Sillage is the trail of perfume left in the air as the wearer passes. It differs from projection, how far a scent radiates around someone standing still, and from longevity, how long it stays on skin. A scent can project strongly yet last briefly, or the reverse.
A trail in a corridor
Someone walks down a corridor and turns into another room. A few seconds later, where they passed, the air still smells. That smell does not come from the skin of anyone nearby, but from molecules that have left the skin and hang in the air. French calls that trail sillage, originally the wake behind a boat.
The short definition: sillage is the trail of scent left in the air as the wearer moves. It differs from longevity, which is measured on the surface itself, skin or blotter, and usually recorded in hours. It also differs from projection, as perfume sellers call it: how far a scent radiates around someone standing still.
The three words are often confused. A bottle praised for strong projection may be strong only in the first hour. A bottle praised for longevity may sit close to the skin, barely noticed by anyone nearby. Separating the three helps read a description correctly, and helps explain why two bottles of the same strength behave differently.
All three depend on molecules. Some molecules evaporate fast, some cling for long, and a small group makes a whole formula spread. This piece looks at that third group.
Sillage, projection, longevity
Longevity is the easiest word to measure. The Lê Mai library calls it tenacity: how long a material or scent stays perceptible on a blotter or on skin. The volatility chart records these hours for each oil Lê Mai sells, using figures from The Good Scents Company: citrus often lasts only hours to a few dozen hours on a blotter, while woods and resins can exceed several hundred.
Projection is harder to measure. It is the ability of a scent to leave a surface and fill the space around it, which the library calls diffusion. By that definition, a material can diffuse well yet last poorly, or last well yet diffuse poorly. The two properties do not travel together automatically.
Sillage is projection plus movement. As the wearer walks, the air around them is drawn along, and the molecules that have diffused are left behind. A scent that projects well usually has a clear sillage; a scent that sits close to the skin leaves a thin trail.
All three are shaped by things outside the bottle: skin temperature, warm or cold air, wind, clothing. So the sillage or projection ratings found online are usually one person's impression in one setting, not a measurement.
Diffusive molecules
A molecule that evaporates fast does not necessarily travel far. Limonene, the main molecule of orange peel, has a molecular weight of about 136 g/mol and leaves the skin very early; the fresh orange note is clear in the first minutes, then thins. It evaporates quickly, but fading fast is not the same as spreading wide.
What decides whether a molecule is noticed at a distance is its odour threshold: the lowest concentration in air at which half of a test panel detects an odour. Thresholds differ by several orders of magnitude between molecules. A molecule with a very low threshold can be noticed by a distant nose even when only a little of it arrives.
Projection also depends on quantity. A material used at an unusually high proportion in a formula, what the trade calls an overdose, has more matter to spread. Hedione is the early example most often cited, as recorded below.
The three synthetic molecules most often cited in discussions of projection are hedione, Iso E Super and ambroxide. All three are much heavier than limonene, with molecular weights of about 226 to 236 g/mol, and all belong to the heart or base. They show that projection and evaporation speed are separate matters.
Hedione and Iso E Super
Hedione, chemically methyl dihydrojasmonate, was characterised and synthesised by Edouard Demole at Firmenich between the late 1950s and 1962, during work on the methyl jasmonate of jasmine. Its scent is transparent jasmine with a light citrus side, airy; the library describes it as giving a sense of space and radiance.
Christian Dior's Eau Sauvage of 1966, composed by Edmond Roudnitska, is usually cited as the first commercial perfume to use hedione at a significant level. Since then hedione has become one of the most used materials in modern perfumery, sometimes a large share of a formula. It lightens and widens the floral, citrus and woody accords around it. Hedione has no dedicated IFRA Standard and is not on the EU list of fragrance allergens requiring labelling.
Iso E Super was developed and patented by IFF in the early 1970s. Its scent is smooth, dry wood with an ambery feel, very transparent. Oddly, many people perceive it only faintly, yet it makes everything around it radiate more widely. The commercial product is an isomer mixture, and a minor isomer with the strongest odour contributes much of the strength.
Iso E Super is used in almost every fragrance family, with some formulas using it at tens of percent. Unlike hedione, it is restricted: IFRA has a Standard for this group of materials, published in 2020, setting limits by product category; under Regulation (EU) 2023/1545 it is on the list of fragrance allergens that must be named above set thresholds.
Ambroxide and mineral radiance
Ambergris is an aromatic mass found at sea. Within it, ambrein slowly oxidises at sea, and one of the products is ambroxide, the most important odorant of ambergris.
In 1950 Max Stoll and M. Hinder at Firmenich synthesised ambroxide from sclareol, a diterpene obtained from clary sage. Several makers sell it under their own names: Ambrox, Ambroxan, Cetalox, Ambrofix. Some newer processes use sugar fermentation to make the precursor instead of sclareol.
Ambroxide smells of dry, clean amber with woody and mineral facets, like warm skin after sun. The library records it as very diffusive and very long lasting, both projecting and lasting, a pairing not every molecule has. At low levels it adds diffusion and a warm skin effect; at high levels it can become the main note of a formula. Ambroxide has no dedicated IFRA Standard.
The pairing of ambroxide and Iso E Super underlies many minimalist woody amber perfumes. Both molecules sit in the base and both radiate widely, so scents built around them tend to keep almost one shape from start to finish and leave a clear trail in the air.
Strong projection does not mean long wear
A scent that projects strongly in the first hour may owe it to a dense top, to alcohol lifting light molecules, or to a material with a low odour threshold. None of these guarantees the scent will remain after five hours. Conversely, a scent that sits close to the skin may still be perceptible the next day, when almost nobody nearby notices it.
The wearer is not a good judge either. Smelling one scent continuously reduces perception, an effect called olfactory fatigue. After a while the wearer may think the scent has gone while someone walking past still notices its trail. With some synthetic musks there are also people who cannot smell them at all, as the piece A scent half the room cannot smell records.
So sillage, projection and longevity are best read separately. A description of strong projection speaks of the air around a person; a description of long wear speaks of the surface of the skin. Both depend on molecules, concentration and setting, and no description replaces smelling over several hours.
The piece How long before a scent leaves continues with longevity: why a scent remains on skin, and what makes it leave early.
A trail of scent is a scent that has left the wearer.
What travels far does not always stay long.
The air keeps what the skin does not.