In a large mixing bowl, combine the baking soda, citric acid, corn starch, and mica powder. Break up any clumps with your fingers or a whisk. Mixing thoroughly at this stage helps prevent uneven color or texture later.
Pour in your lavender and sage fragrance oil and mix until it’s fully combined. You will want to wear gloves if mixing by hand.
Now comes the tricky part — adding just enough moisture. Using your spray bottle, lightly mist the mixture a few times, stirring between sprays.You’re looking for that wet sand consistency — damp enough to hold its shape when squeezed, but not so wet that it starts fizzing. If you drop it from a few inches, the shape should hold. Once your mixture is holding together like damp sand, it’s time to mold. Press the mixture firmly into your silicone mold. The tighter you pack it, the better it holds once dry.
Set the molds in a cool, dry place for 12 to 24 hours. Avoid moisture while they dry, since humidity can cause them to react early.