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. Only the water content is adjusted to get the desired consistency. No additional thickener is added.
Method 2. Dilute to a low 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 add a separate thickener to make the soap more viscous (syrupy).
Method 3a. Table salt (sodium chloride) or other sodium salts
Method 3b. Cellulose- or gum-based thickeners
Method 3c. Synthetic detergents
Method 3d. NaOH (bar) soap
Method 4. Add some types of fragrance to thicken diluted soap.
Getting started
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%, you will need to use Method 3b or 3c to thicken diluted soap made with this type of recipe. Liquid soap that is made with mostly coconut oil or all 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. Play it safe with your soap! Bacteria and fungi do not have to be visible to the human eye to cause infections or disease, as anyone who has had food poisoning will know.
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 and boiled 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
Dilute to a honey-thick consistency for use in a squirt bottle or pump bottle by only adjusting the water content to get the desired viscosity -- no additional thickener is used.
Do not add all the dilution water at once. Many liquid soap making tutorials use the "dilute to honey thick" method. They often create 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 soap is different from the author's soap in the water content or fatty acid profile, your soap may not dilute the same way.
You will get better results if you use a more gradual method to dilute, 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 same 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. (8) Look in Post #1 for the line "To dilute".
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.
Disadvantages 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, using a less concentrated soap has these benefits--
Reducing waste while still cleansing well. Even if half a dollop is enough, many people will still use a full squirt or more.
Less chance of drying or irritating the skin. Concentrated soap can over-clean the skin and strip away desirable skin fats leaving the skin feeling dry and tight, even irritated.
Concentrated soap often does not lather as easily as more diluted soap.
More dilute soap will reducing the cost of your product. Controlling costs is important if you sell.
Method 2. Dilute for use in foamer dispensers.
Dilute to a specific pure-soap content and a watery consistency for use in a foamer dispenser. No additional thickener is added to soap for foamer dispensers because foamers need a water-thin soap to work properly.
Try diluting a sample of your soap to about 10% pure soap content to start with; you can estimate the water needed to dilute the soap using the Liquid Soap Dilution calculator.... Test to see if that dilution works in your particular dispensers. Adjust the water content as needed until the diluted soap foams properly.
Method 3. Dilute and then thicken with a separate thickener.
Dilute to a specific pure-soap content and then add a separate thickener to make the soap more viscous (syrupy). You will have to experiment to find the pure-soap content you like best for the product you want to make.
To get started, I suggest diluting the soap to a 15-20% pure soap content and then choosing one of the following methods for thickening. (1) You can estimate the water needed to dilute the soap using this Liquid Soap Dilution calculator....
Method 3a. Thicken with table salt or other sodium salts
While salt can work well for thickening liquid soap, there can be downsides --
Salts tend to reduce lather and clarity
Some salts such as borax and sodium bicarbonate can cause the soap to decompose and separate
Salt will thicken only within a small range of concentration. Too little salt or too much salt will not work. (3,4)
Not all soap can be effectively thickened with salts. Soap that is moderate to high in oleic acid is a good choice. Salt will do a poor job of thickening soap that is low in oleic acid, such as a 100% coconut oil soap.
Table salt (sodium chloride) is inexpensive and easy to find and is often used for thickening. Other sodium salts can be used such as borax (sodium borate), washing soda (sodium carbonate), and sodium citrate. (10) I have only used table salt as a thickener, so you are on your own if you want to try a sodium salt other than table salt.
Comparable potassium salts, including potassium chloride, potassium carbonate, and potassium citrate (10), may also be effective thickeners.
Do not use salts based on other metals, such as Dead Sea salts (a mixture of various salts based on sodium, potassium, magnesium, calcium, etc.) or epsom salts (magnesium sulfate). You will end up with soap scum, not functional soap!
I also do not recommend using sodium bicarbonate (baking soda) for thickening liquid soap. Although it is a salt, baking soda has the unusual ability to act as a weak acid in soap. This means baking soda can cause soap to decompose into fatty acids. These fatty acids will turn liquid soap cloudy and can separate and float in a greasy layer on top of the soap. (9)
The amount of sodium salt needed will probably be somewhere between 0.2% to 2% by weight. In other words, 0.2 to 2 grams of table salt (or other sodium salt) will be needed to thicken every 100 grams of diluted soap.
If you have not thickened soap with a particular 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.
A"salt curve" is the best way 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 sample 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 is obviously getting thinner. 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. Other thickeners that have been used for thickening liquid soap include guar gum, Crothix, and xanthan gum.
These thickeners can be especially useful if --
You want a thicker gel than you get from dilution alone or from using salt
The soap has a very low or high oleic acid content, so salt-thickening does not work well.
I do not have experience with these thickeners, but Faith Oriold of Alaiyna B. Bath and Body 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. Lye-based soap is an anionic detergent.
The advantages of thickening soap 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 three syndets that can be used to thicken liquid soap.
