Material guide: birch tar — archaeologists call it the oldest synthetic substance humans made, and 30 suppliers sell it today

About a 5 minute read.

The short version

• Name: birch tar oil, CAS 8001-88-5
• Odour type: smoky; odour: smoky, burnt wood, leathery, phenolic
• Strength: high (recommend smelling in a 1.00% solution or less), rank 3; substantivity 400 hours
• Specific gravity 1.13 to 1.35; refractive index 1.522 to 1.590 — both unusually wide
• Suppliers: 30; boiling point 175 °C, flash point 67.78 °C
• GHS: H227 combustible liquid, H315 skin irritation, H319 eye irritation, H335 may cause respiratory irritation
• All three toxicity fields read "Not determined"

The notes field tells you which one to buy

The first sentence of notes is a purchasing instruction:

For perfumery purposes, only the rectified oil of Birch Bark Tar should be considered. Not really an essential oil. The tar is produced from the bark of various birch species by slow destructive distillation.

"Not really an essential oil" is worth stopping on.

An essential oil is the volatile material already present in the plant, distilled out. Destructive distillation is not that. It heats organic matter in a low-oxygen environment until it decomposes, then collects the decomposition products. The molecules you smell were not in the bark. The heat made them.

And the supplier entries in the solubility field put both grades side by side:

Berjé; Birch Tar Crude … Berjé; Birch Tar Rectified

Both are on sale, and the database tells you only the second one counts.

(Grade written into the product name is something I collected in the pine oil article. This record goes further — it names which one to buy.)

Two unusually wide physical ranges

	Range	Width
Specific gravity	1.13 to 1.35	0.22
Refractive index	1.522 to 1.590	0.068

These are the numbers you normally use to confirm that what arrived is what you ordered. Most materials have a specific gravity range spanning the third decimal place (diethyl phthalate in the same database runs 1.115 to 1.118).

1.13 to 1.35 is 19% variation. That is not measurement error. That is "the things sold under this name differ a lot."

And destructive distillation explains why. The product depends on temperature, duration, birch species and bark moisture. This is a material whose composition is set by process parameters, not a material with a fixed composition.

Evaluate below 1%

The odor_strength field reads:

High, recommend smelling in a 1.00% solution or less.

1% is among the lowest recommended evaluation concentrations in this series. Most materials say 10%; acetyl pyrazine says 0.10%.

And the odour description itself was written at that dilution: "1.00% in dipropylene glycol. Smoky, burnt wood, leathery, phenolic."

With 400 hours of substantivity, this is a textbook one-drop-ruins-the-batch material.

(The database's suggested blending partners: cananga, isoamyl phenylacetate, heptyl eugenol, guaiacyl butyrate.)

Its GHS has one uncommon entry

H227 — combustible liquid H315 — causes skin irritation H319 — causes serious eye irritation H335 — may cause respiratory irritation (STOT single exposure, Category 3, respiratory tract)

H335 is uncommon on aroma materials. I measured the coverage of the three toxicity fields earlier and found that the inhalation field has data on only 6.6% of records — and on this record all three toxicity fields say "Not determined," so the respiratory warning comes from the GHS side.

Two fields saying different things again.

Note also what is absent: no H317, skin sensitisation. This material's problem is irritation, not sensitisation, and those are different things.

It may be the first synthetic substance humans made

This section has no direct bearing on perfumery, but it explains what this material is.

Schmidt and colleagues open their 2023 paper in Archaeological and Anthropological Sciences with one line:

Birch tar is the oldest synthetic substance made by early humans. The earliest such artefacts are associated with Neanderthals.

"Synthetic" here has a strict meaning: it was not gathered, it was made by heating one material into another. And that transformation is exactly what the database's notes field calls "slow destructive distillation."

This research line also has a turn I find worth reading — the same first author, two papers pointing opposite ways.

The 2019 one in PNAS is titled Birch tar production does not prove Neanderthal behavioral complexity. The argument:

We show that recognisable amounts of birch tar were likely a relatively frequent by-product of burning birch bark (a natural tinder) under common, i.e. aerobic conditions. … When birch bark burns close to a vertical to sub-vertical hard surface (e.g. an adjacent stone), birch tar is naturally deposited and can be easily scraped from the surface. … usable quantities within a single work session (3 hours, including obtaining birch bark).

In other words: making tar could have been an accident.

Then the 2023 paper takes up the next question — how did Neanderthals actually do it? Comparing two pieces of birch tar excavated at Königsaue, Germany against a reference collection made with Stone Age techniques, the conclusion:

Neanderthals did not use the simplest method to make tar. Instead, they distilled tar in an intentionally created [structure].

First show the simple method works, then show they did not use it. The two together are stronger than either alone, and the order was necessary — without ruling out the accident first, the second paper's conclusion does not stand.

I cite these two not to talk about Neanderthals, but because they establish what this material is: something made by heating a plant into a different substance, and people have been doing that for a very long time.

How to use it

• Buy rectified, not crude. The notes field says so outright.
• Evaluate below 1%. Strength rank 3, substantivity 400 hours.
• It is a foundation material on the smoky/leather line, and it sits on the fougère specificity list (the one from my last article, filtered by accord tags).
• The specific gravity and refractive index ranges are wide, so re-evaluate on a new lot.
• Mind H335. Ventilate; do not open the bottle in a closed space.
• Check IFRA. This material carries polycyclic aromatic hydrocarbon concerns, and all three toxicity fields are empty, so they will tell you nothing.

What this doesn't establish

• I have not smelled it. This is entirely fields, whole-database statistics, and two papers.
• Those two papers are archaeology, not fragrance research. I cite their description of this material and its production method, and touch none of their argument about cognitive evolution.
• "Oldest synthetic substance" is Schmidt 2023's opening claim; I did not check the primary chronological evidence supporting that ranking.
• I did not look up the PAH content of this oil, nor the text of current IFRA standards — "there are PAH concerns" is industry common knowledge, not something established here.
• Attributing the wide gravity and refractive index ranges to process variation in destructive distillation is an inference. The database gives no reason.
• All three toxicity fields are undetermined, so there is no animal or human dose data to cite for this material.
• "One drop ruins the batch" is inferred from the strength rank and recommended concentration, not from an experiment I ran.