15 minute read

The Robb Recipe: A flexible formulation for natural soap

Sara J Robb, Bath Potions and Verdigris Cosmetic Solutions, London, UK admin@drsararobb.info

Dr Sara Robb, a longstanding supporter of our work, offered to tell us how she makes some of the simple, beautiful soaps that she has perfected making over the years. We jumped at the chance! Sara explains the process in full in this blueprint for anyone to follow in crafting some lovely products.

Sara (centre) and her daughters Meggy Jayne (left) and Jasmine (right) busy wrapping some of Sara’s lovely soaps
A selection of oils available Bees for for making Development natural Journal soaps 135 with July The 2020 Robb Recipe. From left to right, bottom row: coconut oil and cocoa butter, top row- sunflower oil, corn oil and olive oil
Images © Sarah Robb

Natural Soap

Soap is a natural cosmetic that beekeepers can make with just two ingredients: oil and hydroxide. Liquid and solid oils are transformed into soap by the chemical process called saponification. The specific oils in the formulation bestow different qualities to the finished product. Coconut oil makes soap with a good lather, while olive oil soap is emollient (moisturising), but does not lather well. These differences arise because each oil is chemically unique, a feature that also affects the formulation of soap. Traditional cold-processed soap recipes outline the exact amounts of specific oils and fats, and substitutions are prohibited.

The Robb Recipe is unique because you can make soaps from different oils using the same recipe - substitutions are allowed! What is more, with my method you do not need any special equipment and can use the soap the day you make it – no waiting for the soap to cure. Several optional ingredients, such as bee products and fragrance, can be added to create unique soap varieties to sell at market. This article will explain the principals of saponification, provide a description of The Robb Recipe, and to get you started, provide instructions for two soap varieties prepared with The Robb Recipe.


The chemical reaction used to synthesise soap is called saponification. Mixing oil and sodium hydroxide (NaOH) starts the soap making process. Hydroxide is a strong alkali, (pH 14), giving the initial blend a high pH. As the chemical reaction progresses, soap and glycerine form and the pH falls. The final bar soap will have a pH of approximately 10.

Saponification is the chemical synthesis of soap. The hydroxide acts like a pair of scissors and cuts the oil molecules, leading to the synthesis of soap and glycerin. During the chemical reaction, hydroxide is consumed and is not a component of the final soap.

Saponification Value

The saponification value (SAP Value) describes how much potassium hydroxide (KOH) is needed to convert one gram of oil into soap. The composition of each oil determines how much hydroxide is required to change that oil into soap. The SAP Value for coconut oil is 191mg per gram oil, while the SAP value for castor oil is 128mg per gram oil. Therefore, it takes more KOH to convert one gram of coconut oil to soap than it does to convert one gram of castor oil to soap. KOH makes soap paste which is used to make liquid soap. To make soap bars, NAOH is used in the formulation.

The Sodium Hydroxide Factor

To formulate a recipe for bar soap, we need to calculate the amount of sodium hydroxide (NaOH) to use. The NaOH Factor for each oil describes how many grams of NaOH is required to convert one gram of oil to soap (note the NaOH Factor is grams hydroxide per gram oil). The table following shows the NaOH Factors for a selection of oils. The values range from 0.129g of NaOH per gram oil for castor oil and rice bran oil to 0.192g of NaOH per gram oil for coconut oil. Many oils and fats fall between these values.

.129   Castor oil, Rice bran oil 

.130   Shea butter

.131   Passionfruit seed oil

.132   Wheat germ oil

.133   Grapeseed oil, Rapeseed oil

.134   Sesame oil, Avocado oil, Borage oil

.135   Walnut oil.

.136   Flaxseed oil, Soybean oil, Sunflower oil

.137 Almond oil, Corn oil, Peanut oil

.138   Apricot kernel oil, Cocoa butter, Neem oil

.139   Cottonseed oil

.141   Palm kernel oil

143   Palm oil

>   < 

.192   Coconut oil

Calculating sodium hydroxide

To determine the amount of NaOH required, multiply the grams of oil in the recipe by its NaOH Factor. Below are the calculations to determine how much NaOH to use to change 100g of castor oil to soap. From the maths below, it takes 12.9g of sodium hydroxide to convert 100g of castor oil to soap and glycerin.

100g Castor Oil x 0.129 = 12.9g NaOH

To determine how much NaOH is needed to saponify 100g of coconut oil using the NaOH Factor, we calculate 19.2g of NaOH is required: significantly more than needed for 100g of castor oil.

100g Coconut Oil x 0.192 = 19.2g NaOH

This comparison gives us an appreciation of the importance of NaOH Factors. If 19.2g of NaOH are used to saponify 100g of castor oil, there would be an excess of 6.3g NaOH in the soap. It is imperative to avoid making soap with excess hydroxide because it will cause burns.


