Adding things to a finished perfume — addition is cheap, subtraction is nearly impossible

Someone on r/DIYfragrance raised an idea most people have had:

"This idea is not original, but was thinking about this when I came across something described as close to Terre D'hermes, without the sportyness. I figure I can just add some sportiness myself. Has anyone tried this and found any new learnings?"

The replies opened with jokes. One person took their bottle of TdH to a bar, reasoning that sitting down and drinking Jack and Diet Coke for hours would deplete the sporty essences by laziness-induced osmosis; the perfume remained stubbornly unchanged. To confirm, they took a vial of their darkest oud to a weekly flag football game to infuse some sporty freshness, and failed again. Another asked what sportiness contains — a question this series has already worked through.

But two replies in the thread are serious, and both can be checked against numbers.

The short version

• Addition is cheap. A 50 mL EDP at 20% holds about 9.5 g of concentrate. Raising bergamot from the corpus median of 10.85% to 15% takes 0.46 g of neat bergamot oil, which is 1.1% of the bottle by weight.
• Subtraction is nearly impossible. Nothing comes out of a bottle; it can only be diluted. Cutting patchouli from 8% to 4% means adding 9.5 g of patchouli-free material — 21.8% of the bottle — and it takes the concentrate from 9.5 g to 19 g.
• So the most practical advice, "increase the bergamot and lily of the valley materials, reduce the patchouli and Atlas cedar", works in its first half and not in its second.
• Someone in the thread has already solved it. The described method of making a very dilute homemade version and blending it 1:1 with the original is, mathematically, subtraction: every material in the original gets multiplied by 0.67.
• "Only works on classic colognes" is supported by the corpus. Citrus and cologne formulas have a median effective material count of 6.5, the lowest of eleven genres; chypre is 10.1, the highest. The same addition moves a much larger fraction of a cologne.
• And adding can make the whole thing weaker. A 1990 study of binary odour mixtures found mixtures smelled less intense than the simple sum of their components, and under periodic exposure showed "compromise" — a mixture can smell less intense than its stronger component alone.

Two you can do, two you can't

Here is the serious advice:

"I've made this formula. I had to replace about 20 per cent of the ingredients with other things, but the result was quite similar to Terre d'Hermès and nice. I'd say that if you want it to be more sporty, I'd increase the bergamot and perhaps lily of the valley materials and reduce the patchouli and Atlas cedar very slightly." (roldarin)

Four instructions. Take them separately.

Addition: a 50 mL EDP at 20% fragrance load, concentrate density around 0.95, holds about 9.5 g of concentrate.

What you want	Neat material to add	Of the bottle by weight
Bergamot 10.85% → 15%	0.46 g	1.1%
Bergamot 10.85% → 20%	1.09 g	2.5%
Lily of the valley materials 2% → 4%	0.20 g	0.5%
Something 0.5% → 2%	0.15 g	0.3%

These numbers are startlingly small. Half a gram of bergamot oil raises its share of the formula by nearly half. That is why the idea of adding to a finished perfume is so attractive: arithmetically, it really is cheap.

Subtraction: you cannot take anything out of a bottle. There is exactly one move available — add something that does not contain it, and dilute.

What you want	"Everything else" to add	Of the bottle	Concentrate becomes
Patchouli 8% → 6%	3.17 g	7.3%	12.7 g
Patchouli 8% → 4%	9.50 g	21.8%	19.0 g
Atlas cedar 5% → 3%	6.33 g	14.6%	15.8 g

Halving one material's share requires adding as much other material as the whole original concentrate. You are not modifying a perfume; you are building a second one next to it and pouring them together.

That asymmetry is the whole point. "Raise A, lower B" reads as two symmetrical moves on a formula sheet, and they are nothing alike on a bottle. roldarin's advice is fine in his own context — he is making the formula, not editing a finished product. It is the transfer to a bottle that breaks the second half.

That "1:1 with a dilute version" method is correct subtraction

Another reply describes what they actually did:

"Yeah I tried this with a few fragrances. I had the most success with Ingenious Ginger, I added some powdery orris (i think mostly Orivone) and some other stuff and it actually turned out really nice! I gave some to a friend and she liked it better than the og."

