A batch of cold process soap receives its essential oils at the last minute, just before it goes into the mould. Six weeks later, the scent on the bar is no longer exactly the scent in the bottle. Some oils almost disappear, some change their voice, some change the colour of the bar, and some remain nearly intact. This piece follows each group of molecules to see why.
A drop of essential oil enters a lye pot
In the six week piece, plant oils are warmed to 38 to 40°C, the NaOH solution cools to the same temperature, then the two meet and are stirred to trace. Essential oils go into the pot at thick trace, together with herbal powders if there are any. From that moment, every aroma molecule has to pass through three stages.
The first stage is the alkaline environment. The freshly stirred batter still holds unreacted NaOH, and its pH is very high. The second stage is heat. In the mould, saponification generates its own heat, and the centre of the block reaches 60 to 70°C during gel phase. The third stage is time. A freshly cut bar has a pH of 9 to 10, then rests on an open rack for six weeks while water evaporates and the pH falls to 8 to 9.
When the essential oil goes in, the batter is at about 38°C, close to skin temperature. At that level, little is lost from the pot itself. The larger losses come later, when the block heats itself in the mould and when the bar sits in open air for six weeks.
These three stages do not act on every essential oil in the same way. What decides is molecular structure: whether a molecule is light or heavy, whether it carries an ester group, whether it carries an aldehyde or a phenol group.
Light molecules leave first
Sweet orange oil is the clearest example. The library records 86 to 93 percent (+)-limonene in Brazilian orange oil and 94 to 96 percent in Sicilian oil. Limonene is a small monoterpene that evaporates quickly. On a blotter it is the brightest part of the opening and also the first to leave.
In soap, limonene meets two disadvantages at once. Gel phase heat drives part of it away within the first day. Six weeks of curing in open air let the rest keep evaporating, and limonene also oxidises over time, as the library notes on its constituent page. This is why a bar scented only with orange oil usually smells clearly orange when freshly cut, then fades week by week.
The easiest way to see this is to note the scent of the same bar on the day it is cut, at week two, week four and week six, in the same way blotters are logged in the blending pieces. The citrus part is usually the line that changes most in the notebook.
Soap makers compensate in two ways. The first is to use more citrus oil within the permitted limit. The second, more common, is to place citrus over a heavier base, so that when the top has faded the bar still has a scent. That base is the subject of a later section.
Esters meet alkali
Lavender, Lavandula angustifolia, contains 25 to 45 percent linalyl acetate and 20 to 38 percent linalool, according to its essential oil page in the library. The two molecules build the scent of lavender together: linalool gives the floral woody side, linalyl acetate the sweet, slightly fruity side.
Linalyl acetate is an ester. Saponification is precisely the cleaving of esters by alkali: the triglycerides in plant oils are esters too, and NaOH splits them into glycerin and fatty acid salts. In the same pot, the esters of an essential oil can be cleaved as well, and linalyl acetate can split into linalool and acetate. How much is cleaved depends on when the oil is added, how much free alkali remains and how hot the mould becomes.
On the nose, the result is a lavender that leans towards linalool: less of the sweet fruity opening, more of the floral and woody side. Lê Mai's Lavender Artisanal Soap No.052 uses Lavandula angustifolia essential oil with dried lavender flowers; the scent on the bar after six weeks is a lavender that has passed through the lye pot, closer to dried flowers than to a freshly opened bottle.
Bergamot is also rich in linalyl acetate; the library's constituent page records it as the main ester of both lavender and bergamot. So the opening of bergamot in soap suffers twice at once: limonene evaporates, and the ester can be cleaved by alkali.
Cinnamon, clove and vanilla: the molecules that colour a bar
Another group does not evaporate but reacts. Ceylon cinnamon bark oil contains cinnamaldehyde, an aldehyde, recorded in the library at up to 75.7 percent. Clove bud oil contains eugenol, a phenol, at up to 96.9 percent. Vanillin in vanilla is also a phenol carrying an aldehyde group.
In an alkaline environment, phenols turn into their salt form, and these molecules readily take part in colour forming reactions. Soap makers note two familiar effects. One is that the batter thickens very quickly after clove or cinnamon oil is added, sometimes within minutes. The other is that bars with vanilla or clove slowly turn beige and then brown during the curing weeks.
Lê Mai's Cinnamon Artisanal Soap No.058 uses Cinnamomum zeylanicum bark oil with cinnamon powder from Sri Lanka. The warm brown of the bar comes mostly from the powder; the essential oil brings the scent, and its amount is kept low because cinnamon bark oil is among the oils the library records as most irritating.
Woods, resins and roots stay longest
At the other end of the evaporation table are large molecules: sesquiterpenes and sesquiterpene alcohols. Patchouli contains 17.5 to 32.7 percent patchouli alcohol. True East Indian sandalwood contains 46 to 60 percent α-santalol. These molecules are heavy, evaporate slowly and carry no ester group for the alkali to cleave. The library notes that patchouli is a good fixative precisely because its molecule is large.
In soap, this group passes through the three stages with little change. They also do the work described in the piece on fixatives: keeping the lighter layers present a little longer. A lasting soap scent is usually built like a small perfume, with citrus on top and wood or resin below.
Lê Mai's Orange Sandalwood Artisanal Soap No.095 uses exactly that structure, through essential oil blend No.095: mandarin and bergamot on top, orange blossom, pepper and rosewood in the heart, geranium, cedarwood and frankincense below. When the orange has faded after the curing weeks, the woody and resinous base is still there.
A heavy base has its own cost. If the lower layer takes up too much, the bar smells of wood from the day it is cut, before the orange has a chance to be heard. The ratio between layers is the same problem as in the piece on top, heart and base ratios, except that soap also has to account for six weeks of curing.
Soap is a rinse-off product
A bar of soap touches the skin for a few dozen seconds and is then rinsed away. IFRA, the association that sets standards for fragrance materials, divides products into categories by how they are used, and each category has its own limits. Soap, shower gel and shampoo belong to the rinse-off category; creams, body oils and perfumes belong to leave-on categories. The same essential oil can have different limits in the two.
The maximum dermal levels recorded in the library, such as 0.07 percent for cinnamon bark oil or 0.5 percent for clove bud oil, are levels for leave-on products. The phototoxicity limit of cold pressed bergamot oil also applies to leave-on products exposed to sunlight, as the piece on citrus and sunlight explains. Soap sits in a different category, but that category still has its own limits, and the amount of essential oil in a formula is still calculated oil by oil.
This also explains why the scent of a soap is usually softer than the scent of the bottle of the same name. The amount used is limited from the start, then six weeks of curing take away more of the fast evaporating part. What remains is the most durable part of the scent.
A lye pot does not filter scents by preference. It filters them by molecular structure.
Light molecules leave first, esters change their voice, phenols change colour, woods remain.
Six weeks later, the scent on the bar is the part that made it through all three stages.