Every timber has a chemical fingerprint. The natural compounds deposited inside the wood — resins, tannins, oils, phenols, terpenes — determine its colour, its smell, its durability, its compatibility with finishes, and even whether it’s safe to breathe. Extractives are the hidden layer behind almost every practical property we’ve discussed so far.
We’ve already met extractives several times in this track: they drive natural durability (Guides 4–5), they affect workability and gluing (Guide 6), and they influence hardness and density in subtle ways. This guide brings all of that together and goes further — into what extractives actually are, why different species produce different ones, and what they mean for anyone selecting, working with, or specifying timber.
What Extractives Are
Extractives are non-structural organic compounds found in wood that can be removed (“extracted”) using solvents such as water, alcohol, or organic solvents.
They are distinct from the three structural polymers that make up the cell wall:
- Cellulose (~40–50% of wood by weight) — the load-bearing fibre
- Hemicellulose (~20–30%) — the matrix binding cellulose fibrils
- Lignin (~20–30%) — the rigid binder that holds cells together
Extractives typically make up 1–20% of the dry weight of wood, depending on the species. In some tropical hardwoods, extractive content can exceed 20%.
They are found in:
- Cell lumens (the hollow centres of cells)
- Resin canals (in softwoods that have them)
- Ray cells (which transport and store chemicals horizontally)
- Cell walls (some extractives are deposited within the wall itself)
Why Trees Produce Extractives
Extractives are not accidental. They are part of the tree’s defence and metabolism.
Heartwood formation
The primary site of extractive deposition is the heartwood. As sapwood cells die and convert to heartwood (Track 1, Guide 6), the tree deposits extractives into the dead cells. These chemicals:
- Protect the dead core from fungal and insect attack
- Seal the cells by blocking pit membranes and filling lumens, reducing water movement through the heartwood
- Store waste products that the tree no longer needs
This is why heartwood is coloured and sapwood is pale. The colour is the extractives.
Wound response
Trees also produce extractives in response to injury. When bark is damaged or a branch breaks, the tree may produce resin, gums, or other defensive chemicals at the wound site to seal the opening and deter pathogens.
Resin canals
Some softwoods (pine, spruce, larch, Douglas fir) have specialised resin canals — tubular passages lined with cells that secrete oleoresin. This resin is a sticky, aromatic mixture of terpenes and resin acids that seals wounds and deters insects.
Not all softwoods have resin canals. Cedar, yew, and most cypresses do not. Among hardwoods, resin canals are rare (though some tropical species have gum ducts).
Types of Extractives
Extractives are chemically diverse. The major categories relevant to timber use are:
Terpenes and terpenoids
These are volatile organic compounds responsible for many of the smells associated with wood.
- Monoterpenes (e.g. α-pinene, limonene) — the sharp, fresh smell of freshly cut pine or spruce. Highly volatile; they evaporate quickly.
- Sesquiterpenes — heavier, less volatile. Contribute to the characteristic scent of cedar and sandalwood.
- Diterpene resin acids (e.g. abietic acid, pimaric acid) — the sticky, non-volatile component of softwood resin. These are what make pine resin gummy and persistent.
Terpenes are dominant in softwood resin and are the basis of turpentine (historically distilled from pine resin) and rosin (the solid residue after distillation).
Phenolic compounds
A large and important group that includes:
- Tannins — complex polyphenols found in high concentrations in oak, sweet chestnut, and many tropical hardwoods. Tannins are responsible for:
- The brown colour of oak heartwood
- Iron tannate staining (black marks when tannin-rich wood contacts iron in the presence of moisture)
- Durability — tannins are toxic to many decay fungi
- Leather tanning — historically, oak and chestnut bark were the primary source of tanning agents
- Flavonoids — contribute to yellow, orange, and red colours in many species. Robinia (black locust) gets its vivid yellow-green heartwood from flavonoid extractives.
- Stilbenes — found in pine heartwood and some tropical species. Contribute to durability and colour.
- Lignans — found in various softwoods and hardwoods. Some have antifungal properties.
Quinones
Powerful bioactive compounds found in several highly durable tropical species:
- Tectoquinone in teak — contributes to its exceptional durability and golden colour
- Lapachol in ipe and other Tabebuia species — strongly antifungal and insect-resistant
- Deoxylapachol and related naphthoquinones in various tropical hardwoods
Quinones are among the most effective natural biocides in wood and are a major reason why species like teak and ipe achieve Class 1 durability.