"The way I did it was by creating a 'ginger stretch' accord that was my interpretation of the core parts of the og with a few tweaks, but very diluted (probably to around 10% or less oils), then I combined that with the og at about a 1:1 ratio. The idea was to make it less potent so you can apply more sprays without it being overpowering, while adding a slightly different character to it." (kdoughboy12)

The stated reason is to make it less potent. Here is what it actually did.

The original bottle: 50 mL, 9.5 g concentrate. Add an equal volume of a homemade liquid at 10% oils, 50 mL, bringing 4.8 g of oils.

	After blending
Total liquid	100 mL
Total oils	14.2 g
Fragrance load	20% down to 16.4%
Every original material's concentration	multiplied by 0.67
The homemade share of final oils	33%

Every material in the original drops to 67% of its former concentration. Patchouli, Atlas cedar, and everything he did not want to touch all get pushed down by a third together. He then uses his own 33% to put back what he wants.

That is the only workable form of subtraction: you cannot lower one material, only all of them at once, and then add back the ones you want to keep. He arrived at it from the "too strong" direction, and it happens to solve the subtraction problem too.

In the subtraction table above, "patchouli 8% → 6%" needs 3.17 g. If that 3.17 g is neat diluent, you get a weaker version of the original. If it is something you blended, you have done subtraction and addition in the same move. Same operation, two uses.

"Only classic colognes" — the corpus supports it

Another reply set a boundary:

"I'd suggest only trying this with fairly classic colognes. Adding to fuller perfumes is unlikely to work well, like hammering a car to make it more aerodynamic (in theory possible, but highly unlikely)." (Wise_Falcon20)

This is checkable, once "fuller" is defined.

I used the effective material count: square each material's weight share, sum them, take the reciprocal. The measure is standard in ecology (inverse Simpson index) and economics (reciprocal Herfindahl). It answers: how many materials are actually speaking in this formula?

A formula with 20 materials where one holds 80% has an effective count of about 1.5 — nominally twenty, effectively one voice.

Across all 954 formulas: median material count 16, median effective material count only 7.1. Half the materials are, by weight, almost silent. That matches the long-tail figure this series computed earlier.

By genre (only genres with 15 or more formulas):

Genre	Formulas	Median materials	Median effective count	Largest single material
Citrus / cologne	109	16	6.5	28.2%
Leather	15	14	6.6	25.0%
Musk	32	16	6.8	27.7%
Woody	87	13	6.9	24.0%
Floral	327	16	7.0	25.9%
Green	99	15	7.4	23.5%
Oriental amber	37	16	7.5	23.5%
Fruity	122	18	7.5	24.5%
Fougère	38	16	7.8	22.0%
Spicy	24	19	7.9	22.0%
Chypre	30	18	10.1	20.1%

Citrus and cologne have the lowest effective material count of the eleven genres (6.5) and the highest single-material share (28.2%). Chypre is the reverse: 10.1 effective materials, largest single material only 20.1%.

The ends differ by 1.55×. A chypre carries 55% more independent voices.

The implication for adding is direct: the same 0.5 g lands in the top few of a cologne dominated by three or four materials, and is the eleventh voice in a chypre where ten materials are each talking.

Wise_Falcon20 did not do this calculation, but his instinct and the corpus point at the same thing.

Adding can make it weaker

The last layer is perception, and it runs against intuition.

In 1990, Schiet and Cain published a study in Perception. Subjects judged the odour intensity of single odorants and binary mixtures, at fixed or varying proportions, presented in an environmental chamber. Exposure was either continuous for 15 minutes, or periodic — once a minute for 15 minutes.

Two results bear on this question.

1. Mixtures smelled less intense than predicted from the simple sum of their unmixed components. This is hypoadditivity, and its degree was about the same for the four odorant pairs studied.
2. Periodic exposure produced "compromise" — in the paper's words, the mixture can smell less intense than its stronger component alone.

