Many soapers use one of three approaches for diluting their soap --
Method 1. Dilute to a honey-thick consistency for use in a squirt bottle or pump bottle. The water content is controlled to get the desired consistency. No additional thickener is added.
Method 2. Dilute to a specific pure-soap content for use in a foamer dispenser. The consistency is watery. No additional thickener is added.
Method 3. Dilute to a specific pure-soap content and then thicken with an additional thickener to the desired consistency. There are two classes of thickeners that can be used --
Method 3a. Table salt (sodium chloride) or other salts
Method 3b. Cellulose- or gum-based thickeners
Method 3c. Synthetic detergents
Use the right soap recipe
If you want your soap to be honey-thick by dilution alone (Method 1) or if you want to use salt for thickening (Method 3a), the soap should contain about 50% oleic acid.
If the oleic acid content is a lot lower than 50%, it will be near impossible to get thick soap by dilution alone (Method 1) or by using salt (Method 3a). A separate non-salt thickener must be added (Method 3b).
Liquid soap that is all or largely made from coconut oil is an example
If the oleic acid content is a lot higher than 50%, the soap might be pourable for a short while after dilution, but it is likely to revert back to a non-pourable jelly, even after adding a lot of water. Avoid this problem by reformulating the recipe to lower the oleic acid to about 50%.
A 100% olive oil (castile type) liquid soap is an example
Use only water for dilution
Never dilute liquid soap with honey, aloe, milk, beer, or other liquids that contain sugars, starches, or proteins.
These sources of food greatly increase the risk of microbial growth in the soap. Remember -- microbes do not have to be visible to the human eye to cause infections or disease, as anyone who has had food poisoning will know. Play it safe with your soap!
Use only water to dilute liquid soap, preferably distilled water or reverse osmosis water. I recommend distilled or reverse osmosis water for best results. Filtered rainwater can be used in a pinch. If you only have drinking water for dilution, it can be used, but be aware that minerals dissolved in drinking water may make the soap cloudy and reduce the lather.
Heat is helpful but not required
I dilute my soap paste using warm water but I otherwise do not heat the soap. This is a hands-off method that is energy efficient. The tradeoff is the soap will need several hours to a day to fully dilute.
Other people want faster results so they use a hot water bath (bain marie), oven, or a crock pot. The added warmth speeds dilution, but you do need to pay more attention to the process.
Check the thickness of the soap at room temperature
Always evaluate the consistency of diluted soap at normal room temperature before deciding whether to dilute further.
If you thicken the soap enough to suit you while it is warm, the soap will most likely be too thick at room temperature. Liquid soap normally thickens and sometimes becomes less clear when it cools.
Method 1. Dilute to honey thick
Do not add all the dilution water at once. Many liquid soap making tutorials use the "dilute to honey thick" method. They often give the impression you can measure X amount of water in proportion to the paste, mix the whole amount of water with the paste, and end up with a perfect honey-thick soap every time. If your recipe varies from the tutorial, your soap may not dilute the same way as the author's soap.
You will get better results if you remember "slower is faster" when diluting, especially when making a recipe new to you. Keep good notes about how much water you use, so the dilution process will go faster the next time you use the recipe.
Add about 1 part warm water by weight to 2 parts soap paste. Mix the water and paste together as best you can. A potato masher works well to break up large lumps in the beginning. Later on, a whisk, spatula, or even a stick blender will be helpful.
When this first portion of water is fully mixed into the paste, check the thickness and consistency. If more water is needed, add 1/2 part warm water to the mixture. Mix and check. If more water is needed, add 1/4 part water. Check. Continue diluting with decreasing amounts of water until the soap is close to the desired thickness. Near the end of dilution, you may be adding 1 teaspoon of water at a time.
Sometimes lumps of firm paste or a floating layer of firm soap refuse to dilute nicely. If that happens, you can remove those stubborn bits and dilute them in a separate container. This reduces the chance you will over-dilute the main portion of soap.
Susie explains how she uses this method of dilution in her tutorial. Look in Post #1 for the line "To dilute" in this tutorial.
For a tried-and-true recipe, you can speed the dilution process a bit --
Add about 3/4ths of the total dilution water you used the last time.
Check the thickness, add a bit more water if needed, check again, and so on.
Pros and cons of the "dilute to honey thick" method
When I dilute liquid soap as described above until the soap has a honey-thick consistency, the amount of pure soap in the finished product ranges from 30% to 40%.
For general-purpose handwashing and bathing, soap this concentrated is not necessary for these reasons --
Concentrated liquid soap is wasteful. When people wash with bar soap, they usually rub the bar enough to get the right amount of soap for the job. When people use a liquid soap product, they tend to squirt the same number of dollops onto their hand or washcloth. Even if half a dollop is enough, many people will still use a full squirt or more.
Concentrated soap can over-clean the skin and strip away desirable skin fats. The result is the skin can feel dry and tight, even irritated. This will be especially true if the liquid soap recipe includes a high percentage of coconut oil (in other words, a soap high in lauric and myristic acid).
