In a blend, the base notes are slowest to evaporate. Aromatherapy oils such as sandalwood and vetiver that takes longer to evaporate. They take longer to absorb into the body if you're putting them in massage oil. They are slowest to evaporate, and they give depth to the blend and hold the scent. If you are making a blend, ideally you would like to do is have a top, middle and base note. The top note is the first and the quickest to evaporate, so it takes the scent out quickly, and usually top notes are hot or cold. For example, sharp aromatherapy's like citruses and eucalyptus would be a top note. They carry the scent out, but they disappear quickly. You're middle notes are the hard notes of the blend. Usually if you're making a blend, you would have the predominance of middle or hard notes. Your base notes are the slowest to evaporate, the slowest to penetrate and they act like a fixative to the blend. As the others disappear, what remains is more of your base notes. They would be things like the vetiver, sandalwood, and patchouli, which are your heavier base notes. So, together you have a harmony. When you do it properly, the top notes will carry the blend out. The base notes slow it down, so it doesn't evaporate as fast and there is nothing left. Then the hard notes are kind of the focus of the blend and I like to think of it like a train. I like to think of the engine as your top notes, all the cars in the middle are your middle notes, and then the caboose is the base note. The hard note is like the center of the blend. The predominance of the blend would be the hard note.
