For your safety and maximum shelf life, it is best to add a preservative when you are making a formulation that contains water or water rich ingredients. Products that contain water are called hydrous products. Water rich ingredients include water, hydrosols, and aloe. Examples of water rich products include creams, lotions, toners, and masks. Anytime you add water to a product, it can become contaminated with microbes within a matter of days, if the product is not preserved.
For home or personal use, some skin and hair care recipes that contain water rich ingredients can be made preservative free, but they need to be used up immediately or refrigerated and used up within a couple to a few days. For example, when you make a homemade fruit based mask, it is best to make only make enough for one use. Or if you make enough for 2-3 masks, then label and refrigerate it and use it up within 2-3 days. And use clean spoons or cosmetic spatulas when handling the mask.
If you don't wish to use preservatives, then make anhydrous products. Anhydrous products are waterless products. A few examples include balms, oil based perfumes, facial oils, and body oils. Anhydrous products usually do not need a preservative. This does not mean they can't become contaminated, but they are less likely to. To help the shelf life of an anhydrous product, I highly recommend adding an antioxidant, such as vitamin E or rosemary oil extract (note: vitamin E and rosemary oil extract are not preservatives).
In anhydrous products, you need to be careful when using them, to make sure that no water gets into them. If there is a possibility that water may get into an anhydrous product, it is best to add a preservative.
Li Wong has been studying plants for over twenty six years. She is a longtime qualified aromatherapist trained in clinical aromatherapy and advanced aromatic medicine. She is also a professional natural cosmetic formulator, a natural perfumer, and a longtime qualified herbalist trained in family, community, and clinical herbalism. She holds two degrees in environmental science/biology. Her environmental focuses include botany, ethnobotany, and conservation. Plant Alkemie is her educational website and school. http://www.plantalkemie.com