Tar Properties
Part of Adhesives
Understanding the physical and chemical properties of wood tar to select and apply it correctly for waterproofing, preservation, and adhesive use.
Why This Matters
Wood tar is one of the most versatile materials a rebuilding community can produce. It waterproofs boats, preserves timber, protects rope, treats leather, caulks seams, and serves as a base for adhesive pitch. But tar is not a single substance — it is a complex mixture of hundreds of chemical compounds, and its properties vary dramatically depending on the wood source, production temperature, and fraction collected. Using the wrong tar for a job leads to failure: too-thin tar runs off surfaces, too-thick tar cracks in cold weather, creosote-heavy tar poisons soil around fence posts.
Understanding tar’s physical and chemical properties lets you select the right fraction for each application, modify tar with additives for specific performance, and troubleshoot when a tar product fails. This knowledge separates competent tar use (which built the world’s navies and preserved its infrastructure for millennia) from haphazard smearing that wastes time and material.
Wood tar was so economically important that entire regions specialized in its production. Scandinavian “tar pits” exported millions of barrels. The word “Stockholm” tar became a quality standard. The British Navy’s demand for tar shaped colonial policy. Understanding why tar works, and how, is understanding one of civilization’s foundational material sciences.
Chemical Composition
Wood tar is produced by destructive distillation — heating wood without oxygen. The heat breaks down cellulose, lignin, and other wood polymers into a complex mixture of smaller molecules.
Major Component Classes
| Component Class | Percentage | Properties | Role |
|---|---|---|---|
| Phenols (creosote, guaiacol, cresol) | 20-35% | Antiseptic, toxic to organisms, strong odor | Preservative action |
| Organic acids (acetic, formic) | 10-20% | Acidic, corrosive, volatile | Evaporate during processing |
| Polycyclic aromatic hydrocarbons (PAHs) | 5-15% | Dark color, carcinogenic, persistent | Waterproofing, UV resistance |
| Terpenes and terpenoids | 5-15% | Aromatic, solvent properties | Penetration into wood |
| Resin acids | 10-20% | Sticky, thermoplastic | Adhesive properties |
| Water | 10-30% (in raw tar) | Dilutes tar, must be removed | Evaporates during settling |
| Methanol and acetone | 2-5% | Volatile solvents | Evaporate early |
| Pitch residue | 5-15% | High molecular weight, solid | Forms the body of pitch |
How Source Wood Affects Composition
| Wood Source | Tar Character | Best Applications |
|---|---|---|
| Pine heartwood | Resinous, thick, aromatic, high in terpenes | Waterproofing, adhesive pitch |
| Pine sapwood | Lighter, more acidic, lower resin content | Wood preservation, rope treatment |
| Birch bark | Contains betulin, very sticky, flexible | Adhesive (birch tar pitch) |
| Hardwood (oak, beech) | Thin, acrid, high in acetic acid | Pyroligneous acid for pickling, not adhesive |
| Fatwood (resin-saturated) | Very thick, dark, resin-rich | Best quality tar for all purposes |
Best Raw Material
For adhesive and waterproofing tar, always prioritize resinous conifer wood — pine stumps, fatwood, and heartwood. Hardwood tar is useful for other purposes (wood preservation via creosote) but makes poor adhesive.
Physical Properties
Viscosity
Tar viscosity varies enormously depending on composition and temperature:
| Condition | Viscosity | Comparable Substance |
|---|---|---|
| Raw tar, warm (40°C) | Low | Cooking oil |
| Raw tar, room temperature | Medium | Honey |
| Reduced tar (50% evaporated) | High | Cold honey / molasses |
| Pitch (fully reduced) | Very high | Solid at room temperature |
| Cold pitch (-10°C) | Brittle | Glass-like, shatters on impact |
Temperature sensitivity is one of tar’s most important properties. It softens and flows with heat, stiffens and hardens with cold. This thermoplastic behavior makes tar easy to apply (heat it up) and durable in place (it sets as it cools), but it also means tar-coated surfaces can soften in extreme heat.
Density
Raw wood tar has a density of approximately 1.05-1.15 g/cm3 — slightly heavier than water. This means tar sinks in water, which is important when treating underwater surfaces (it stays in place rather than floating away).
Adhesion
Tar adheres well to porous materials through mechanical interlocking — it flows into wood grain, fiber gaps, and surface roughness, then hardens in place. Adhesion to non-porous surfaces (metal, glazed ceramics, glass) is poor. For these surfaces, tar must be used in combination with mechanical fastening.
Water Resistance
Tar is hydrophobic — it repels water effectively. A properly applied tar coating prevents water penetration into wood for years. However, tar is not permanently waterproof in continuous submersion. Underwater surfaces (boat hulls, bridge pilings) need periodic re-tarring, typically annually.
Tar Fractions
During distillation, different compounds come off at different temperatures. Collecting tar in stages yields fractions with different properties.
