Superfat and lye discount in soapmaking are terms used interchangeably. And it refers to the amount of oils left unsaponified in a soap recipe which can be achieved by reducing the amount of lye.
A precise amount of lye is necessary to transform a specific combination of oils used in a recipe into soap. Since lye is a caustic chemical, we need a ‘buffer’ amount of oils to make sure that the soap doesn’t turn out lye heavy. The free oils also make the soap emollient on the skin. We do this by reducing the amount of lye (lye discount) by a certain amount. A safe and acceptable range for superfatting is 5-8%.
What happens below or above this superfat limit?
Superfatting below 5% could make a harsh soap because it means that almost all the oils are used up by the lye. It is also important to note that lye purity can affect superfat to a certain extent. For example, 50g lye at 99% purity will saponify more oils compared to that of the same amount of lye at 95% purity. So, using less than 5% superfat in this case, will increase the lye amount, leave lesser unsaponified oils and result in a harsh soap. Anything above 8% superfat means that there is a considerable amount of free oils in the soap. Soap itself doesn’t spoil or go rancid but free unsaponified oils will. This could potentially lead to Dreaded Orange Spots (DOS) in soap. A high superfat also dampens the lather. However, many experienced soapmakers sometimes experiment with upto 10%.
Exception to superfat
An exception to this rule is with soap recipes high in coconut oil. Coconut oil in soap has a drying effect. Consequently, to counteract the dryness, upto 20-25% superfat can be incorporated in the recipe. And it also helps that coconut oil has a good shelf life too. Here’s a 100% coconut oil soap recipe should you wish to try it.
Using a soap calculator is a good way to watch the amount of lye reducing by increasing the superfat.
For example, when you plug in the Basic Palm Oil Recipe from our collection of Cold Process Soap Recipes for Beginners, you may note that a 6% superfat gives the following recipe;
Olive Oil – 125gCoconut Oil – 125gPalm Oil – 125gCastor Oil – 25g
Sodium Hydroxide – 57gDistilled Water – 114g
Whereas, when you increase the superfat to 8%, the recipe will change as follows;
Olive Oil – 125gCoconut Oil – 125gPalm Oil – 125gCastor Oil – 25g
Sodium Hydroxide – 55.8gDistilled Water – 112g
Note that the change is very slight (1.2g)! But it makes a significant difference to the superfat in a recipe. As a result, it is important to use a well calibrated and accurate weighing scale for measurements while making soap.
Have I covered everything you need to know about superfat & lye discount in soapmaking? Let me know if you have any additional questions in the comments below! Always happy to help!