In a small bowl or jar, mix together the zinc oxide, sea salt, baking soda, and bentonite clay.
Slowly stir in the witch hazel (or water) until it reaches a creamy consistency.
Mix in the essential oil and preservative if using.
Store in an air-tight jar in the fridge and use within two weeks (one week if using water in place of witch hazel).The recommended preservative is broad-spectrum, all natural, and used at a 4% rate. It's only necessary if you're not mixing up the dry powder with liquid right before use.