipe is much this something you know about? I also notice that basil is very pungent when used at a high rate…i found this out the hard way recently when I added some to a soap!

    • Hi Ana! You are right, some basil essential oils contain constituents, such as estragole, which are considered carcinogenic. But not all basil essential oils are equal and it also depends on various factors, such as if and how much of a constituent is in an essential oil. Most of the widely used basil essential oils is not of the estragole chemotype, but of the linalool chemotype which is not carcinogenic. Additionally, we are also talking about soap here, which is a rinse off product. Something that does not remain on your skin (as opposed to the leave on products such as moisturisers). In any case the amount of basil essential oil in this soap is 1%. For more Information visit

    • Thanks, Kelly! And the green clay is a great suggestion! For those intending on using it, mix about 1/2 to 1 tablespoon max with equal amount of water before adding at trace.

  4. Hi Jackie,

    I love your soap blogs thank you for sharing! Do you have a really nice recipe to share for a creamy rich body ?

    • Hi Nicolle! So happy you like my blog. Body lotions, creams and other moisturisers are coming to my blog this year. Keep checking back!

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