fragrance

This is a continuation of the education pieces I've been posting recently. If you missed them here are the first two:

• An Example of how Commercial Fragrances are Composed

• Notes on Notes

Everyone seems to have a fragrance they love that just doesn’t last. I see online posts all over the place expressing dismay and even anger about how short lived certain fragrances are (“If I pay $200 for a fragrance, it should last all day!”)

There are an awful lot of ideas about the reasons for why fragrances last as long (or as short) as they do: EdPs last longer than EdTs. God forbid you get an EdC! That smell will be gone before you’re out the door! I see people saying that higher quality, more expensive fragrances with lots of naturals will last longer. I also see people saying that you have to go with synthetics in order to get good longevity.

It’s both more simple and more complicated than most people think.

Let’s take few step back. You smell a fragrance because molecules of it have evaporated into the air and are floating around. When you inhale, the fragrance molecules get sucked into your nose and bump against your scent receptors. The scent receptors send information to your brain about what you’re smelling and, voila! You can smell the bread baking in the kitchen.

These scent receptors aren’t infinitely sensitive. There has to be a certain concentration of a material in order for your olfactory receptors to register it and for your brain to interpret it as a smell. That concentration is called the “threshold of detection” and it’s measured in parts per million/billion/trillion. The threshold of detection is different for different materials, often several orders of magnitude different. Geosmin, one of the molecules responsible for petrichor (the after-the-rain smell) is detectable in parts per trillion. A single drop of it in a kilo of fragrance concentrate leaves a prominent smell.

Beyond the threshold of detection is the threshold of identification. This is the concentration at which you can not only determine that there’s something is present, you can identify what it is.

So one could describe the longevity of a fragrance in technical terms as being the amount of time that some part of it remains in the air in a concentration that exceeds its threshold of detection.

The second important factor in longevity is the evaporation rate of a particular material, which is determined by the vapor pressure. I mentioned Geosmin earlier as being a material that humans are incredibly sensitive to, but it doesn’t last a long time in fragrances because it evaporates and disperses very quickly.

The longest lasting materials are the ones that evaporate slowly and that people are good at smelling.

Typically larger molecules evaporate more slowly than smaller molecules. Musks, for example, are very large molecules. On paper test strips, they can last for weeks.

Limonene, on the other hand, is smaller. It evaporates much more quickly, only lasting a few hours on paper.

There are other factors that affect evaporation rates as well. For example some molecules stick together more than others, making them slower to evaporate. Often the presence of larger molecules slows down the evaporation rate of other, lighter molecules as well. This is what people are referring to when they say that a material is a fixative. Perfumers will quite often use small amounts of materials that are good fixatives, just for this effect (iso E Super and white musks, for example).

Heat is another factor. As a material heats up, it evaporates more quickly. This is why a fragrance that’s seems perfect in cooler weather can seem powerful enough to be cloying as temperatures rise.

Surface area exposed to air is another. A narrow necked bottle with a liter of water in it will evaporate more slowly than a liter of water spread out on the ground in a thin layer. This is one of the reasons that most fragrances have atomizers. A spritz from an atomizer spreads the fragrance out in a nice thin layer over a large area, maximizing the surface area.

Finally, the amount present affects longevity. A drop of water will generally evaporate more quickly than a liter.

I could write much more on this topic (and might at some point in the future), but this should be enough to allow for a discussion about longevity.

Oh, actually, there’s one other related topic that doesn’t explicitly affect longevity, but is important to understanding the behavior of a fragrance as it evaporates.

If you double the concentration of limonene molecules hitting your olfactory receptors, does that make the limonene smell twice as strong? It turns out that it doesn’t. Every material has a different “slope,” or the rate at which perceived intensity increases as concentration increases. The average is somewhere around 1.2.

In other words, when you double the concentration of a material, it only smells 1.2x as strong, not 2x as strong. This meant that in order to double the perceived intensity of most materials, you’re looking at increasing the concentration by closer to 10x than 2x.

Some materials have an even lower slope than that. When you combine a low slope and a slow evaporation rate, you end up with a material that seems to have a relatively consistent strength over a long period of time...it doesn’t start out strong and then get weaker quickly. Instead it will often seem to become more prominent over time as the more powerful (but shorter lived) materials burn off. Musks are a good example of this.

So now, after reading all of this, you should have the understanding needed to discuss fragrance longevity in more practical terms.

The concentration of a fragrance (the amount of fragrance base in carrier) matters much less than the composition of the fragrance. I’m not go into great detail into the differences between Eau de Cologne, Eau de Toilette, Eau de Parfum and Parfum in this post, but I will point out that the difference between them is not always just the concentration. Quite often, the formula changes as well. When the EdP version lasts a lot longer than the EdT, that’s more likely the result of difference in formulation than it is a difference in concentration.

I often say that concentration is nowhere near as important as people think it is when it comes to longevity. I always get strange looks when I say so because i’m directly contradicting common Internet wisdom. Think about it though:

The entire reason that alcohol is used as a carrier is that it allows for a fine mist of fragrance oils to be dispersed across the skin...and then it evaporates.