Methods 2 and 3. Getting started -- decide on the pure-soap content
The "dilute to honey thick" method is simple, but it is not always the most ideal method. As an alternative, consider diluting the product until the pure-soap content is 8% to 20%. (1) Here are some of the advantages of diluting the soap to a specific pure-soap content --
Reducing product waste while still cleansing well
Reducing the chance of drying or irritating the skin
Building lather more easily
Controlling costs if you sell
I created a simple calculator to help you estimate the water needed to get a diluted liquid soap with a specific concentration of pure soap --
This calculator will open in a new browser window. If you are interested in the math, the formulas in the calculator are explained in the Extra credit section below.
Method 2. Dilute for use in foamer dispensers.
The soap will probably need to be diluted to about 10% pure soap content, but this percentage is just a rough number to start with. You will want to experiment to find the pure-soap content that works best for your particular foamer dispensers.
Method 3. Dilute and then thicken with a separate thickener.
If you want the soap to have a thick syrup or gel texture, I suggest diluting to a 15-20% pure soap content and then using a separate thickener to increase the viscosity to the desired consistency. Again, you will have to experiment to find the pure-soap content you like best for the product you want to make.
Method 3a. Thicken with table salt or other salts
Anionic surfactants, including soap, can be thickened with salts. Anionic means the surfactant has a negative charge.
Soap that is moderate to high in oleic acid is a good choice for thickening with table salt (sodium chloride).
A soap that is low in oleic acid, such as a 100% coconut oil soap, is a poor candidate for salt thickening. You should consider using a cellulose- or gum-based thickener instead (Method 3b)
If salt works as a thickener, the amount needed will probably be somewhere between 0.2% to 2% by weight. In other words, 0.2 to 2 grams of table salt will be needed to thicken every 100 grams of diluted soap.
Other salts can also be used to thicken liquid soap. Possibilities include potassium carbonate, sodium carbonate, potassium citrate, sodium citrate, and borax.
There are downsides to using any salt in soap --
Salts tend to reduce the lather
Salts can reduce the clarity of the soap
Some salts such as borax can cause the soap to decompose and separate
Salt will thicken only within a small range. Too little salt will not work. Too much salt will not work either. (3,4)
If you have not thickened soap with table salt before or if you are working with a recipe new to you, experiment with a small sample of soap first! After you find the amount of salt needed to thicken the sample, use that information to thicken the main portion of the soap.
Do a "salt curve" to find how much salt is needed to thicken the soap --
Dilute the soap to the desired viscosity or to the desired percentage of pure soap as described above.
Weigh a small amount of the diluted, room temperature soap into a container. A 50-100 gram sample is a good size. Record the weight of this sample.
Add a weighed amount of dry salt into the sample. Stir with a spoon or spatula until the salt is fully dissolved. Avoid making lots of foam, so do NOT use a stick blender or whisk.
Start with a 0.2% dose of dry salt. This would be 0.1 gram salt added to a 50 gram soap sample or 0.2 gram salt added to a 100 gram sample.
Record the weight of dry salt added.
Let the mixture sit for about 1 hour and then evaluate the thickness. Do not be in a rush -- thickening can take some time to fully develop.
Stir in another 0.2% dose of dry salt into the sample, let the mixture sit, and evaluate the thickness.
Repeat until you have added a total of 2% salt to the soap or until the soap becomes thick enough for your needs. Record the percentage of salt that worked best.
Another way to do a salt curve that requires more soap, but takes less time --
Set up 10 containers, each containing the same weight of diluted liquid soap. I suggest using 50 to 100 grams of soap per container.
Add 0.2% dry salt to the first sample, 0.4% to the second, 0.6% to the third, and so on until the last sample gets a 2% dose of salt.
Let the samples sit for at least 1 hour and then evaluate the thickness of each sample. Record the percentage of salt that worked best.
After you learn the percentage of salt needed to thicken the sample(s), use that same percentage of salt to thicken the main portion of diluted soap.
Method 3b. Control thickness with cellulose- or gum-based thickeners.
A separate cellulose-based thickener such as HEC (hydroxyethyl cellulose) or HPMC (hydroxypropyl methylcellulose) can be used to thicken liquid soap. Some makers have tried thickening liquid soap with guar gum or xanthan gum with variable success.
These thickeners can be especially useful if --
You want a thicker gel than you get from dilution alone (Method 1) or from using salt (Method 3a)
The soap has a very low or very high oleic acid content, so you cannot use Methods 1 or 3a
I do not have experience with these thickeners, but Faith Oriold has several good tutorials to guide you. (2)
Method 3c. Control thickness by adding synthetic detergents.
Some synthetic detergents (syndets) can thicken mixtures of anionic (an-eye-on-ick) detergents. Anionic means the detergent has a negative charge. True soap is an anionic detergent. Adding certain types of syndets may also help the soap become milder to the skin.
The advantages of thickening with syndets can include --
The product may be milder to the skin than soap alone
Some syndets may be able to thicken soap with a low oleic acid content
A syndet-soap mixture may lather better than soap without syndet
Cocamidopropyl Betaine (CAPB), Sodium Lauryl Sulfoacetate (SLSa), and Coco Glucoside are several syndets that can be used to thicken liquid soap.