While it is dangerous to have excess hydroxide in soap, it is desirable to make soap with a bit of excess oil. A surplus of oil, called super-fatting, makes a more gentle and moisturising soap. There are two ways to super-fat soap. The first is to add extra oil to the recipe; the second is to reduce the amount of NaOH used in the formulation. For example, using 10% less NaOH than the amount calculated with the NaOH Factor, results in 10% excess oil – soap that is 10% super-fatted.

The Robb Recipe

The Robb Recipe is a unique cold-processed formulation allowing substitutions. Part of the formulation is fixed; do not change the quantities of lye and coconut oil. However, the other oils in the recipe are interchangeable, creating endless combinations.

0.129 to 0.143 = 11% to 15%.

Being able to make substitutions in a soap recipe is an advantage for those with access to limited varieties of oils. The oils and fats available for soap making will vary geographically. In East Africa, sunflower oil might dominate the supermarket shelves, while in the Caribbean, corn and soybean oils might be predominant. 

Flexible formulation

An ideal formula makes a soap that is gentle, has a good lather and is emollient. I have written a flexible recipe for Bees for Development that achieves this goal. The first ingredient is 500g of coconut oil to make a bubbly soap. To the coconut oil, add 500g oil or fat from the teble on page 4. It does not matter how many oils are mixed providing 500g of coconut oil and 500g of other oils and fats are used. To make soap with The Robb Recipe, follow the rules below and the method provided in this article.

Robb’s Rules

1. The lye cannot be changed! Use 143g of NaOH in 260g of water.

2. The amount of coconut oil cannot be changed. Use 500g of coconut oil.

3. Add a total of 500g oil(s) from the list below (several different oils can be used but the total weight mustadd up to 500g).

Almond Oil

Apricot Kernel Oil

Avocado Oil

Borage Oil

Castor Oil

Cocoa Butter

Corn Oil

Cottonseed Oil

Flaxseed Oil

Grapeseed Oil

Neem Oil

Palm Kernel Oil

Palm Oil

Passionfruit Seed Oil

Peanut Oil R

apeseed Oil

Rice Bran Oil

Sesame Oil

Shea Butter

Soybean Oil

Sunflower Oil

Walnut Oil

Wheat Germ Oil

4. Bee products added as follows

Beeswax: 15g maximum

Honey: 15g maximum

Propolis: 5g maximum (raw or tincture)

Pollen: 5g maximum

5. Adding fragrance, colour and botanicals Essential oils or fragrance oils: up to 15g Pigments and dyes added to colour the soap (add to the desired hue)

Botanicals can be added up to 15g

Robb recipe cards Natural Soap Lavender Lime Honey Soap Sweet Orange Protein Bar

Adding pollen to the Sweet Orange Protein Bar, gives this soap a rich, caramel colour. A honeycomb pattern was made on the bars by pressing bubble wrap onto the surface of the raw soap
Pale golden bars of Lavender Lime Honey Soap. With a refreshing scent and topped with lavender buds, this soap is a delight to use

Super-Fatting and The Robb Recipe

Unlike other soap formulations, the amount of hydroxide and coconut oil remains constant in The Robb Recipe while the other oils are variable. Consequently, the amount of super-fatting will change as the oils selected are changed.

Use the calculations below to determine the amount of NaOH needed for complete saponification using 500g of coconut oil and 500g of castor oil. The coconut requires 95.9g of NaOH . The NaOH Factor of castor oil is 0.129, the smallest of the oils listed in Robb’s Rules number 3. 500g of castor oil requires 64.5g of NaOH. Added together, the total NaOH for 100% saponification for coconut and castor oil is 160.4g. The Robb Recipe calls for 143 grams NaOH, resulting in a soap that is 11% super-fatted.

500g Castor Oil       x    0.129     =    64.5g NaOH                                                                                500g Coconut Oil    x    0.192     =    95.9g NaOH                                                                                                                         Total     =    160.4 NaOH

143g/160.4g NaOH = 89% NaOH 11% Super-fatted

Now compare the above with soap made with 500g of coconut oil and 500g of palm oil. Palm oil has the highest NaOH Factor listed in Robb’s Rules number 3. The calculations are below. The 500g of coconut requires 95.9g of NaOH. Using the NaOH Factor for palm oil (0.143), 71.5g of NaOH is necessary to convert 500g of palm oil to soap. The total NaOH required to convert both the coconut and palm oil to soap is 167.4g. The Robb Recipe uses 143g of NaOH, resulting in the coconut and palm oil soap being 15% super-fatted.

