Two boards can be the same species and still behave differently. Chemistry is the reason.
Wood is not just cellulose and lignin arranged into fibres.
It is also a chemical system.
Trees manufacture and deposit hundreds of low‑molecular compounds into wood.
Some are water‑soluble.
Some are oily.
Some are volatile.
Some are toxic to fungi.
Some react instantly with iron.
These compounds are collectively called extractives.
They are a major reason wood has:
- colour and darkening patterns
- smell
- natural durability (or a lack of it)
- treatability differences
- “mystery” gluing and finishing problems
This guide explains what extractives are, where they live, and how to think about them like a maker.
What Extractives Are (A Practical Definition)
Extractives are wood chemicals that are not part of the structural cell wall polymers (cellulose, hemicellulose, lignin).
They are typically lower‑molecular‑weight compounds that can be removed (“extracted”) with solvents.
They include many chemical families: terpenes, phenolics (including tannins), resin acids, oils, waxes, and more.[1]
Workshop translation:
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If you ever asked “why does this wood smell, stain, bleed, resist glue, or last outdoors?” the answer is often: extractives.
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Where Extractives Live (And Why Heartwood Is Different)
Extractives are usually most concentrated in heartwood.
Heartwood and sapwood can have very similar basic structure.
What makes heartwood behave differently is largely its chemistry.
It is the deposition of extractives that gives heartwood many of its distinguishing traits.[2]
Sapwood vs heartwood (chemistry lens)
- Sapwood: active transport zone, generally lower extractive content.
- Heartwood: older inner zone, typically higher extractives and often higher natural durability.
This is why:
- sapwood is usually less durable outdoors
- heartwood can be rot resistant even when it is not “hard”
What Extractives Do (The Four Big Effects)
1) Colour and colour change
Many characteristic colours in timber are extractive‑driven.
Extractives also change with oxidation and UV exposure.
This is why:
- cherry darkens dramatically
- walnut can lighten
- some tropical species dull or brown over time
Colour is partly anatomy.
But it is often chemistry first.
2) Smell
Volatile extractives (and related compounds) are what you smell when you plane cedar, pine, oak, or sapele.
Smell is a real identification channel because chemistry is species‑specific.
3) Natural durability (decay and insect resistance)
A large body of research treats extractives as a major driver of natural durability.
Extractives can have antifungal, insecticidal, or otherwise bioactive roles.
A review of naturally durable wood extractives notes they are often concentrated in heartwood and can impart resistance to wood‑destroying organisms.[1]
4) Permeability and treatability
Extractives can reduce treatability by:
- physically blocking pathways
- changing wetting behaviour
- interacting with preservatives and carriers
This is one reason heartwood is often more difficult to pressure treat than sapwood.
The Main Extractive Families You Will Actually Encounter
This is not exhaustive.
It is the set that explains most real-world behaviour.
Tannins (and tannin‑like phenolics)
Tannins are especially relevant in oak and some other species.
They explain two common issues:
- Iron staining (blue‑black or grey): A reaction between tannin-like extractives and iron can produce blue–black discolouration, especially when moisture enables the reaction.
- Oak and several other species are particularly prone because they have high tannin-like extractives.[3]
- Run‑off staining: Tannins can leach with water, leaving tea‑coloured stains on nearby materials.
Workshop rule: if you are working wet oak or exterior oak, assume iron + water = stain.
Oils and waxes (lipophilic extractives)
“Oily” woods are oily because of extractives.
These species can:
- resist wetting
- interfere with glue adhesion
- create finishing surprises
A standard wood adhesives reference (USDA/FPL Wood Handbook, adhesives chapter) notes that for some surfaces (e.g., teak), wood extractives can interfere with direct adhesive contact and reduce bond strength.[4]
Workshop rule: surface prep and timing matter more on oily wood.
Resins (especially in softwoods)
Resins and resin acids show up as:
- pitch pockets
- bleed-through under heat
- tool gumming
- adhesion problems in finishes
Resin is also a defence chemistry.
You are seeing the tree’s protective system.
Extractives and Finishing (Why “The Same Finish” Behaves Differently)
Extractives influence finishing through:
- wetting and penetration
- stain uptake (especially on ring-porous woods)
- colour shift under UV
- bleed (resin/extractive migration)
Practical decisions extractives force you to make:
- whether you need a barrier/sealer
- whether you should avoid reactive iron solutions on tannin‑rich woods (unless you want ebonising)
- whether you need grain filling (anatomy + chemistry)
Extractives and Gluing (Why Some Bonds Fail)
Extractives can:
- reduce surface energy and wetting
- migrate to the surface after planing
- create chemically weak boundary layers
If a glue-up fails on a wood known for oiliness or extractives, do not assume “bad glue.”
Assume chemistry until proven otherwise.
A Simple Diagnostic Framework
When a wood behaves “oddly,” ask four questions:
- Is this heartwood or sapwood? (chemistry likely differs)
- Is the behaviour water-related? (water-soluble extractives, tannins)
- Is the behaviour heat-related? (resin bleed, extractive migration)
- Is the behaviour adhesion-related? (oils/waxes affecting wetting)
This framework prevents most extractive surprises.
Media and Image Recommendations
- Iron stain demo
- oak + water + steel wool (or iron solution) showing blue‑black staining
- Heartwood vs sapwood comparison
- same species, show colour/durability contrast
- Resin pocket example
- pitch bleed through a finish on a warm day
- Finish penetration comparison
- same finish on low-extractive vs high-extractive species
What’s Next
Guide 6 — Wood Degradation Processes — explains what extractives are defending against: fungi, UV, hydrolysis, oxidation, and the slow breakdown pathways that turn wood into “old wood.”
🔗 Knowledge Network
Glossary Terms
- Extractives
- Heartwood
- Sapwood
- Tannins
- Phenolics
- Oils
- Waxes
- Resins
- Iron stain
- Treatability