CAPB is an amphoteric (am-fo-teh-rick) detergent, meaning it can be negatively or positively charged depending on the pH. In a soap mixture, CAPB will behave as a negatively charged cationic (cat-eye-on-ick) chemical. CAPB is very mild and does not produce a lot of foam. Reputable sources have theorized CAPB may not thicken a low-oleic soap, but I cannot confirm this is correct.
Saranac, a member of the Soap Making Forum, reported CAPB worked very well to thicken liquid soap, but the mixture had a fishy odor. (5)
I also found CAPB easily thickened a sunflower-coconut liquid soap with a 64% oleic acid content. To my nose, CAPB adds a slight fishy odor, but I think the scent is mild enough it can be masked with an appropriate scent. You will have to decide whether the odor is a deal-breaker or tolerable for you.
CAPB is sold as a liquid with 30% active ingredient. I suggest starting with 2% by weight; in other words, add 2 grams of CAPB liquid per 100 g diluted soap. Pour the CAPB into room temperature soap and gently mix with a spatula or spoon to avoid making a lot of foam. I saw results very quickly.
SLSa is an anionic detergent that is relatively mild and lathers well. Saranac used SLSa to thicken liquid soap with good results (6), as have I. SLSa does not add an odor, but it is harder to incorporate into the soap. I suggest starting with 2% SLSa powder (2 grams SLSa per 100 grams of liquid soap).
SLSa is sold as a powder. I recommend using good ventilation and/or respiratory protection when working with this dusty, irritating powder.
SLSa does not dissolve well in room temperature liquid soap . Warm the soap to about 100 F / 38 C, sprinkle the powder over the surface of the soap, and gently use a whisk to break up lumps and incorporate the powder into the warm liquid.
The mixture will be thinner when warm than at room temperature, so be sure to let the mixture cool to room temperature before you evaluate the thickness.
Coco Glucoside is another detergent that Saranac reported worked well for thickening. (6) I have not tried it, so cannot share my experiences with this detergent.
A syndet that does not work to thicken soap -- Sodium C14-16 Olefin Sulfonate.
Adding fragrance can alter the thickness
If your finished product is the paste, scent the paste. If the finished product is the diluted soap, then scent the diluted soap.
Use scent with a light hand. Most people report they prefer to scent liquid soap more lightly than bar soap, so start with 0.5% to 1% by weight of the finished product and see what you think. It is easier to add more scent than it is to remove too much.
Test the fragrance first in a sample to learn if the fragrance alters the texture or appearance of the soap. Some fragrances thicken liquid soap, others thin it out, a few cause the texture to become clumpy or grainy, and some scents do absolutely nothing. There is no way to know what will happen except to test a sample.
1. Jane Barber. Active surfactant calculator. Making Skincare. http://makingskincare.com/surfactant-calculator/
2. Faith Oriold. Tutorials -- Liquid Soap. Alaiyna B. Bath and Body. http://alaiynab.blogspot.com/search/label/tutorial
3. Perry Romanowski. Why does salt thicken shampoos? Chemist's Corner. https://chemistscorner.com/why-does-salt-thicken-shampoos/
4. Belinda Carli. How to thicken surfactants with salt: Make shampoo or body wash. The Institute of Personal Care Science. https://www.youtube.com/watch?v=y5z_zAnSEZg
(5) Saranac. 9 February 2018 post. Soap Making Forum. https://www.soapmakingforum.com/threads/formulating-liquid-soap.69070/post-686941
(6) Saranac. 8 February 2018 post. Soap Making Forum. https://www.soapmakingforum.com/threads/formulating-liquid-soap.69070/post-686745
Extra credit. The formulas used in the Liquid Soap Dilution calculator are described here. These formulas can be used with grams or weight ounces. Use only one unit of weight for everything -- do not mix ounces and grams.
1. Determine the pure soap weight, paste weight, and percentage of pure soap for the entire batch.
Total pure soap weight = Fat weight in the batch + KOH weight in the batch
(Get these weights from your recipe)
Total paste weight = Weight of the entire batch of soap paste
(Weigh the whole batch of finished paste after it is made)
Pure soap % in the paste = Total pure soap weight / Total paste weight X 100
2. Choose the percentage of pure soap you want in the diluted soap. I am calling this the "Dilution%" in the formulas below. This percentage must be equal to or less than the "Pure soap % in the paste" that you just calculated.
3. Measure out the weight of soap paste you want to dilute. I am calling this the "Paste weight" in the formulas below. You can dilute the whole batch of paste or only part of the batch -- that is up to you.
4. Calculate the weight of pure soap in this amount of soap paste --
Pure soap weight = Paste weight X Total pure soap weight / Total paste weight
5. Calculate the water needed to dilute the paste to the Dulution% you chose --
Dilution water weight = Pure soap weight X 100 / Dilution% - Paste weight
6. Calculate the total weight of the diluted soap --
Total diluted weight = Paste weight + Dilution water weight
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