Bài 3 · Ba tầng của một mùi

Twenty drops of essential oil, split into three groups by how fast they leave a blotter. Lesson three of the blending course sets a starting frame for top, heart and base, then shows why that same frame has to be adjusted.

A frame to start from

The evaporation chart built in lesson two sorted the bottles on the table into three groups: those that leave a blotter within hours, those that last through a day, and those still recognisable after three days. The three groups are top, heart and base. The next question is how much of a blend each group should take.

Blending manuals usually offer a starting frame. The library's Blending 101 page gives the classic ratio of 30 percent top, 50 percent heart, 20 percent base, with a beginner exercise: 3 drops of bergamot, 5 of lavender, 2 of cedarwood. The library's pyramid tool opens at 25, 45 and 30 percent. Other manuals quote a range of 20 to 30 percent top, 40 to 50 percent heart, 25 to 35 percent base.

The figures differ by a few percent, and that difference is the first piece of information. No single ratio is right for every blend. The frame is only a starting point, something to correct. A blend without a frame gives no direction for correction; a blend with one makes every change measurable.

The frame also shifts with the product. The same Blending 101 page gives cologne a heavier top, around 50 percent, while eau de parfum moves more weight into heart and base. The product decides how far the scent has to travel, and the tier ratio follows that decision.

A drop is not a drop

A drop is a volume, about 0.05 ml. The library works on 20 drops per ml, with a margin of about 20 percent depending on viscosity: thick oils such as vetiver or sandalwood form larger drops. Even at the level of volume, two drops from two bottles are not equal.

The larger difference lies in how much odour each drop carries. A 30, 50, 20 ratio counted in drops quietly assumes that every drop contributes the same. It does not. The pyramid tool says so plainly: one drop of ylang ylang and one drop of sweet orange do not give the same amount of odour.

The simplest test uses two bottles: star anise and sweet orange. Star anise oil holds 71 to 92 percent (E)-anethole; sweet orange is almost entirely limonene, 86 to 96 percent depending on origin. Put one drop of anise and ten drops of orange in a small vial and dip a blotter. In this test the anise usually sets the direction of the blotter. The ten drops of orange form a bright ground underneath, but the shape of the scent belongs to the single drop.

The ratio on paper is one part anise to ten parts orange. The ratio the nose hears is close to the reverse. This is why a formula written only in drops does not yet describe its scent.

Odour strength and threshold

Every molecule has its own odour threshold: the lowest concentration at which half of a test panel detects it. The library's glossary notes that thresholds differ by orders of magnitude between molecules. The constituent that fills most of the volume does not necessarily decide the smell.

Perfumery adds one more figure: the ratio between a compound's concentration and its threshold, called the odour activity value. When the ratio is above 1, the compound is likely to contribute to the overall smell. A constituent present at a few percent with a very low threshold can still override one that makes up nine tenths of the oil.

Odour strength and evaporation speed are two separate axes. Evaporation tells how long a scent stays; strength tells how loud it is while it stays. Sweet orange evaporates fast and is not especially strong, so on a blotter it recedes early. Patchouli evaporates very slowly, stays recognisable for 400 hours or more on the library's volatility chart, and a small share of it is enough to colour a whole blend.

The nose also tires. After a few minutes of continuous smelling, perception of an odour drops; the term is olfactory fatigue. Each evaluation needs a pause, in the rhythm of smell, rest, write practised since lesson one.

So the tier ratio is only half the problem. The other half is the intensity of each bottle, which no dropper can measure. Only the blotter and time can.

When the top is too thin

A blend with too thin a top opens already heavy. The blotter is barely dipped and the first thing to reach the nose is flower or wood, with no bright stretch before it. Usually the ratio on paper is not wrong; the top oils simply leave too fast.

The library's volatility chart gives cold pressed lemon about 4 hours on a blotter, sweet orange an estimated 6 hours, coriander seed 8 hours. Rose is listed at 168 hours. By the fifth hour the citrus has nearly left the blotter, while the floral part has covered only a short stretch of its time.

The first correction is to add drops to the top, but it has a limit: more orange does not make orange last longer, it only makes the opening louder. The second is to choose top oils with a longer tail. Pink pepper sits between top and heart; the chart lists 147 hours, taken from a close species. The third is to let the heart carry some of the brightness, as clary sage does with linalyl acetate at 45 to 74 percent.

When adding to the top, record both the drop count and the moment the top note disappears from the blotter. The top is judged by its length, not only by its volume.

When the base covers everything

The opposite error is more common. The base is made of large, slow molecules, many of them intense: patchouli, vetiver, resins, heavy woods. In the first hour they seem silent. After a day they are all that remains.

The pyramid tool notes that strong oils such as cinnamon, clove and vetiver need only a small share. Blending 101 gives the opposite reminder: a blend of top and heart alone is gone within one to two hours, and at least 10 percent base is needed to hold it. Between the two reminders lies a narrow band.

The sign of too heavy a base is a freshly dipped blotter that smells of earth or resin from the first minute, with a heart that never comes through. The correction is to remove base one drop at a time, not by half. One drop of patchouli in twenty is 5 percent; two drops is 10 percent. Each small step is enough to change the shape of the blotter at the twenty fourth hour.

Sometimes the problem is not the drop count but the material. Freshly distilled patchouli has a camphor edge; the library notes that aged patchouli is smoother and rounder, one of the few oils valued for age. At the same ratio, two lots of patchouli of different ages give two different bases.

Testing on the pyramid tool

The library's pyramid builder splits a total drop count by tier share. Oils are grouped in advance using the volatility chart; any bottle can be moved to another tier with the arrow buttons. The default shares are 25, 45, 30 percent.

The way to use it for this lesson is to build two versions. The first follows the default frame, twenty drops in total. The second keeps the same bottles but changes the ratio according to what the blotter showed: one drop less of the strongest oil, one drop more of the weakest. Dip two blotters at the same time and note them at five minutes, one hour and one day, as in lesson two.

The drop counts in the tool are ratios between oils, not the concentration used on skin. Before any skin contact the blend must be diluted, and each oil must stay below the limit given on its page. Star anise, for example, has a maximum dermal level of 1.75 percent and is contraindicated in pregnancy, breastfeeding and for children under 5. The one drop of anise to ten of orange test above is for the blotter only.

After the two versions the scent notebook gains a new line: not the right ratio, but the distance between the ratio on paper and the ratio the nose hears. Lesson four narrows the problem to two bottles and measures that distance across nine ratios.

Ratios are written in drops. Scent is not counted in drops.

On paper the three tier frame stands still. On a blotter it moves by the hour.

A good formula is one that has been corrected many times.