Why shouldn't you reuse the amount of lye specified in another recipe?

Two natural soap recipes may contain the same oils, but in different proportions. Consequently, their requirements for caustic soda (sodium hydroxide) will differ. This is why a recipe designed for olive oil cannot simply be adapted by replacing the olive oil with coconut oil, even if the total weight of the oils remains the same.

In the cold-process soap-making method, you cannot swap one oil for another without checking its saponification value (SAP), as every oil has a unique SAP value. If a specific amount of caustic soda is required for "Recipe A", replacing one of the oils necessitates a completely new calculation for "Recipe B". An error in calculating the amount of caustic soda can make the finished soap dangerous: an excess of lye will result in a harsh, caustic soap, while an insufficient amount may prevent the saponification process from occurring correctly.

Therefore, any change made to a recipe requires a new calculation, using the saponification values ​​(SAP) corresponding to the specific oils chosen. This is why there is no universal amount of caustic soda that applies to all oils; they are simply not identical.

To determine the amount of lye (NaOH) needed for a recipe, the saponification value is used. This figure represents the amount of caustic soda required to saponify a given quantity of oil.

For oil A, 13.4 g of NaOH is required to saponify 100 g of the oil. For oil B, 18.3 g is required.

Examples of NaOH Saponification Values

This list includes the oils most commonly used in soapmaking. Please note, however, that these are approximations; you may encounter different values ​​for the same oil when consulting other online sources. It should also be noted that coconut oil requires more sodium hydroxide (lye) than olive oil.

What if you mix several oils in a single recipe?

To create a standard soap recipe, you use a variety of different oils. For each oil, you must calculate the exact amount of caustic soda (sodium hydroxide) required, based on its specific saponification value (SAP value). You then add these results together to determine the total amount of caustic soda needed for the recipe.

Let’s imagine a recipe consisting of two oils:

• 400 g of olive oil, with an SAP value of 0.134.

• 100 g of coconut oil, with an SAP value of 0.183.

The calculation is therefore performed in two steps.

For the olive oil:

400 × 0.134 = 53.6 g of NaOH.

For the coconut oil:

100 × 0.183 = 18.3 g of NaOH.

Next, add the two results together:

53.6 + 18.3 = 71.9 g of NaOH.

Thus, 71.9 g of caustic soda is required to fully saponify these two oils. Please note that this recipe should not be used as-is, as it is merely an example. To make this recipe functional, you would need to incorporate a "superfat" and calculate the appropriate amount of water.

Conclusion: understanding SAP indices means learning how to formulate

Every oil requires a very specific amount of sodium hydroxide (lye). The saponification value, or "SAP value", allows you to account for these differences and is an essential metric when developing a recipe. Once you understand its role, you can begin creating your own formulas and adjusting the proportions of your oils. The next step in learning how to formulate your own soap recipes is to understand "superfatting" and how to determine the appropriate amount of water based on the type of natural soap you wish to create.