Tropolones
- Thujaplicin (β-thujaplicin, also known as hinokitiol) in Western Red Cedar and Alaskan Yellow Cedar — a powerful antifungal compound that gives these low-density species their surprising durability
- Tropolones are unusual in nature and highly effective against a broad spectrum of decay fungi
Oils and fats
- Teak oil — not a true drying oil, but a mixture of extractives that give teak its greasy feel and water-repellent surface
- Oleoresins in softwoods — mixtures of terpenes and resin acids
- Various fatty acids and triglycerides present in small quantities in most species
Oily extractives affect gluing (by blocking adhesive penetration), finishing (by repelling water-based products), and moisture uptake (by reducing wettability).
Sugars and starches
Found primarily in sapwood, these are food reserves stored by the living tree:
- Starch in ray cells and axial parenchyma
- Simple sugars in recently active sapwood
These are not protective — in fact, they attract insects. Powderpost beetles (Lyctus spp.) specifically target sapwood with high starch content and large pores (ring-porous hardwood sapwood is especially vulnerable).
Inorganic deposits
- Silica (silicon dioxide) — deposited in the cells of some tropical hardwoods (teak, iroko, keruing). Causes rapid tool dulling, as covered in Guide 6.
- Calcium carbonate and calcium oxalate — crystal deposits found in some species, contributing to blunting and occasionally to health risks from dust.
What Extractives Do: A Summary of Effects
| Effect | Mechanism | Key species examples | | — | — | — | | Colour | Phenolics, quinones, flavonoids absorb light at specific wavelengths | Walnut (brown), Purpleheart (purple), Padauk (red-orange), Robinia (yellow) | | Smell | Volatile terpenes and other compounds evaporate from the surface | Cedar (thujaplicin), Pine (monoterpenes), Camphorwood (camphor) | | Durability | Toxic extractives kill or inhibit decay fungi and insects | Teak (tectoquinone), Ipe (lapachol), Cedar (thujaplicin), Oak (tannins) | | Iron staining | Tannins react with ferrous iron to form black iron tannate | Oak, Sweet Chestnut, Walnut | | Gluing difficulty | Oily extractives form a surface barrier to adhesives | Teak, Cocobolo, Rosewood, Ipe | | Finish compatibility | Oils repel water-based finishes; tannins can bleed through paint | Teak (oily), Cedar (tannin bleed), Redwood (tannin bleed) | | Tool dulling | Silica crystals abrade cutting edges | Teak, Iroko, Keruing | | Health hazards | Allergenic, irritant, or toxic compounds in dust or on skin contact | Cedar (plicatic acid), Cocobolo (quinones), Mansonia (toxins) | | Corrosion | Acidic extractives corrode ferrous metals | Oak, Sweet Chestnut, Western Red Cedar | | Colour change over time | Extractives oxidise or break down under UV light | Cherry (darkens), Walnut (lightens), Padauk (dulls), Purpleheart (browns) |
Resin in Softwoods: A Closer Look
Resin is the most visible and tactile extractive in everyday woodworking. Anyone who has worked with pine, spruce, or larch has encountered it.
What resin is
Softwood resin (oleoresin) is a mixture of:
- Volatile terpenes (turpentine fraction) — thin, aromatic, evaporate over time
- Non-volatile resin acids (rosin fraction) — sticky, solidify when terpenes evaporate
Fresh resin is fluid and sticky. As the volatile fraction evaporates, it becomes hard, brittle, and amber-coloured.
Where resin occurs
- Resin canals — normal longitudinal and radial canals in pine, spruce, larch, and Douglas fir
- Traumatic resin canals — formed in response to injury; can appear in species that don’t normally have resin canals
- Resin pockets — lens-shaped cavities between growth rings, filled with liquid resin. Common in spruce and sometimes in Douglas fir. These can be a significant defect in joinery timber.
Practical problems with resin
- Bleeding: Resin can ooze from knots and resin pockets, especially when timber is heated (e.g. by sun exposure or during kiln drying). This damages paint films and stains surfaces.
- Gumming: Fresh resin gums up saw blades, planer knives, and sandpaper. Blade cleaner and slower feed rates help.
- Finish interference: Resin on the surface prevents paint and some coatings from adhering. Surfaces should be cleaned with white spirit or a resin-dissolving solvent before finishing.
- Kiln drying: High-temperature kiln schedules are sometimes used to “set” resin — driving off the volatile fraction so that it’s less likely to bleed later. This is standard practice for construction-grade pine and spruce.