Light Fraction (First Runnings)
- Temperature: 100-200°C
- Appearance: Thin, watery, yellowish
- Composition: Mostly water, acetic acid, methanol, acetone, light terpenes
- Uses: Pyroligneous acid (wood vinegar) — used as an antiseptic, insect repellent, and soil acidifier. Not useful as adhesive
- Handling: Corrosive, strong vinegar smell, flammable vapors
Middle Fraction (Primary Tar)
- Temperature: 200-350°C
- Appearance: Dark brown to black, moderately viscous
- Composition: Phenols, creosote, resin acids, heavy terpenes
- Uses: The main product — waterproofing, wood preservation, rope treatment, adhesive base
- Properties: Strong antiseptic, good penetration, moderate viscosity
Heavy Fraction (Pitch)
- Temperature: 350-500°C
- Appearance: Black, very thick to solid
- Composition: High molecular weight PAHs, polymerized compounds
- Uses: Caulking, adhesive pitch, roofing, heavy waterproofing
- Properties: Highest waterproofing, lowest penetration, most brittle
Testing Tar Quality
Without laboratory equipment, you can assess tar quality through simple field tests:
Drop Test
Place a drop of tar on a smooth surface at room temperature. After 1 hour:
- Good waterproofing tar: drop holds its dome shape, doesn’t spread
- Too thin: drop flattens and spreads — needs further reduction
- Good adhesive pitch: drop is solid and doesn’t deform when poked
Stretch Test
Warm a small amount between your fingers:
- Good flexibility: stretches into thin threads without breaking
- Too brittle: snaps without stretching — needs more oil or wax
- Too soft: won’t hold shape — needs further cooking or more charcoal filler
Burn Test
Touch a flame to a tar sample:
- High terpene content: ignites easily, burns with sooty yellow flame
- Well-reduced pitch: harder to ignite, burns more slowly
- Heavily oxidized/old tar: difficult to ignite, smolders rather than flames
Smell Test
- Fresh pine tar: strong, pleasant pine/turpentine aroma
- Creosote-heavy tar: sharp, acrid, medical smell — high preservative content
- Overcooked tar: burnt, acrid — some degradation has occurred
- Rancid/spoiled: unusual sour smell — water contamination, bacterial action
Applications by Property
| Application | Required Properties | Tar Type | Additives |
|---|---|---|---|
| Boat caulking | Thick, waterproof, flexible | Heavy pitch | Beeswax for flexibility |
| Rope preservation | Penetrating, antiseptic | Middle fraction, warm | None — apply by soaking |
| Timber preservation | Penetrating, creosote-rich | Middle fraction, hot | Thin with turpentine if needed |
| Roofing | Thick, weather-resistant | Reduced tar / pitch | Fiber for reinforcement |
| Adhesive | Strong tack, gap-filling | Pitch + charcoal | Charcoal powder, beeswax |
| Leather treatment | Flexible, waterproof | Middle fraction + fat | Tallow or lard, 50/50 |
| Medical (wound seal) | Antiseptic, thin film | Light middle fraction | Apply very thin |
Health and Safety
Tar contains compounds that are hazardous with prolonged exposure:
- Creosote and phenols: skin irritants, toxic if ingested, carcinogenic with chronic exposure
- PAHs (polycyclic aromatic hydrocarbons): carcinogenic — avoid prolonged skin contact and inhalation of hot tar fumes
- Volatile compounds: flammable vapors during production and hot application
Practical Precautions
- Work outdoors or in well-ventilated areas when heating tar
- Avoid skin contact with hot tar (burns) and prolonged contact with cold tar (chemical irritation)
- Do not use tar-treated wood for food contact — cutting boards, bowls, food storage containers should never be tarred
- Wash hands with oil (fat, vegetable oil) after handling tar — water alone won’t remove it
- Do not burn tar-treated wood indoors — toxic smoke
Removing Tar from Skin and Tools
| Surface | Removal Method |
|---|---|
| Skin | Rub with animal fat or vegetable oil, then wash with soap/ash water |
| Metal tools | Heat gently and wipe with cloth; residue dissolves in turpentine |
| Clothing | Freeze (if possible) and scrape; soak in oil then wash |
| Wood surfaces | Scrape while warm; sand if needed |
Storage and Shelf Life
Tar stores well but changes over time:
- Sealed containers: lasts years with minimal change. The key is preventing both evaporation (which thickens tar) and water ingress (which weakens it)
- Open containers: surface oxidizes and thickens; stir before use
- Temperature: store in cool location. Tar in hot storage slowly polymerizes and becomes increasingly difficult to work
- Never add water to tar for storage — it causes separation and spoilage
Wood tar’s properties — waterproofing, preservation, adhesion, antiseptic action — made it one of the foundational materials of pre-industrial civilization. Understanding these properties lets a rebuilding community extract maximum utility from a single production process, matching the right tar fraction to each application for reliable, long-lasting results.