How is a perfume made, and what are top, heart and base notes?

A perfume is made by blending fragrant ingredients, some pulled out of plants and some built in a laboratory, into a mixture designed to change as it dries on your skin. Nobody wants a scent that smells the same for ten seconds and then vanishes, so perfumers compose in three stages, using a musical metaphor of top, middle and base notes, which together make the harmonious scent accord.

The ingredients come first. Synthetic odorants are made by organic synthesis and purified, while odorants from natural sources need various methods to extract the aromatics from the raw materials. In steam distillation, steam is passed through the raw material and drives out its volatile fragrant compounds. The Persian chemist Ibn Sina is credited with introducing the extraction of oils from flowers by distillation, and he first experimented with the rose. Citrus peels are different: their oil is abundant enough to be extracted by squeezing.

Then comes the order of the performance. The top note is the immediate impression, built from small, light molecules that evaporate quickly. As it fades, the middle or heart notes emerge, the main body of the perfume, which also mask the often unpleasant first impression of the base notes. Finally the base notes appear, bringing depth and solidity, usually not perceived until 30 minutes after you put the perfume on.

Perfumers also choose how strong to make it. Pure perfume contains 15 to 30 percent aromatic compounds, while an eau de cologne has only 2 to 3 percent. And sometimes a single ingredient defines a legend: Chanel No. 5, launched in 1921, owes its crisp, modern character to its use of aldehydes as aroma boosters.