Clay is incredibly hard to preserve, so it's always best to make a clay mask just before using it by adding it to a small bit of lotion or a toner or something similar. In my recent botanical extract (part one) e-zine, I made a bunch of these by creating the clay portion, then adding a gel or toner to it before applying to my face. Add your hydrolyzed proteins, hydrosols, and other water soluble ingredients to the toner or gel, and add powdered botanicals to the dry clay mask portion.
If you want to add clay to a product that you'll preserve, please use a really strong, broad spectrum preservative designed for hard to preserve products, like Phenonip or Germaben II at 1%, and only keep it for a week or so. After that, you're asking for contamination.
red reef clay from Windy Point Soap in Alberta. It's a type of kaolin clay. It's an awesome burgundy colour, but note that a little goes a long way. Make sure you do a patch test before doing a full, thick facial mask.
As an aside, please think twice before adding essential oils to your facial products. I know they can offer some pretty awesome properties, but they also add a fragrance that is pretty hard to wash off when you're sick of it. I know lavender seems lovely at first, but picture wearing it all day just under your nose. If you wish to use EOs this way, please start with 0.05% or 0.1% and see what you think.
You have so many to choose from, and they're so inexpensive when compared to other ingredients. As someone with oily, sensitive skin, I like white and red reef kaolin, pink clay, and glacial clay. Buy a few and see what you like best! Remember to keep great notes!