Metallic Scent In Perfumery

by: Matvey Yudov

From time to time, when experiencing perfumes or household product smells, we notice a metallic aspect. People often call it a "metallic" smell, saying it smells like metal or steel. But does metal have a smell? To perceive a smell it has to be volatile; molecules have to physically fly into your nose, come into contact with a receptor and launch a chain of biochemical processes that the brain interprets as a certain type of odor. But it does not seem that metals are excessively volatile, with the exception of mercury. Besides that, smell is a characteristic of substances with a molecular structure, but substances with an ionic lattice (for example, inorganic salts) or a metallic lattice (iron) should not smell.

Wait a minute, you say, but metallic things do smell! They have a very specific and very recognizable smell which is hard to misinterpret. Door handles, swings, coins, keys, screwdrivers, spoons... Now take a good look at the list, do you see a certain pattern? Yes, all of these metallic things get in contact with our skin! That is the secret of a metallic smell.

The main cause of a metallic smell is hidden in fats, which are oxidized under the impact of oxidoreductases (a group of enzymes which catalyze the transfer of electrons from one molecule to another, which is called oxidation for losing electrons, and reduction for gaining them). Iron cat-ions Fe2+ decompose lipid peroxides, producing intensively smelling substances. Only a small amount of iron ions is needed to produce such an amount of volatile substances which we are able to smell.

For the same reason, blood has a characteristic metallic smell; iron contained in hemoglobin launches a similar reaction. During evolution we have acquired a special sensitivity to this smell: some predators can sense it miles away.

What are these substances which cause a metallic smell? Usually, these are unsaturated (often conjugated) aldehydes and ketones. One of them, oct-1-en-3-one, or amyl vinyl ketone you can meet in the perfume lab. In its pure form, it has an earthy metallic smell with mushroom and vegetable nuances, and some kind of fishy note, a similar smell you sense while preparing freshly caught sweet water fish. 

The smell of unsaturated aldehydes is also very common. There are many of them in nature. The specific smell of cilantro is caused by conjugated unsaturated aldehydes (E)-2-Decenal, (E)-2-Dodecenal, (E)-2-Tetradecenal and others. Some insects use similar substances as their protective chemical weapon. For example, more than 50 years ago, (E)-2-Decenal and (E)-2-Octenal (a lighter homolog of the named compound) were isolated in stink bug secretion.
 

Recently, scientists of the Linkoping University in Sweden lead a comprehensive research on the smell of blood. They distinguished the key component of the smell of blood, it appeared to be trans-4,5-Epoxy-(E)-2-decenal, a "bloody aldehyde". Our sensitivity to this compound is extraordinary – it could even be sensed in the dilution of 0,07-0,3 ppt (part per trillion). Similar compounds, – trans-4,5-epoxy-(E)-2-octenal, trans-4,5-epoxy-(E)-2-nonenal, trans-4,5-epoxy-(E)-2-undecenal, – have a similar smell to blood.

In the perfumer's palette, there are many materials with a certain metallic character. Many would agree that you can find a certain metallic nuance in Rose oxide, a minor component of the rose smell. Geranium contains an even bigger quantity of rose oxide, that is why geranium oil smells metallic at some point. Rose oxide participates in the forming of a characteristic green-metallic note in the aroma of the white wine Gewürztraminer.

We can find rose oxide as a chemical defense in the Aromia Moschata, or musk beetle. Mark Buxton loves using rose oxide in his works: Comme des Garçons 3 has a serious overdose of it.

Nitriles are another group of substances which smell kind of metallic. Nitriles are quite stable and inert, and often used as odorants for bleaches and aggressive acids for household use. Sometimes nitriles are used for adding a specific metallic context to a perfume, like in several provocative creations by Antoine Lie.

Some people smell a metallic nuance in lavender. In a classic fougere, a cool lavender metal very often comes along with a metallic nuance of geranium, and balanced with the warmth of coumarin and balsamic materials. In fougeres of the  new generation, conceived by Davidoff Cool Water, the metallic context is enhanced by additional Dihydromyrcenol and Allyl amyl glycolate (the latter smells like a can of pineapple preserve).

Do you like metallic notes in perfumery? Are they addictive, irritable or frightening to you?