CAPB, cocoamidopropyl betaine, is an amphoteric (am-fo-TEH-rick) detergent, meaning it can be negatively or positively charged depending on the pH. CAPB is very mild and does not produce a lot of foam.
In soap, CAPB will behave as a positively charged cationic (cat-eye-on-ick) chemical.
Some cosmetic chemists have thought CAPB may not thicken a low-oleic soap. I have not yet tested this hypothesis to know one way or the other.
Saranac, a member of the Soap Making Forum, reported CAPB worked very well to thicken liquid soap, but it added an objectionable fishy odor to the mixture. (5) I do not know the oleic acid content of the soap Saranac used.
I found CAPB thickened a liquid soap with 64% oleic acid content. Like Saranac, I found the CAPB added a slight fishy odor. I thought the odor was mild enough it could be masked with an appropriate fragrance, but more sensitive noses might not agree.
CAPB is sold as a liquid with 30% active ingredient. To start, try adding about 2% of this liquid 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.
Once you have a sense of how it thickens your soap, adjust the amount of CAPB to get the viscosity you want.
SLSa, Sodium Lauryl Sulfoacetate, is an anionic (negatively charged) 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 compared with CAPB.
SLSa is sold as a powder. Use good ventilation and/or respiratory protection when working with this dusty, irritating powder.
I suggest starting with 2% SLSa powder (2 grams SLSa per 100 grams of liquid soap).
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 a mild non-ionic (no charge) detergent that Saranac reported worked well for thickening. (6) I have not tried it, so cannot share my experiences with this detergent.
Some synthetic detergents do not thicken soap. I tested Sodium C14-16 Olefin Sulfonate, an anionic detergent. It had no effect on thickness. I also tested Polysorbate 80, a non-ionic detergent, and found it also did not thicken liquid soap.
Method 3d. Thicken with NaOH (bar) soap
Sodium soap (bar soap made with NaOH) will sometimes work to thicken liquid soap (soft soap made with KOH). The chemistry involved is simlar to thickening liquid soap with any other sodium salt (see Method 3a).
The amount of thickening will depend on the fatty acids in the liquid soap, so it is wise to thicken a sample first to see if sodium soap will work well as a thickener or not.
Liquid soaps that are rich in oleic, stearic, and/or palmitic acids are the best candidates. A soap that is low in these fatty acids, such as a 100% coconut oil soap, may not thicken well with bar soap.
If you have not used bar soap as a thickener for liquid soap or if you are working with a liquid soap recipe new to you, experiment with a small sample first! After you find the amount of bar soap needed to thicken the sample, use that information to thicken the main portion of the soap.
For a start, add grated bar soap at 2-3% based on the weight of diluted liquid soap. In other words, add 2-3 grams of grated bar soap to 100 grams of diluted liquid soap. (7)
You can gently warm the liquid soap to 100-120 F / 40-50 C to help the grated soap dissolve more quickly. Stir gently to avoid making a lot of foam.
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.
After you learn the percentage of bar soap that works best to thicken the sample, use that same percentage of bar soap to thicken the main portion of liquid soap.
Method 4. Fragrance may thicken diluted soap
Some fragrances thicken soap, others thin the soap, some cause the texture to become clumpy or grainy, and other scents do absolutely nothing. There is no way to know what will happen except to test a sample.
If you have a fragrance that does thicken your soap, scent the sample to get the right level of fragrance, let the soap sit for at least a few hours to let the thickness stabililze, and then evaluate the results.
Add 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.
You may need to add another type of thickener. It is possible the added thickener may interact with the fragrance in ways you cannot predict, so it is wise to test a fragrance and thickener combination using a sample of soap.
Reference s
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. Formulating liquid soap. 9 February 2018 post. Soap Making Forum. https://www.soapmakingforum.com/threads/formulating-liquid-soap.69070/post-686941
6. Saranac. Formulating liquid soap. 8 February 2018 post. Soap Making Forum. https://www.soapmakingforum.com/threads/formulating-liquid-soap.69070/post-686745
7. Nisha. How to thicken liquid soap: Using soap base. Soap Lab Malaysia. Version dated 28 February 2018. http://www.soaplabmalaysia.com/2018/02/how-to-thicken-liquid-soap-using-soap.html
8. Susie. Cold process liquid soap. Soap Making Forum. https://www.soapmakingforum.com/threads/cold-process-liquid-soap.49852/
9. Saranac, Ngian, GalaxyMLP, DeeAnna, et al. Citric acid as a chelator. Soapmaking Forum. https://www.soapmakingforum.com/threads/citric-acid-as-a-chelator.56643
10. Faith Oriold and Lady-of-Four. How to thicken liquid soap. Soapmaking Forum. https://www.soapmakingforum.com/threads/how-to-thicken-liquid-soap.23771 See posts 28, 45, and 47.
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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