De-resinating
For high-quality joinery, resin-rich surfaces can be treated:
- Wipe with white spirit, acetone, or methylated spirit to dissolve surface resin
- For knots, apply knotting solution (shellac-based) to seal the resin before painting
- Heat can mobilise resin from deeper in the wood — this is why resin sometimes bleeds through paint on south-facing surfaces months after installation
Tannin Bleed and Iron Staining
Two of the most common practical problems caused by extractives are tannin bleed and iron tannate staining.
Tannin bleed
When tannin-rich timber is exposed to water (rain, condensation, or even high humidity), water-soluble tannins can migrate to the surface and stain it — or stain surfaces below.
- Cedar and redwood cladding often produce brown tannin runs on masonry, render, or paving below when first installed, especially during wet weather
- Oak can produce dark stains on adjacent lighter materials
- Tannin bleed through paint occurs when water-soluble tannins dissolve and migrate through porous paint films, leaving brown marks. Prevention: use a shellac-based primer (“stain block”) before painting tannin-rich species.
Iron tannate staining
When tannin-rich wood contacts ferrous metal (iron or steel) in the presence of moisture, a chemical reaction produces iron tannate — a deep blue-black compound.
This is the same chemistry used historically to make iron gall ink (the standard writing ink of medieval Europe).
In practice:
- A steel clamp left overnight on damp oak produces a vivid black mark
- Steel nails and screws in oak or chestnut develop black halos
- Iron filings or steel wool residue on oak surfaces create black spots
Prevention:
- Use stainless steel or non-ferrous fasteners (brass, bronze, silicon bronze) in tannin-rich species
- Keep steel machinery tables and clamp faces clean, or use protective pads
- Remove iron staining with oxalic acid solution (a mild bleach for wood) — this dissolves the iron tannate and restores the natural colour
Colour and Colour Change
Extractives are responsible for the colours of timber — and for the changes those colours undergo over time.
Initial colour
The colour of freshly cut heartwood is determined by the type and concentration of extractives:
- Walnut: Rich chocolate brown (phenolics and quinones)
- Cherry: Warm pink-salmon (flavonoids)
- Padauk: Vivid red-orange (pterocarpans)
- Purpleheart: Deep violet (oxidised extractives)
- Yellowheart: Bright yellow (flavonoids)
- Ebony: Dense black (high concentrations of various phenolics)
- Robinia: Yellow-green (flavonoids)
- European Oak: Golden tan (tannins)
Sapwood in nearly all species is pale cream to white — the absence of extractives means the absence of colour.
Colour change over time
Almost all timber changes colour with exposure to light and air:
- Cherry darkens dramatically from pink to a rich, warm reddish-brown. This is one of the most pronounced colour shifts and is driven by UV-induced oxidation of extractives.
- Walnut lightens from dark chocolate to a more golden brown over time.
- Padauk dulls from vivid orange-red to a muted brown as the surface extractives oxidise.
- Purpleheart shifts from bright purple to a darker brown-purple.
- Teak weathers to silver-grey when exposed to UV and rain outdoors (the extractives leach and the surface lignin degrades).
- Oak darkens slightly and develops a warmer, deeper tone.
These changes are primarily driven by UV radiation and oxidation. They occur fastest in direct sunlight and can be slowed (but not fully stopped) by UV-inhibiting finishes.
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Practical tip: If you’re making furniture from cherry, walnut, or any colour-changing species, expose all components to light equally before final assembly. Otherwise, a leaf from an extension table — stored in a dark cupboard — will be a different colour from the rest of the table when it’s brought out.
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Extractives and Finishing
The interaction between extractives and surface finishes is one of the most practically important topics for woodworkers and specifiers.
Oily species
Timbers with oily extractives (teak, rosewood, cocobolo, ipe, olivewood) resist water-based finishes. The oil sits on the surface and prevents the finish from wetting and bonding to the wood.