The second point is worth pausing on. Adding something to a bottle of perfume can make the whole bottle read weaker than it did before. Not "one more smell in there" — total intensity moving down.

The exposure regime changed the result. Continuous exposure came closer to simple additivity and showed no compromise. The authors' account is that under continuous exposure, mixtures tend to exhibit less adaptation than their components. Their closing line is good: mixtures may seem to lack potency relative to their unmixed components, but may in fact compensate through an increase in durability.

Wearing perfume is periodic exposure. You spray, you walk away, you notice it again later. That is the condition under which compromise appeared.

The limit of this paper should be stated. It measured binary mixtures, not a sixteen-material perfume plus a seventeenth. Extrapolating from four odorant pairs to a whole bottle is mine, not theirs. What it establishes is a direction — mixing is hypoadditive, and adding material is not adding intensity — not any coefficient you can put into a formula.

Schifferstein and Kleykers in 1996 add an angle and a limit. Testing the interaction model Olsson published in 1994, they found the data showed asymmetrical mixture suppression and hypo-additivity, and that the deviations from symmetry and the presence of compromise violate two of the model's principles, so the model cannot accurately describe interactions in mixtures of dissimilar components. They also note that these violations probably occur for other mixture types too.

But this study used tastants — sucrose and citric acid mixtures — not odours. I cite it for the phenomenon of asymmetrical suppression, not to move a taste result onto smell.

My reading changed once this round

My starting assumption was that adding to a finished product is a bad idea because the quantities are too small to move anything. So I computed "how much do you have to add" first, expecting an absurdly large number.

It came out at 0.46 g. Addition is far easier than I thought.

The real problem only appeared when I computed subtraction, and only because roldarin's advice contains two "reduce" instructions that I had not initially separated from the two "increase" ones. Separating them exposed the asymmetry: four operations in one sentence, two of which cost 0.5 g and two of which cost 9.5 g.

That mistake is worth recording, because it is the gap between formula language and bottle language. On a formula sheet, "take A from 8 to 4" and "take B from 10 to 15" look equally easy; both are one number. On an already-blended liquid, the first one asks you to redo half the work.

What you can actually do

• Split off a portion before you add anything. One reply puts it plainly: "Just split a portion off and mess around with that, for when you inevitably hate it after your changes." That is the most useful sentence in the thread.
• Addition: half a gram moves the material you want up. Start at 0.1 g, because there is no undo.
• Subtraction: do not try to lower one thing. What you can do is lower everything together and add back what you want — which is the 1:1 dilute-version method.
• Choose the target: the closer to a classic cologne, the better the odds. In something with an effective count of 6.5, one addition takes a place. In a chypre at 10, it does not.
• Expect weaker, not stronger. Mixing is hypoadditive, and wearing perfume is exactly the periodic exposure under which compromise showed up.
• Keep the original bottle. It is the only thing you have to compare against.

What this doesn't establish

• Every calculation assumes a 20% fragrance load and a concentrate density of 0.95. Real products run from 8% for an EDT to 30% for an extrait, and changing that number changes everything. These are orders of magnitude, not a formula.
• Fragrance load is not the same as "oils". A finished product also contains alcohol, water, UV absorbers and so on; treating the whole non-alcohol portion as concentrate is a simplification.
• Corpus shares are weight shares within the formula. I used them as a proxy for proportions inside a finished bottle, which holds only while the fragrance load is constant — and adding something changes the load. The tables give the new concentrate totals.
• The effective material count is a weight measure, not a perceptual one. A cologne with an effective count of 6.5 does not mean you can smell 6.5 things in it. It measures how concentrated the weight distribution is.
• Genres are applied from title keywords, not from smelling. Leather (15) and spicy (24) are small samples and those rows' medians will be unstable.
• Schiet and Cain 1990 measured binary mixtures, not complex perfumes. The extrapolation to a whole bottle is mine, not the paper's.
• Schifferstein and Kleykers 1996 used tastants. I cite the phenomenon of asymmetrical suppression, not a taste result treated as an odour result.
• I have not run this experiment. This piece is arithmetic and literature, not a report on modifying a bottle of Terre d'Hermès.