Which trials are worth keeping to see if they improve: a criterion you can look up

Someone posted a short piece on r/DIYfragrance that is essentially a wail:

"A few months ago I was trialing some new ingredients and I made a perfume from some of my new ingredients and some of my old stuff. I think around that time I was even experimenting with diluting resins on my own. Now after a few months macerating it is literally the best perfume I've ever made. It is sweet, leathery, animalic and complex enough that I truly couldn't recreate it if I tried. ALWAYS WRITE YOU RECIPES!!!"

Someone replied with the line: "The difference between 'science' and 'screwing around' is writing it down."

But two questions in that thread went unanswered, and both are answerable.

The two questions

The first, asked by one person and echoed by another with "same thought came up to my mind as well":

"How do you know when to actually keep and store a blend you've done even when it doesn't smell perfect at first? I mean if I had to store all my blends in order to 'let the magic happen' I'd so quickly run out of storage lol so I tend to throw out stuff a lot. But hearing you I'm thinking I might be missing out some surprisingly interesting maturations!"

The second, from someone else:

"For these situations, is it possible to have GSM lab to analyze it and report back?"

Nobody took either. This article does.

The short version

• "Maturation" is not one vague thing; it contains specific reactions. The most lookup-able is the Schiff base: anthranilates (primary amines) condense with aldehydes, losing a water molecule.
• Across 954 public formulas, 102 (10.7%) contain both classes. In roughly one formula in ten, that reaction is possible.
• And 11 (1.2%) buy a ready-made Schiff base as a single ingredient. The industry makes this deliberately and sells it.
• The reaction is reversible and sensitive to conditions. A 2022 study demonstrated controlling its hydrolysis through the medium — which also means having the formula does not guarantee reproducing it unless conditions match.
• Recovering a formula by GC-MS works, and returns you half of it. The last article did the arithmetic: analysts get 70–90%, and what is left is the small-weight, hardest-to-identify end.
• The practical criterion: keep trials containing aldehydes plus amines, resins, or natural extracts. Do not keep ones built purely from stable single molecules whose problem you can already name.

About a 9 minute read.

What actually moves during maturation

"Leave it a few months and it gets better" is just waiting unless you can name a mechanism. So here is one that can be named.

A Schiff base (imine) forms by condensation of an aldehyde or ketone with a primary amine, removing a water molecule. In perfumery this is not theory — it is a product you can buy.

Nicastro and colleagues, in the International Journal of Molecular Sciences in 2022, studied four Schiff bases applicable to the fragrance industry. Their amine was methyl anthranilate, and their four aldehydes were cyclamal (cyclamen aldehyde), helional, hydroxycitronellal and triplal — all highly volatile odorants.

Three of those four aldehydes have material guides in this series. (Cyclamen aldehyde, helional, hydroxycitronellal.)

They tracked the stability of these Schiff bases over time by HPLC-MS, in neutral and acidic conditions, both in solution and trapped in low molecular weight gels.

One line in their introduction is the crux of the whole thing: Schiff bases are central molecules in several biological processes for their ability to form and cleave by small variation of the medium.

Their conclusion: it is possible to control the hydrolysis of the Schiff bases in the gel environment, thus tuning the quantity of aldehyde released and the persistency of the fragrance.

What that means for your bottle: if your formula contains both an anthranilate and an aldehyde, then over time some of them combine into a Schiff base, and where that equilibrium sits depends on the medium — acidity, water content, temperature. This is not a metaphor about flavours melding. It is a reaction with a name, a product, and a chromatographic trace.

How often that is even possible

Computable, so I computed it.

Taking the 954 public formulas and finding those containing both an anthranilate (aromatic primary amine) and an aldehyde:

	Formulas	Share
Containing an anthranilate	122	12.8%
Containing both a primary amine and an aldehyde	102	10.7%
Listing a ready-made Schiff base as an ingredient	11	1.2%

In one formula in ten, that reaction can happen.

The most frequent pairings:

Formulas	Amine	Aldehyde
17	Methyl anthranilate	Alpha-hexyl cinnamaldehyde
14	Methyl anthranilate	Ethyl vanillin
13	Methyl anthranilate	Heliotropin
13	Methyl anthranilate	Lyral
11	Methyl anthranilate	Hydroxycitronellal

That last pairing deserves its own look. Methyl anthranilate plus hydroxycitronellal are the two starting materials for the classic commercial Schiff base sold under a trade name. Eleven formulas put the two components in separately; another eleven buy the finished product.

The same thing done two ways, and the difference between them is time. In the first group the reaction runs slowly in the bottle; in the second it was completed in a factory.

This also explains why the original poster cannot reproduce theirs. They mention diluting resins themselves around that time, and the result was "sweet, leathery, animalic". Resins are compositionally complex and batch-variable, and animalic character often comes from things that need time to show. Even with the formula written down, the same recipe with a different resin batch under different conditions does not guarantee the same bottle.

The natural materials are moving too

The other direction of change is oxidation, and unlike Schiff base formation, it mostly does not go somewhere good.

Najdoska-Bogdanov and colleagues, in Natural Product Communications in 2016, compared the volatiles of fresh and aged sweet fennel fruits using hydrodistillation with GC, and static headspace with GC.

The finding: as the fruit ages there is a gradual loss of volatile components — lower essential oil yield and lower volatile content. Differences in the main components (trans-anethole, estragole, fenchone, limonene) between the two methods were negligible.