Solutions:
- Wipe the surface with acetone or naphtha immediately before finishing to temporarily dissolve surface oils
- Use oil-based finishes (tung oil, Danish oil) which are chemically compatible with the wood’s own oils
- Shellac is an excellent sealer for oily species — it bonds well and provides a base for topcoats
- Epoxy is the most reliable adhesive for oily species
Tannin-rich species
Painting or coating tannin-rich species (oak, cedar, redwood, chestnut) can result in:
- Tannin bleed through water-based primers and paints
- Brown staining on the surface of light-coloured coatings
Solutions:
- Apply a shellac-based stain block primer before painting
- Use oil-based primers which are less susceptible to tannin bleed than water-based ones
- Ensure timber is dry before coating — moisture mobilises tannins
Resinous species
- Clean resin from the surface with solvent before finishing
- Seal knots with knotting solution (shellac) before painting
- Avoid dark-coloured finishes on resin-prone areas — resin can lift the finish film
Blotch-prone species
Some species absorb stain unevenly because of variations in extractive distribution, cell density, or grain porosity:
- Cherry — notorious for blotchy stain absorption
- Birch — uneven density between earlywood and latewood
- Pine — resin-rich areas repel stain; adjacent areas absorb heavily
- Maple — tight grain absorbs stain unevenly
Solutions:
- Apply a pre-stain conditioner (a thinned sealer that partially fills the wood surface, evening out absorption)
- Use gel stains (which sit on the surface rather than penetrating, giving more uniform colour)
- Use dye-based stains instead of pigmented stains — dyes penetrate more evenly
Extractives and the Species Database
On Timber Logic, every species page will note key extractive characteristics:
- Durability class (driven by extractives)
- Known staining or bleed risks
- Gluing notes (oily species flagged)
- Health hazards from dust
- Expected colour change over time
- Silica content (where relevant)
Understanding extractives helps you read a species profile holistically — not as a list of isolated numbers, but as an interconnected picture of how the timber will behave from sawmill to finished product.
Media and Image Recommendations
- Photo series: extractive-driven colour range
- Freshly planed samples of walnut, cherry, padauk, purpleheart, robinia, and ebony — showing the natural colour palette created by different extractive chemistries
- Photo: colour change over time
- Cherry or walnut samples at day 1, 3 months, and 12 months of light exposure — showing the progression
- Photo: iron tannate staining on oak
- Before and after: a steel clamp mark on wet oak, and the same mark after oxalic acid treatment
- Photo: tannin bleed on cladding
- Cedar or redwood cladding with brown tannin runs on the masonry below
- Close-up photo: resin pocket in spruce
- Cross-section showing a lens-shaped resin pocket between growth rings, with liquid resin visible
- Diagram: where extractives are found in a trunk
- Cross-section showing sapwood (low extractives, pale), outer heartwood (high extractives, rich colour), and inner heartwood (moderate extractives, sometimes less durable)
The Key Idea
<aside> 💡
Extractives are the chemical identity of a species. They create colour, smell, and durability. They determine how timber interacts with finishes, adhesives, metals, and biological organisms. They also pose health risks when inhaled as dust. You can’t see most extractives directly — but you see their effects every time you look at, smell, cut, or finish a piece of timber.
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What’s Next
In Guide 8 — Toxicity and Wood Dust, we focus on the health side of extractives. Which species produce hazardous dust? What are the specific risks — respiratory, dermal, and systemic? And what practical measures should every woodworker take to protect themselves?
🔗 Knowledge Network
Species Pages
- Walnut — colour, iron staining, colour change over time
- Cherry — dramatic UV-driven colour change
- Padauk — vivid colour, rapid oxidation dulling
- Purpleheart — colour shift from oxidised extractives
- European Oak — tannins, iron tannate staining, corrosion
- Teak — oily extractives, tectoquinone, silica, gluing difficulty
- Western Red Cedar — thujaplicin, tannin bleed, corrosion
- Sweet Chestnut — tannins, iron staining
- Robinia — flavonoid-driven yellow-green colour
- Ebony — dense phenolic extractives
- Pine / Spruce — monoterpenes, resin bleed, resin pockets
- Douglas Fir — resin canals, oleoresin
- Larch — resinous softwood
- Cocobolo — oily extractives, gluing difficulty
- Rosewood — oily extractives, finish interference
- Ipe — lapachol, oily surface, silica content
- Iroko — silica deposits, tool dulling
Glossary Terms
- Extractives
- Terpenes
- Monoterpenes
- Tannins
- Flavonoids
- Quinones
- Tectoquinone
- Tropolones
- Thujaplicin
- Oleoresin
- Turpentine
- Rosin
- Iron Tannate
- Tannin Bleed
- Silica
- Resin Canal
- Resin Pocket
- Knotting Solution
- Oxalic Acid
Calculators
- None for this guide
Related Guides
- Track 3 – Guide 4 – Wood Durability Classes
- Track 3 – Guide 5 – Rot Resistance and Fungal Decay
- Track 3 – Guide 6 – Workability of Timber
- Track 3 – Guide 8 – Toxicity and Wood Dust
- Track 1 – Guide 6 – Heartwood vs Sapwood
- Track 3 – Guide 9 – Stability Differences Between Species
- Track 2 – Guide 10 – Dimensional Stability