A freshly flayed animal hide does not smell of what people call “leather”. That smell comes from what humans do to the hide: tanning agents, oils, smoke, resins. The leather note in perfume is therefore the smell of a craft more than of an animal. It can be smoky and tarry, soft as suede, or green and bitter like damp earth.
This article goes through the materials that build a leather note, and why the most famous of them may only be used in purified form.
Russia leather and birch tar oil
The most classic leather note in perfumery imitates Russia leather (cuir de Russie), leather dressed with birch tar oil to make it waterproof and supple. The oil is not steam-distilled like an essential oil. Birch wood and bark are heated with little air, a process called dry distillation, and the liquid tar that runs off is collected.
It smells of smoke, tar and antiseptic. Most of that comes from phenols formed as the lignin in the wood breaks down under heat: guaiacol, creosol and the cresols. This is the same group of compounds as in straw smoke and smoked meat, only far more concentrated.
But dry distillation also produces polycyclic aromatic hydrocarbons (PAHs), compounds that can cause cancer. For that reason the IFRA Standard prohibits crude birch tar oil as a fragrance ingredient. Only rectified oil meeting the PAH limits may be used: benzo[a]pyrene and 1,2-benzanthracene together must not exceed 1 ppb in the finished product. Cade oil, dry-distilled from the wood of Juniperus oxycedrus, is under the same rule. Both standards belong to the 47th Amendment, published in 2013.
In practice, a retail bottle of “birch tar oil” that does not state it is rectified is a bottle that should not go on the skin.
Other materials
| Material | The leather note it brings |
|---|---|
| Rectified birch tar oil | Smoke, tar, Russia leather |
| Isobutyl quinoline | Green, bitter, earthy leather; very strong |
| Labdanum | Warm leather, honey, dark resin |
| Castoreum (now mostly substitutes) | Animalic leather, slightly smoky |
| Synthetic saffron odorants | Soft, slightly metallic, suede-like leather |
Isobutyl quinoline is a synthetic with a bitter leather smell, somewhat like roots and damp soil. It is so strong that a tiny amount changes the character of a whole formula. Many of the green, dry leather notes in classic perfumes rely on it.
Labdanum sits on the border between leather and amber. Smelled alone, many people describe it as old leather and dark honey. Paired with vanilla it becomes amber; paired with smoky phenols it becomes leather. The article on amber covers the other side of this material.
Castoreum was once taken from the scent glands of beavers. Today it is almost always replaced with reconstructions, for ethical reasons as well as cost.
Suede and clean leather
Not every leather note is smoky. Many recent perfumes use a “suede” note: soft, powdery, a little like iris and musk, closer to the inside of a new leather bag than to a tannery. These notes are usually built from synthetic musks, saffron odorants and a touch of the iris family, with almost no phenols.
The two styles are easy to tell apart side by side: Russia leather makes you think of fire, suede makes you think of cloth.
Leather in cars and everyday objects
The “new leather” smell of a car seat or a handbag is yet another story. It is the sum of tanning agents, softening oils, surface finishes, adhesives and the plastics around them. The article on new car smell explains which parts of that smell come from interior materials. The leather note in perfume picks only the pleasant part of that story.
Sources
- IFRA Standard: Birch wood pyrolysate (crude and rectified). Amendment 47, 2013. ifrafragrance.org
- IFRA Standard: Cade oil (crude and rectified). Amendment 47, 2013. ifrafragrance.org
- Ohloff G., Pickenhagen W., Kraft P. (2012). Scent and Chemistry: The Molecular World of Odors. Verlag Helvetica Chimica Acta, Wiley-VCH.