Fatty aldehydes. The smell of clean.

Smelling one aldehyde alone

Fatty aldehydes are chain molecules: a row of carbon atoms, one end carrying an aldehyde group. Perfumers name them by carbon count. C10 is decanal. C11 is undecanal. C12 is dodecanal, also called lauric aldehyde. There are branched forms too, such as 2 methylundecanal, known in the trade as MNA.

Put a drop of concentrated C10 on a paper blotter and the first impression can be harsh, oily, almost unpleasant. Diluted, it becomes tangerine and orange peel, a little soap. C11 is softer, waxy, metallic. MNA is cold and bright, with incense and tangerine rind; some say it smells like snow.

They are not foreign to nature. Orange peel, mandarin, coriander leaf and kaffir lime all hold a little. But there they are a small part of a larger smell. Only when isolated and used at higher doses do they show their character.

And that character is hard to name with objects. People speak of a candle just blown out, a hot iron on cotton, sheets fresh from the wash. All moments, not things.

From the laboratory to N°5

Chemists knew fatty aldehydes in the nineteenth century, and by the early twentieth they appeared in a few perfumes. But they were hard to purify, unstable, and usually used sparingly.

In 1921 Ernest Beaux made N°5 for Gabrielle Chanel. According to Nez magazine, he used a blend of C11, unsaturated C11 and C12 at about one percent, far more than earlier perfumes. Good quality aldehydes had only just become available from Givaudan in Lyon.

N°5 was not the first perfume with aldehydes. But it made them the subject. Beneath lies a large bouquet: jasmine, rose, ylang ylang, orris. Above it sits a dry, slightly metallic light that makes the bouquet resemble no garden. It resembles a room that has been carefully tidied.

The French perfumers' society later gave this structure its own branch: floral aldehydic. N°5 is its landmark.

Aldehydes as a state

Most perfume materials can be named by object. Rose oil is rose. Sandalwood is wood. Vanillin is vanilla. Fatty aldehydes resist. What they evoke is not a thing but a condition: dry, clean, bright, cold.

Perhaps that is why they travelled so far from perfume. The smell of detergent, fabric softener, hotel towels relies largely on the same family. Over the twentieth century they taught people to smell cleanliness.

This makes fatty aldehydes something like an abstract noun in the language of smell. No one can point at clean. But everyone recognises it.

Is clean a smell, or no smell?

We tend to think a clean room is a room without smell. But step into one room wiped with plain water and another wiped with floor cleaner. The second feels clean. Cleanliness has its own smell.

That smell is the result of a century of learning. Nothing makes candle wax and orange peel naturally suggest fresh laundry. The link was built, through N°5, through soap, through detergent, through millions of repetitions.

So fatty aldehydes are an unusual material. They record not an object but a human habit: the wish that the surfaces around us look untouched. Each time we smell clean, we smell the trace of a task just finished.

In the Lê Mai library, fatty aldehydes are recorded as molecules found in small amounts in citrus peel and coriander leaf. At that level they cannot do what N°5 did. They only remind us that the dry light was already there in a piece of fruit peel.