To start with, there’s two main categories of product when we’re talking shelf life: things that are 100% oil based (lip balms, body butters, salves), and things that contain water (lotions, sprays) or might come into contact with water (body scrubs designed to be used in the bath).
For things that are 100% oil based
The kind of spoilage we’re concerned about here is rancidity, which is oils oxidizing and starting to smell funky (like old crayons). Some oils oxidize faster than others, so if you add an oil with a short shelf life (but anything less than one year is considered short), that will shorten the shelf life of your product. You can extend the life of 100% oil based products by adding an antioxidant, like vitamin E, during the cool down phase.
Adding anything that contains water (not just water, but hydrosols, teas, extracts, etc.) will bump your concoction to the other category and will drastically shorten the shelf life.
For things that contain water
Anything that contains water needs a broad spectrum preservative, but those are not infallible. The more delicious bug food we add to our potions, the faster we’ll override that preservative and the faster our products will spoil. Here’s a list of things that will speed spoilage (it is by no means exhaustive):
- Any kind of food (milks, flours, starches, honey, syrup, sugar, nuts, etc.)
- Any kind of plant matter (aloe juice, hydrosols, etc.)
- All herbs and herbal infusions
Some of these things will spoil faster than others. Dairy milk is notoriously difficult to preserve and I would not recommend including it in anything that you don’t intend to use up that day. Clay masks are also very hard to preserve, so I would recommend only mixing the dry parts up in advance.
The amount of bug food you use is important, too. 1% honey is very different from 10% honey. If you want to include some oat milk or hydrosol in a lotion recipe, try replacing only half or a quarter of the water with it instead of all of the water.
You should also take a look at commercially produced herbal infusions and extracts rather than homemade to extend shelf life.
For things that could become contaminated with water
Honestly, the easiest thing to do here is to not allow the concoction to get wet. Scoop out however much scrub you need for a single use and take that to the tub in a small plastic dish. Put bath oils in pump top bottles. Use a dry finger or popsicle stick to remove cleansing balms from their containers.