Let’s talk about water


One of the most common questions I get asked here in New Zealand is “Do I need to use distilled water when I make soap?” To answer this, we need to take a closer look at the water you use. Where does your water come from? When rain water falls, it is mostly free from impurities, but as it makes its way through the soil and rocks, the water dissolves and picks up minerals, such as calcium and magnesium. Water containing minerals is called hard water, whereas rain water and treated water very low in dissolved minerals is called soft water.

In your home, hard water is the cause of lime scale and soap scum, which is when the minerals in the water bond with the soap forming a greasy film. Another effect that hard water has on soap is that it tends to reduce the quality and amount of lather in your soap.

In soap making, hard water can also affect the saponification process, i.e. when the dissolved minerals in the water react with the other chemicals, leading to inconsistencies  and poorer quality of your soaps. However the most common problem with hard water is the increased chance of getting DOS (= Dreaded Orange Spots) in your soaps, a term used in the soap making community to describe the localised oxidation (orange spots) of unsaturated soft oils, such as olive oil, sunflower oils and rice bran oil. These DOS not only affect the appearance of the soap, but will also cause the oxidised oils in the soap to go rancid.

glass of water

However, the good news is that most of the tap water here in New Zealand is rain water or comes from a rainwater reservoir. So we can, with ease of mind, use tap water to make soap. For those on bore water (water from underground aquifers), don’t despair! You can either use distilled water or you can add chelators to your soap. Distilled water is available from your supermarket or hardware store, and is fairly cheap.

To find out if you are on bore or reservoir water, check with your water supplier or local council!

If you want to use chelators to soften your water, the most common chelators used in soap making are citric acid, sodium citrate and EDTA (ethylenediaminetetraacetic acid). They work by binding the minerals, basically rendering them inactive, and thus improving the lather and effectiveness of the soap. Citric acid is probably the easiest to get hold of here in New Zealand, with the added bonus that it is also considered a natural additive. If you intend to use citric acid to soften your hard water, usage is 10 g of citric acid for every 200 to 300 ml of water (roughly the amount of water used in a 1000 g oil recipe). Mix the citric acid into the water before you add the caustic soda. However, because citric acid also neutralises, you will need to add extra caustic soda to compensate. For each 10 g of citric acid used, you will need to add an extra 6.24 g NaOH to your recipe.


Another dilemma many beginner soap makers have is should you weigh or measure your water? As you may have noticed in the previous paragraph, or in my tutorials and workshops, I usually give the water amount in millilitres. Water is the only ingredient in your soap that doesn’t matter if you are a couple of grams out, or even 10 grams!

The reason for this lies in the purpose of water in the soap making process. If I were to add the caustic soda, which comes in pellets or flakes, directly to my oils and fats, guess what would happen? Nothing. Water is the medium I need to dissolve the caustic soda, so that its molecules can react with the fat and oil molecules to produce soap (and glycerin). That is the only purpose of water. It isn’t part of the actual chemical reaction during the saponification process, and eventually the water needs to be removed again, which is done through curing.

So how much water I use doesn’t really matter, as long as I use enough to dissolve all the caustic soda, right? Wrong. Even though water isn’t part of the chemical reaction itself, it does have an impact on the speed and temperature of the soap making process. Saponification is an exothermic reaction, meaning it produces heat. The amount of water will let you control both the speed and the temperature of the chemical reaction. By adding more water to a recipe, you can slow down the saponification process and lower the temperature of the soap during this process. You add more water if you need more time, such as when doing colour work and swirling or when you have a lot of fast moving oils in your recipe. You also add water if you need to lower the temperature of the process and prevent the soap overheating, for example when using certain ingredients, particularly those containing sugar, such as honey. And when using additives like clays and flours, which absorb water, or when you add fresh ingredients or water containing ingredients to your soap, you will also need to consider the amount of water you are using in your recipe.

Although adding more water in a recipe can be useful, it does have its drawbacks. More water slows the saponification process, which means the soap will take longer to thicken and reach trace. And all the water you add to your soap will eventually need to evaporate again during the curing to make for a nice, hard bar of soap. More water equals longer curing time. More water can also make a soap softer and stickier, making it more difficult to unmold. And another major disadvantage is that soaps with a high water content are prone to forming glycerin rivers. Although perfectly fine to use, the translucent streaks or rivers are usually regarded as an undesired effect in soaps.

Using less water is known as adding a ‘water discount’ to your recipe. Where more water slows down the process, less water will speed it up. This is an advantage when using a lot of slow moving oils and those with long curing times, such as olive oil. My olive oil soap recipes usually have water discount, which cuts down the curing time significantly.  However, discounting your water in a recipe will speed up the chemical reaction and increase the temperature, making it more difficult to control the soap making process, which is why I usually recommend to attempt water discounting to advanced soap makers only. In rare cases, a water discount can cause a soap to overheat to the point of quickly expanding and rising out of the mold, a phenomenon  known as the Volcano Effect. Although spectacular to witness, the design of the soap will be ruined (although the soap can still be used).

How much water you should use in a recipe depends on the oils you use (slow or fast moving), the design of your soap (i.e. colour work or swirling), and your additives and the effect they have on the saponification process. You’ll notice different soap makers and soap calculators will use different methods to calculate their water. Some use a set amount of water, usually something between 250 – 300 ml of water per 1000 g of oils. Others calculate a percentage of the total weight of oils or total recipe, somewhere between 22% to 33%. The most accurate method, however, is to calculate the amount of water in relation to the amount of caustic soda in the recipe.

The standard amount I use in my soap recipes is a 33% lye solution, which is twice as much water as caustic soda. So for example, if a recipe calls for 130 g of caustic soda, I would use 260 ml of water. Most of my recipes will use a 33% lye solution. If I add a water discount to a recipe, for example when using certain ingredients or to avoid glycerin rivers, I might go up to a maximum of a 40% lye solution. Anything more than 40% and you’ll end up with a very fast moving (accelerating), high temperature chemical reaction, which would be very difficult to control. Also never use a lye solution stronger than 50% (equal parts of water and caustic soda), because there won’t be enough water to dissolve the caustic soda.

How to calculate your water:

25% lye solution
weight of caustic soda (g) x 3 = water (ml)


33% lye solution
weight of caustic soda (g) x 2 = water (ml)


40% lye solution
weight of caustic soda (g) x 1.5 = water (ml)


Note: 1 gram of water equals 1 ml water

To understand and see the effects different strengths of lye solutions can have on soap, check out this blog post by Auntie Clara, who created a stunning Ghost Swirl soap by using only different strengths of lye solutions. And while you’re there, have a browse through her soap gallery. Her soaps are absolutely amazing!

Lastly, a friendly word of caution: Adding a water discount will increase the strength of your lye solution, making your lye more potent and more dangerous! Be careful when working with lye. Always wear protective gear, such as safety goggles and disposable gloves to protect your eyes and skin. Stay safe!

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