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A smelling strip dipped in lemon oil loses its scent after about four hours. Another dipped in patchouli is still recognisable after more than four hundred. Both figures sit in the same library table, and they answer the question of how long a perfume lasts better than any single figure: there is no one duration, only many molecules leaving at their own pace.

A question with no single number

The question of how long a perfume lasts usually expects an answer in hours. But a perfume is dozens to hundreds of molecules, each with its own evaporation rate. What is called longevity is really the time the slowest molecules remain in sufficient quantity for the nose to notice.

The library's volatility chart ranks the 57 oils Lê Mai sells by the hours their scent stays recognisable on a blotter at full strength, using data from The Good Scents Company. The spread between the two ends is wide: rosemary and expressed lemon about four hours, lavender and mandarin about eight, rose about 168, frankincense about 188, while patchouli, sandalwood, vanilla and myrrh reach the source's ceiling of 400 hours.

The library itself states the limits of these figures. Blotter hours are not wear time on skin, in soap or in candles. Temperature, dilution, raw material lot and the sensitivity of the nose all shift the figure. The chart supports relative comparison between materials, not a forecast of when a particular perfume will fade.

Light molecules, heavy molecules

A molecule's volatility is measured by its vapour pressure. According to the library's glossary, high vapour pressure molecules reach the nose first and fade early; low ones linger. A rough proxy is molecular weight: limonene, the main molecule of citrus peel, weighs about 136 g/mol; linalool about 154; khusimol from vetiver about 220; ambroxide about 236.

That is why bergamot oil, with limonene at about 27 to 52 percent, appears as a top note in many classic perfumes the library lists, from Farina's Eau de Cologne of 1709 to Eau Sauvage of 1966. It opens a scent brightly, but it is not given the job of staying.

At the other end is the base. The library's vetiver page describes the roots being distilled for 24 to 48 hours, far longer than most materials, because time is needed to draw the heavy compounds out of the root. The oil is rich in sesquiterpenoids such as khusimol and the vetivones, and is recorded as a lasting fixative. Ambroxide, a constituent Firmenich synthesised from sclareol in 1950, is described by the library as very diffusive and very long lasting.

Weight is not everything. Vanillin weighs about 152 g/mol, lighter than linalool, yet vanilla sits at the ceiling of the volatility chart. The shape of a molecule and the way it holds to a surface matter too. Weight is only the first way in to understanding why citrus is brief while woods and resins endure.

Concentration and solvent

The same scent formula can be made at several concentrations. According to the library's glossary, eau de cologne typically carries about 2 to 5 percent concentrate, eau de toilette about 5 to 15 percent, eau de parfum about 15 to 20 percent, extrait about 20 to 30 percent. The library stresses that these are industry conventions, not legal standards; each house sets its own levels.

Higher concentration usually means a longer lasting scent, because more slow molecules sit on the skin and take longer to fall below the detection threshold. But the relationship is not a straight line. An eau de toilette rich in woods and musks can outlast an eau de parfum built mostly on citrus and light florals. The library also notes that the EDT and EDP of one perfume name can be two different formulas.

The solvent is the rest of the story. Perfume uses high strength alcohol as its solvent; the alcohol evaporates within the first minutes and takes some of the lightest molecules with it. A perfume oil, made in a carrier oil instead of alcohol, evaporates more slowly and stays closer to the skin. With the same blend of oils, changing the solvent changes the whole rhythm of the scent's departure.

Skin, hair, fabric

The surface receiving the scent is a variable too. Skin is warm, so molecules on it evaporate faster than on a cool surface. Skin also absorbs part of the fragrance and has its own odour, so one perfume can open differently on two people. That is why the volatility chart measures on paper: a blotter is neutral, without body heat, and lets materials be compared with each other.

Fabric holds scent differently. Its fibres are porous, cool, absorb nothing into a bloodstream, and generally hold heavy molecules longer than skin. On the other hand, some coloured materials, such as amber brown oils, can mark light fabric. Hair sits in between: a fibre, but warm from the scalp, and constantly moving.

There is a safety note tied to surfaces. Expressed citrus oils such as bergamot can cause phototoxic reactions on skin in sunlight, according to the library's phototoxicity page. Wanting a scent to last is no reason to spray more on sun exposed skin; the piece on lime peel and sunlight sets out the specific limits.

The wearer is often the worst judge

The nose adapts very quickly. According to the library's glossary, olfactory fatigue is the reduced perception of a smell after continuous exposure, and the way to recover is to step away from it for a while. The person who has just sprayed a perfume stays inside that scent for hours, and is therefore often the first to decide it has gone, while those nearby can still smell it.

Many complaints that a perfume fades fast are really a nose that has stopped paying attention. The sense of smell is organised to notice change, not to monitor a steady odour. A simple check is to ask someone else, or to smell the wrist after leaving the room for a few minutes.

There is another phenomenon: specific anosmia. The library notes that some people cannot smell one particular compound while their sense of smell is otherwise normal, and that this is fairly common with certain synthetic musks. A perfume built on a musk base can last a very long time on skin while its wearer never perceives that part at all.

Longevity should also be kept apart from projection. The library defines sillage as the trail of scent left in the air as the wearer passes, while tenacity is how long the scent stays on the surface itself. One scent can cling to the skin for a long time yet sit very close; another can project far in the first hour and then retreat.

Testing longevity on a strip

Instead of folk tips, the most reliable method is to observe directly with a smelling strip. Dip or spray a strip of paper, write the name and time on it as the library recommends, then leave it somewhere airy and out of the sun. Smell again after fifteen minutes, after an hour, after four hours, after eight, and the next day. Note when the character changes, and when only the base remains.

To compare two bottles, use two identical strips, the same amount, the same moment. Between sniffs, walk away from the strips so the nose can recover. The same test can be repeated on a small piece of fabric and on the wrist, to see how one perfume behaves differently on three surfaces.

The test does not produce an absolute figure, but it produces a map: which part of the scent leaves early, which stays, and what the bottle's true base really is. For anyone who wants a scent to last longer, that map is usually more useful than a number of hours printed in an advertisement. The piece on the materials that make scent stay continues into the base of a perfume.

A scent does not leave all at once.

It leaves molecule by molecule, light first, heavy last.

How long only has an answer once the question becomes: which part of the scent.