500g Coconut Oil   x    0.192    =    95.9g NaOH                                                                                   500g Palm Oil        x    0.143   =    71.5g NaOH                                                                                                                          Total    =     164.7 NaOH

143g/167.4g NaOH = 85% NaOH 15% Super-fatted

The Robb Recipe will produce soap that is super-fatted between 11% and 15% regardless of which oils are chosen from the list in Rule 3. The table at the top of page 5 shows the relationship between the NaOH Factor and the levels of super-fatting in The Robb Recipe.

Soap varieties made with The Robb Recipe

Creating multiple soap varieties soap is possible with The Robb Recipe. Two variations are provided below. The recipe cards for Lavender Lime Honey Soap and for Sweet Orange Protein Bar are on page 5 and the bars are shown above. Although both soaps follow The Robb Recipe, they are distinctly different.

Soap making equipment

The two essential items required to be a successful soap maker are an accurate scale that weighs in increments of one gram and an electric hand mixer. Also, bowls, some sort of soap mould, towels, a heavy jar and spoons. – items that could be readily available in the kitchen. Importantly, gloves and goggles are needed to wear when making soap.

Note: after use, plastic containers used to make soap may retain the fragrance and will no longer be suitable for food preparation and storage.

Method for making soap with The Robb Recipe

Prepare the lye solution

• Put on gloves and goggles

• Place the glass jar on the scale and weigh 260g of water into the container

• Carefully weigh 143g NaOH into a bowl

• Add NaOH to the water and stir until the crystals are in suspension (The solution will warm up as the NaOH dissolves into the water) Do not inhale the fumes

Prepare the oils, fats and waxes

• Weigh 500g of coconut oil, and the other oils you have chosen into a bowl

• Melt any solid oils before transferring them to the bowl

• If beeswax is used melt with the solid oils

Add the optional ingredients

• Weigh each of the optional ingredients and add to the oil mix

• At this point, all ingredients, except the lye solution, should be in the bowl

• Prepare soap mould and have towels to hand

Adding the lye to the oil mixture

• Pour the lye solution into the oil mixture while stirring slowly (mixer powered off)

• Once all the lye has been added, start mixing with the power on

• The soap mixture will begin to thicken

• Blend until the soap mixture resembles cake batter

Prepare your lye solution, wearing your gloves and goggles. Always add the solid hydroxide to the water to avoid the danger of the lye boiling over. Note the lye in the picture is cloudy. When the sodium hydroxide has completely dissolved, it will be clear

Pouring the soap

• Pour the soap mixture into the mould

• Layer towels on top of the mould to keep in the heat

Saponification and the gel phase

• Check the soap occasionally for progression of the gel (see photo below)

• When the gel reaches the edge of the soap mould, saponification is complete

• Unwrap the soap and let it cool

Prepare soap bars

• Cut the cool soap into uniform bars

• Prepare product labels

• Place the bars in chosen packaging and apply labels


Handmade, natural soap is an excellent value-added product that beekeepers can sell alongside their honey. The Robb Recipe provides an easy-to-follow, flexible recipe to make soap with bee products and local oils. There is no need for specialist equipment and the soap can be used the same day as it is made- an advantage for making soap to take to market.

Additionally, The Robb Recipe can be used to make several different soaps. The sample formulations for Lavender Lime Honey Soap and Sweet Orange Protein Bar demonstrate the flexibility of The Robb Recipe. When you choose oils from the approved list, the soap will be 11-15% super-fatted. Combining different oils and optional ingredients produces endless varieties of soap.

Sweet Orange Protein soap going through the gel phase. The gel starts in the centre as heat accumulates from saponification. The gel spreads to the edge of the soap. When the gel has reached the edges, the oil and hydroxide have all been converted to soap and glycerin

I hope this article will inspire you to make natural soaps.

Supplementary Information

Super-Fatting calculations for Lavender and Lime Honey Soap

500g Coconut Oil       x     0.192    =    95.9g NaOH                                                                              250g Shea Butter       x     0.130    =    32.5g NaOH                                                                            125g Cottonseed Oil  x     0.139    =    17.4g NaOH                                                                        125g Olive Oil             x     0.136   =     17.0g NaOH                                                                                                                          Total   =     162.2 NaOH

143g/162.8g NaOH = 88% NaOH 12% Super-fatted

Super-Fatting calculations for Honey and Orange Protein Bar

500g Coconut Oil      x     0.192    =    95.9g NaOH                                                                              250g Castor Oil         x     0.129    =   32.3g NaOH                                                                                250g Olive Oil           x     0.136    =    34.0g NaOH                                                                                                                          Total    =    162.2 NaOH

143g/162.2g NaOH = 88% NaOH 12% Super-fatted


Robb,S.J. (2009) Dr Sara’s Honey Potions. Northern Bee Books, Mytholmroyd, UK. Robb,S.J. (2012) Beauty and the Bees. Northern Bee Books, Mytholmroyd, UK.

Sarah’s new book is reviewed in the Bookshelf section