If it is an expensive lavender EO, I'd save it for lotions and things like that where I can enjoy the scent. I have a couple of lavender varietals that I use only for special lotions. I'd never use them in soap because their distinctive aroma would be lost after the lye gets done with them.
But if the cost isn't an issue or if the EO is tolerably inexpensive, I'd use a true lavender EO (lavandula angustifolia aka L. officianalis) at 3-6% ppo in soap (15-30 grams EO per 500 grams of oils). It's considered to be GRAS, generally recognized as safe.
Just to be thorough -- it's Lavender, not Lavandin, right? Lavandin is a skin sensitizer and is not GRAS.
edit: I just read Soapmaker's post, and I agree with their suggestion to also use rosemary oleoresin (ROE) to prevent rancidity. Lavender EO is one that can accelerate rancidity if the EO is oxidized.
Thank you all for your opinions. It was sold at a huge discount because the producers on Hvar had zero sale. So I may use it in a half kilogram batch.
But I have never used ROE either and will have to research its availability and price for me (Slovenia, Europe).
Yes, I've gotten all-over rancidity in soap from both EOs. The experience was an unintentional experiment early on in my soap making misadventures. I stirred up a small batch of soap, divided it in half, and added mint EO to one half and lavender EO to the other. The only difference was the EO. The mint triggered rancidity slower than the lavender, but both halves became entirely orange and musty smelling in just a couple of months. It was an impressive lesson, and not in a good way.
This is an example where an effective antioxidant (such as the ROE we're talking about) would have been more helpful than a chelator (such as EDTA or citrate or sodium glutamate), if I had to use only one or the other.
More often a chelator is the better choice if you can only use one, but obviously this story proves that's not always effective.
I nowadays add ROE to stored liquid oils and other fats that don't otherwise have an antioxidant (store-bought lard has antiox added). And I also add a chelator when making soap. That covers all the bases.
I can't find ROE locally either, although I realize it's a lot easier and probably cheaper for me to order it than it is for you, @Dawni.
Some people use tocopherols (aka vitamin E) in their fats as an antioxidant. There are a couple of problems with that idea --
There are at least eight flavors of tocopherols. Gamma tocopherol appears to be the safest and most effective antioxidant for fats, based on my reading, if you're going to use tocopherols for this purpose. The others are useful for use on skin or inside the body, but not as useful to prevent rancidity in fats.
The other problem, even if you can find the right tocopherol, is that many fats naturally contain tocopherols. Some fats that oxidize and go rancid fast -- hemp, grapeseed, pumpkin, etc. -- are examples of fats that naturally have high levels of tocopherols. That seems odd if you are of the mind that tocopherols are supposed to be ANTI-oxidants. More is good, right?
The thing is adding too much of an antioxidant can cause fats go rancid even faster -- a process called pro-oxidation.
So my thinking is ... don't use tocopherols unless you can use the right kind AND you can measure the natural tocopherol content in the fat to know if you should add more.
Rather than deal with all that, I "KISS it" and stick to ROE.
The chemicals that make ROE a good antioxidant aren't naturally found in regular fats, so there's not as much risk in overdosing with ROE as long as I measure carefully and follow the dosing guidelines.
I had another question about tocopherol I was gonna make a post for but you already kinda answered it haha thank you @DeeAnna!
And yes, the closest I can find ROE that will be easy to ship aka hand carry is from India but that will have to wait til they lift flight suspension Shipping from non ASEAN countries is horrendous here...