Do I need a preservative?
Short answer
Any product containing water does. Anhydrous products (oils, butters, scrubs, bath salts) don't, as long as water never gets in.
Explanation
Water, including hydrosols, lets microbes grow, so every water-containing formula in the book has a broad-spectrum preservative. The book names specific systems: Geogard Ultra (about 1%), Leucidal Liquid (2%, a 'natural' option), Geogard 221 (1%), Geogard ECT (0.5–1%, stable at acidic pH), Euxyl PE 9010 (0.8%) and LiquaPar Optima (0.8–1.2%). Caprylyl glycol boosts preservatives, and disodium EDTA protects peptides by binding metal ions.
Oil-only products rely on antioxidants such as vitamin E and rosemary CO2 extract to slow rancidity.
What to do
- Add the preservative at about 35 °C, after the actives.
- If you can't get a specific preservative, use another named system at its maker's recommended dose.
Source: The Big Book of Homemade Skin Care — All water-based formulas in The Big Book of Homemade Skin Care