I talked about vapor pressure earlier. Ambrettolide, a very long lasting base note, has a vapor pressure of 0.00016. It lasts on skin for several hours. Linalool, a top note, has a vapor pressure of 0.016. It lasts on skin for about an hour. Ethanol has a vapor pressure of 55 (mmhg@25C, not Kpa). It lasts for a minute or two, then it’s gone.

There are math equations that you can do to calculate how long it’s going to be before all that ethanol that’s gone. Within a couple minutes it’s just gone.

Go ahead and spray fragrance on your skin. You can watch the ethanol evaporate.

Once it’s gone, the fragrance you just sprayed on is now at 100% concentration, regardless of whether it started out at 5%, 10%, 15% or 100% when it was in the bottle.

If you have a bottle of a particular fragrance at 20% and another bottle of the exact same formula at 10% and you do one spray of the first and two of the second over the exact same area, after 5 minutes the fragrance applied will be pretty much the same.

The amount of ethanol that used to be in a fragrance, back when it came out of the bottle 8 hours ago does not in any way affect the behavior of the pure materials left on your skin 8 hours later.

So what does?

The composition of the fragrance. Fragrances with low vapor pressure, low threshold of detection materials will last the longest How much fragrance base you apply (I.e the amount left after the alcohol evaporates). Two sprays of a 10% fragrance will apply more material than one spray of a 15% fragrance. Also, atomizer volume (the amount of material sprayed out in each spray) can affect this. Olfactory fatigue.

We haven’t talked about olfactory fatigue yet. When you constantly smell a smell, you get used to it. Your brain starts to edit it out. You stop smelling it.

Some materials cause olfactory fatigue more quickly than others. And some materials are found all over the place, like, say laundry products. A lot of people are walking around in a constant haze of olfactory fatigue to certain materials.

It turns out that the laundry industry snatches up just about every material that smells fresh and can survive 45 minutes in hot soapy water followed by an hour in a hot dryer. A lot of people spend 24 hours/day, 7 days a week with cloth that’s soaked in fragrant materials less than a foot away from their nose and then they’re puzzled because a fragrance that’s supposed to last for 8 hours fades after 2, because they already had olfactory fatigue to half the materials that make up the drydown.

Also, to be frank, a lot of these materials are fairly subtle smelling and a lot of people who do smell them may not even recognize them as being part of that citrus/lavender/geranium melange that they applied several hours ago.

I’ve noticed that the fragrances that are perceived as lasting the longest often tend to get their longevity from materials that people don’t have as much olfactory fatigue to due to laundry products, e.g. vanilla or oud.

Appendix with some notes:

On reformulations with shorter longevity:

I’ve seen GCMS readouts from a good number of famous, modern fragrance. Without a single exception, every modern fragrance for which i’ve seen a GCMS, from Aventus to Sauvage to Coco Madmoiselle to Eternity has buckets of iso e super, white musks of various sorts and ambroxan. They’re relatively cheap, powerful and pleasant. They’re practically used as a filler. They also last a very, very long time.

I regularly see claims that a supposedly reformulated version of a particular fragrance “smells the same buy only lasts for a couple of hours.” Reformulation paranoia is another subject that i’ll likely touch on later, but even if the fragrance WAS reformulated, quite often the longest lasting ingredients are some of the cheapest. IFRA changes are also often cited as reasons for these supposed reformulations that kill longevity, but these cheap, long lasting materials are generally allowed at very high levels by IFRA. By “very high,” I mean that they could make up 100% of the fragrance base without running afoul of IFRA regulations.

To be honest, I don’t know where these claims come from, though I have a few suspicions.

On carriers other than alcohol:

If you’re using a fragrance that uses an oil based carrier like fractionated coconut oil, or an oil-like carrier like jojoba, the carrier doesn’t evaporate right away. Instead it stays behind and acts as a fixative. You’ll likely get better longevity but a more muted odor.

On using Iso E Super to make fragrances last longer:

Your fragrance is probably already drenched in Iso E Super. It’s a really, really inexpensive material. The perfumer didn’t add more of it for a reason. When you use too much it can make everything else smell flat and muted. Will it make the fragrance last longer? Maybe, but not that much longer. There’s already probably enough in there to get a good fixative effect.

On whether natural materials last longer than synthetics:

Some naturals last a long time. Most of the time, we can synthesize most of the molecules that make up these materials in a lab. Synthetics can also include all sorts of other stuff too though. I can think of materials for which the natural version smells better (deeper/richer/more complex), but I can’t think of any where the natural outlasts the synthetic. I can, however, think of plenty of materials for which there is a longer lasting synthetic version.

The type of fragrance being made (e.g. a heavy oriental vs. a citrus floral) typically has a lot more to do with longevity than whether it’s made mostly from synthetics or naturals, but if you’re using both you have a wider palette to work from and more choices for long lasting materials at each step along the way.