Thermally Modified Ayous: The Complete Guide to Its Process, Properties and Benefits
Thermal modification is a controlled heat-treatment process that permanently alters the chemical structure of wood without using toxic chemical preservatives. The process improves the timber’s durability, dimensional stability, and resistance to biological degradation while producing a rich, darker appearance.
Overview of the thermal modification process
Ayous (Triplochiton scleroxylon) is particularly well suited to thermal modification because of its low density, straight grain, fine and uniform texture, and excellent machining characteristics. Its homogeneous cellular structure responds consistently to heat treatment, producing a lightweight timber with exceptional dimensional stability, improved durability, and a uniform, attractive appearance.
The process typically occurs in a sealed kiln or specialized thermal modification chamber where oxygen levels are kept very low by using steam, nitrogen, or a vacuum. This prevents the timber from burning while allowing high temperatures to alter the wood chemistry.
The three main stages
Stage 1 - Drying
The timber is gradually heated from ambient temperature to approximately 100–130°C.
During this stage:
Free water is removed from the wood cells.
Bound moisture begins leaving the cell walls.
Moisture content is reduced to nearly 0%.
Steam is often introduced to prevent excessive checking and cracking.This stage can take many hours depending on, timber thickness, species, initial moisture content.
Stage 2 - Thermal modification
The temperature is increased to approximately 160–220°C.
Typical temperatures include:
160–180°C – Improved stability
190–210°C – Most common for exterior products
210–220°C – Maximum durability & darker colourThe timber remains at peak temperature for 2–4 hours, but total heating cycles commonly last 24–72 hours. The atmosphere contains little or no oxygen while steam serves several purposes including, heat transfer, oxygen displacement, crack reduction and moisture control.
Stage 3 - Cooling and conditioning
The timber is slowly cooled. Controlled humidity or steam is added to raise the moisture content to approximately 4–7%.
Without conditioning, the timber would become excessively brittle.
The timber is then machined or graded before shipment.
What happens inside the wood?
The heat permanently changes the wood chemistry.
- Hemicellulose degradation
Hemicellulose is the least thermally stable component of wood. It begins to decompose first.
Effects include:
- Reduced hygroscopicity
- Lower equilibrium moisture content
- Greater dimensional stability
- Reduced fungal food source
This is the primary reason thermally modified wood absorbs far less moisture than untreated timber.
- Cellulose changes
Cellulose largely remains intact.
However:
- Some amorphous cellulose becomes more crystalline.
- Crystallinity increases.
- Water absorption decreases.
- Excessively high temperatures can begin degrading cellulose, reducing strength.
- Lignin modification
Lignin softens during heating before partially re-polymerizing.
The result is:
- Stiffer cell walls
- Greater biological resistance
- Improved dimensional stability
- Extractives change
Natural extractives within the wood are modified during the thermal treatment process. Some volatile compounds evaporate or are chemically altered, resulting in a more stable and uniform material.
For Ayous, this means:
- Minimal extractive migration due to its naturally low resin content
- Improved coating and finish adhesion
- Reduced risk of staining or extractive bleed
- More uniform colour development throughout the timber
- Enhanced long-term appearance in exterior applications
Why fungi struggle to attack thermally modified wood
Decay fungi require:
- Moisture
- Oxygen
- Suitable temperature
- Digestible carbohydrates
Thermal modification reduces two of these requirements.
The wood contains:
- Less available moisture
- Fewer digestible sugars from degraded hemicellulose
- As a result, decay organisms have much greater difficulty colonizing the timber.
Improvements in dimensional stability
One of the largest advantages is reduced movement.
Compared with untreated Ayous:
| Property | Untreated Ayous | Thermally Modified Ayous |
| Water absorption | Moderate | Much lower |
| Swelling | Moderate | Reduced by approximately 50–70% |
| Shrinkage | Moderate | Reduced by approximately 50–70% |
| Warping | Low to moderate | Very low |
| Cupping | Can occur with moisture cycling | Significantly reduced |
| Twisting | Low | Minimal |
| Seasonal movement | Moderate | Very low |
| Equilibrium moisture content (EMC) | Approximately 10–12% | Approximately 4–7% |
| Dimensional stability | Good | Excellent |
Improved biological durability
Ayous is already relatively stable before treatment. After thermal modification it becomes exceptionally stable.
Typical improvements include:
- 50–70% reduction in swelling
- Excellent resistance to cupping
- Minimal twisting
- Very low seasonal movement
However, thermal modification does not make the timber suitable for continuous ground contact or permanent immersion in water unless additional protection measures are used.
Aesthetic benefits
Heat causes chemical reactions similar to slow natural ageing.
Colour changes include:
- Pale yellow → golden brown
- Brown → chocolate brown
- Uniform colour throughout the section
The darker the treatment temperature:
- Darker appearance
- Higher durability
- Lower mechanical strength
If exposed outdoors without a UV-protective finish, the timber will gradually weather to a silver-grey, similar to untreated wood.
Practical benefits
Moisture Performance
One of the most important changes is equilibrium moisture content (EMC).
Untreated Ayous:
- Approximately 10–12% EMC in many temperate environments
Thermally modified Ayous:
- Approximately 4–7% EMC
This means the timber remains significantly drier throughout its service life.
Improved Machining
Advantages include:
- Cleaner machining
- Less resin clogging
- Better sanding
- Better coating adhesion
However:
Sharp tooling is essential. Brittle edges can chip during machining.
Environmental Benefits
Unlike pressure-treated timber, thermal modification:
- Uses heat rather than biocidal preservatives.
- Introduces no heavy metals or toxic chemicals into the wood.
- Allows the timber to be handled without concerns associated with preservative treatments.
- Can extend the service life of fast-growing plantation species, such as Radiata Pine, reducing demand for naturally durable tropical hardwoods.
Common applications of thermally modified Ayous
- Exterior cladding
- Decking
- Doors
- Pergolas
- Outdoor furniture
- Interior wall linings
- Ceiling panels
- Sauna interiors
- Architectural facades
- Screening and fencing
Advantages and limitations of thermally modified timber
Advantages
- Excellent dimensional stability
- Lower moisture absorption
- Improved resistance to fungal decay
- Reduced resin bleed
- Attractive dark hardwood-like appearance
- No added preservative chemicals
- Improved coating performance
- Longer service life in above-ground exterior use
- Better resistance to warping and twisting
- Sustainable use of plantation-grown timber
Limitations
- Reduced bending and impact strength
- More brittle than untreated timber
- Higher production cost due to the energy-intensive process
- Requires careful machining and handling
- Not intended for permanent ground contact or marine environments without additional design considerations
Thermal modification transforms Ayous from a relatively low-durability, moisture-sensitive softwood into a premium architectural timber with enhanced stability, durability, and aesthetics. By heating the wood to 160–220°C in a low-oxygen environment, the process permanently alters hemicellulose, cellulose, and lignin, reducing moisture uptake and improving resistance to fungal decay.