For some perfumes, the greatest challenge is getting them to last. Natural floral aromas, especially, evaporate quickly—they last no more than an hour or two on the skin—and need to last longer to be viable.
It has long been the supposition of perfumers, master and otherwise, that by combining a rapidly evaporating substance with a slowly evaporating one that the evaporation rate of the quickly evaporating compound is slowed down. With this in mind, heavy, thick and viscous substances are added to perfumes. Such substances as benzoin, musks, oppoponax, sandalwood and a myriad of others retard the evaporation of top and middle notes. Overdoing it—adding too much of a viscous substance—will flatten the perfume and create too much restraint.
Another approach is to recreate the perfume using heavier notes such that there’s a backup perfume profile to take over when the first evaporates. In other words, say you add benzoin. Because this compound is persistent, it will remain in the dry down and give the impression that the perfume is long-lasting, when in reality the first part of the perfume—that part from which it finds its identity—will have evaporated.
When I was investigating other iris perfumes when I was working on Green Iris, I found a number of perfumes that started out with a strong iris accord, but ended, hours later, smelling like something else entirely.
Perhaps the most famous fixative of all, is ambergris, confusingly referred to as “amber.” A small amount of tincture added to a perfume gives a life-likeness and vibrancy that otherwise would be missing. I noticed this with Magnolia, how after a few weeks from blending it seemed to blossom. Few perfumers use ambergris anymore which is strange since, while expensive, it is a lot cheaper than other things we still use. Substitutes such as ambroxan, ambrinol, ambercore, and andrane are typically used to provide this essential “amber” note. These are long-lasting compounds and, like ambergris, help fix the perfume.