The interesting part is what appeared: limonene oxides, carvone and carveol were detectable in the headspace of aged fruits but not in the hydrodistilled fresh samples. Products with retention times longer than trans-anethole also showed up.

One of their conclusions is that static headspace GC can be used to assess the maximum storage time of fennel fruit still suitable for human consumption.

This is fennel fruit, not perfume, at a different scale and in a different matrix. It is here because it shows the other half: ageing does two things at once — the original components decrease, and new ones appear. We have written the mechanism of oxidation, and the geraniol study where oxidation changed sensitizing potency is the same fact from another angle.

So "it improves with time" is not a general rule. It holds for some combinations, and for others it is spoilage.

So which ones to keep

Putting those two together gives a criterion — not by feel, but from the ingredient list.

Worth keeping:

• Anything containing both aldehydes and anthranilates. Schiff bases form slowly and the change is real. About a tenth of formulas qualify.
• Anything containing resins, absolutes or natural extracts. Compositionally complex, full of slow reactions, and variable batch to batch anyway.
• Anything with a high proportion of musks or other low-vapour-pressure materials. Those need time to show at all; on day one you cannot judge their proportion.
• Anything you cannot yet say what is wrong with. "Not perfect but I can't name why" usually means you are not yet at the point where you can judge it.

Not worth keeping:

• Anything wrong in an identifiable direction. If you can tell that one material is overdosed, it will still be overdosed in three months. Time does not change ratios.
• Anything built purely from stable single molecules whose problem you have already diagnosed. There is no slow reaction to wait for.
• Anything you do not intend to remake. That sounds obvious, and it is exactly how the dump-out jar grows.

One reply described that jar:

"I'm sad because my dump-out jar actually smells good on a candle level right now, and I have no idea what I dumped in there anymore."

That jar is this article's cautionary tale and its best evidence at the same time.

Can GC-MS recover it

The second unanswered question.

Yes, and it returns about half.

The last article quoted a GC analyst who reverse-engineers fragrances at a Givaudan-owned company: they are expected to get 70 to 90% of the way, with the perfumers finishing by nose.

And I computed the weight structure: across 953 formulas the median is 16 materials, of which 9 carry 90% of the weight and the top three carry 54.7%.

So GC-MS gives you back the large structure. The materials that carry weight get detected and identified.

What it does not give back is the other end. Materials dosed under 1% are 14.3% of the count and 1.01% of the weight — the hardest to quantify accurately, and frequently exactly the part that makes something "too complex to recreate".

Two practical problems too. One, commercial GC-MS analysis costs money, and for a home trial it is usually not worth it. Two, it tells you what is in the bottle now, not what you poured in — if Schiff bases have formed and things have oxidised, the report shows products rather than starting materials.

So writing it down remains the cheap option. But if that bottle genuinely matters, this route exists, and it gives you most of it.

What counts as writing it down

Since the conclusion is "write it down", here is what to write.

The minimum: each material's name, its concentration (neat, or a 10% stock), the weight, and the date.

The concentration column is the one most often missed, and it is the commonest cause of a failed reproduction. "Vanillin 0.5 g" and "vanillin 10% stock, 0.5 g" differ by twentyfold.

Three more fields save a lot of trouble:

• The material's batch or purchase date. Batch differences are real, especially for naturals.
• The solvent used for dilution. DPG, ethanol and IPM behave differently downstream.
• Why you added it. If you cannot say what a material is doing in a formula, it usually is not doing anything — and three months from now, you will need the you of three months ago to explain.

Someone in the thread recommended note-taking software, and someone else turned up to plug their own tool. The tool is secondary; the fields are what matter. A sheet of paper or a spreadsheet will do, as long as those columns are on it.

And one reply is the most practical line in the thread:

"If you write it down, remember where you put it."

What this doesn't establish

My first pass counted indole as an amine, which is wrong. Indole's nitrogen sits in the ring; it is not a primary amine and does not form Schiff bases the way anthranilates do. Including it inflated the "possible" count from 102 to 179. The figures above are the corrected ones, restricted to anthranilates.

"Contains both classes" is not "the reaction has happened". I computed chemical possibility, not measurement. How much actually forms depends on concentration, medium, acidity, temperature and time — which is precisely what Nicastro's paper is about controlling. I measured no bottles.

Nicastro worked in gels and solution, not perfume. Their aim was controlled release (making profragrances), not describing how perfume ages. I cite the reaction itself and its property of forming and cleaving with small variations in medium, not their application.

The fennel study is fruit, not formula. It shows that oxidation both removes old components and creates new ones; it does not generalise to any perfume's ageing curve.

The keep/discard list is inference. It follows from mechanism rather than from an experiment. I have not run a keep-versus-discard comparison, and I have not seen one.

The GC-MS figures come from the previous article. The 70–90% is that analyst's own account, and the weight structure comes from my public formula corpus; both carry their own limits.

References

G. Nicastro et al., Controlled Hydrolysis of Odorants Schiff Bases in Low-Molecular-Weight Gels, International Journal of Molecular Sciences, 23(6), 3105 (2022). PMID 35328526. doi:10.3390/ijms23063105

M. Najdoska-Bogdanov, J. B. Bogdanov, M. Stefova, Changes in Volatile Compounds during Aging of Sweet Fennel Fruits, Natural Product Communications, 11(3), 423–429 (2016). PMID 27169197

Related: Storage and oxidation, The same name is not the same material, Can jamming GC-MS protect a formula, One per slot.